Hydrophilic and superhydrophilic surfaces and materials

Abstract

The term superhydrophilicity is only 11–12 years old and was introduced just after the explosion of research on superhydrophobic surfaces, in response to the demand for surfaces and coatings with exceptionally strong affinity to water. The definition of superhydrophilic substrates has not been clarified yet, and unrestricted use of this term to hydrophilic surfaces has stirred controversy in the last few years in the surface chemistry community. In this review, we take a close look into major definitions of hydrophilic surfaces used in the past, before we review the physics behind the superhydrophilic phenomenon and make recommendation on defining superhydrophilic surfaces and coatings. We also review chemical and physical methods used in the fabrication of substrates on surfaces of which water spreads completely. Several applications of superhydrophilic surfaces, including examples from the authors' own research, conclude this review.

Full text

Hydrophilic and Superhydrophilic Surfaces and Materials
*
Jaroslaw Drelich,
1)
Emil Chibowski,
3)
Dennis Desheng Meng,
2)
and Konrad Terpilowski
3)
1)
Department of Materials Science and Engineering
2)
Department of Mechanical Engineering - Engineering Mechanics
Michigan Technological University
Houghton, MI 49931, USA
3)
Department of Physical Chemistry-Interfacial Phenomena
Faculty of Chemistry
Maria Curie-Sklodowska University
20-031 Lublin, Poland
Abstract
The term superhydrophilicity is only 11-12 years old and was introduced just after the explosion
of research on superhydrophobic surfaces, in response to demand for surfaces and coatings with
exceptionally strong affinity to water. The definition of superhydrophilic substrates has not been
clarified yet, and unrestricted use of this term to hydrophilic surfaces has stirred controversy in
the last few years in the surface chemistry community. In this review, we take a close look into
major definitions of hydrophilic surfaces used in the past, before we review the physics behind
superhydrophilic phenomenon and make recommendation on defining superhydrophilic surfaces
and coatings. We also review chemical and physical methods used in fabrication of substrates on
surfaces of which water spreads completely. Several applications of superhydrophilic surfaces,
including examples from the authors’ own research, conclude this review.
Keywords: hydrophilic material, hydrophilic surface, superhydrophilic surface,
superhydrophilic coating, superhydrophilicity
1. Introduction
The terms “hydrophilic surface” and “hydrophobic surface” appeared in the literature many
decades ago and they are commonly used to describe opposite effects of the behavior of water on
a solid surface. A hydrophilic surface has strong affinity to water whereas hydrophobic surface
repel water. This simple definition however, is too general for the classification of a variety of
different solids having different wetting characteristics, typically studied in three-phase systems
with water and air or water and oil as fluids. Surprisingly, a variety of different definitions of
hydrophilic and hydrophobic surfaces is used by the diverse scientific community. We found it
important to briefly review the most common definitions in this paper.
The interest in manipulating hydrophilicity and hydrophobicity of solid surfaces and producing
coatings with either strong or poor affinity to water exploded in the last twenty years, especially
after a wide acceptance that liquid spreading control can simply be accomplished through
*
Published in: Soft Matter, Vol. 7, No. 21, 2011, pp. 9804-9828.
1

changes in surface roughness and topography. Superhydrophobicity, superhydrophilicity, and
superwetting are now the most popular topics in wetting studies and many research groups
attempt to understand and reveal the physics behind liquid penetrating (or suspending on) the
surfaces of complex geometry and structure, often controlled at sub-microscopic level. The
fundamentals of superhydrophobicity, fabrication of water-repelling surfaces and coatings and
their applications were reviewed by several authors on a number of occasions since 2005.
1-15
There is however, almost no review of research on superhydrophilic surfaces, and this paper
intends to fill this gap.
The term superhydrophilicity appeared for the first time in the technical literature in 2000, in
four papers published by three different research groups from Japan.
16-19
Roots of this term could
probably be dated back to 1996, when Onda et al.
20,21
published two, currently highly-cited,
papers on wettability of fractal (rough) surfaces in which the terms of superhydrophobic and
superweeting surfaces were proposed. In 1997, Fuijishima et al.
22
demonstrated superhydrophilic
effect on a glass slide coated with a thin TiO
2
polycrystalline film (Figure 1). The spreading of
water was the result of both hydrophilic properties of anatase exposed to UV radiation and
submicroscopic roughness of coating, although the effect of water spreading was entirely
attributed to photoinduced self-cleaning capability of TiO
2
at that time, and the term
superhydrophilicity was not used.
Since 2000, number of papers published on preparation of superhydrophilic surfaces and
coatings persistently increases every year. Figure 2 shows the number of papers published
between 2000 and 2010, in which either term “superhydrophilic” or “superhydrophilicity” were
used, per search tool of ISI Web of Knowledge.
This paper reviews the last-decade of the research in this new field, and goes beyond it. It is
organized as follows: First we review the definitions of hydrophilic solids and surfaces,
including the most common misconceptions used, to show that there is a necessity for better
quantification of this term. In the first section, we also provide examples of naturally occurring
hydrophilic solids, which in recent years, are sometimes incorrectly called superhydrophilic.
Then, we analyze the issue of complete water spreading on hydrophilic surfaces. High quality
superhydrophilic surfaces cannot be fabricated without control over the hydrophilicity of
materials used. For this reason we provide a brief overview of the methods commonly used for
enhancing hydrophilicity of surfaces. Since all surfaces, particularly hydrophilic ones, are prone
to contamination, this topic is also briefly reviewed. In the second half of the paper, we define
superhydrophilic surfaces and briefly discuss the means of enhancing spreading of a liquid over
not-smooth surfaces. Because roughness and topography of the surface are critically important to
the design of smart superhydrophilic surfaces and coatings, we critically review basic models
that describe the behavior of liquid on rough surfaces. For all of the current advancements over
the last several years, superhydrophilic coatings are still in their infancy but are just now moving
toward several possible applications and commercialization. To appreciate this progress, in the
last segment of this paper we review the research on superhydrophilic surfaces and coatings, as
applied to different possible products and devices.
2. Defining Hydrophilic Surfaces and Examples of Hydrophilic Materials
2

The term hydrophobicity originates from two words of ancient Greek; hydro (water) and phobos
(fear) and was originally ascribed to a property of molecules. The hydrophobic molecules, when
dispersed in water, are either repelled from it towards the water surface or aggregate into
micellar structures. However, they dissolve easily in an nonpolar solvent, e.g. alkane, whose
molecules are also hydrophobic. Hydrophobic property results from the absence of any
permanent or induced electrical dipole of the molecule and lack of ability to form hydrogen
bonds. Therefore, hydrophobic molecules interact with water only through London dispersion
force. The same is true for hydrophobic surface formed of solely apolar (i.e. hydrophobic)
molecules, on which a water droplet will exhibit large contact angle. It was arbitrary assumed
that on a hydrophobic surface the contact angle has to be 90
o
or larger.
23
In fact, the above
definition of hydrophobicity can be generalized respectively in order to definite lyophobic
molecule and lyophobic surface, i.e. the surface to which a liquid has ‘phobos’ (does not like it)
and forms drop if used in small amount.
From thermodynamic point of view, the solid/liquid system, free of any chemical reaction, tends
to minimize its free energy what can be realized via physical interactions (physical bonding
formation). In case of a hydrophobic molecule suspended in a polar medium (water), the medium
will repel the molecule because neither polar nor hydrogen bonds can develop. Water molecules,
on the other hand, form hydrogen bonds between themselves. Hydrophobic interaction (effect) is
the result of these two competitive interactions.
24
Free energy (ΔG) associated with mixing
apolar molecules with water depends on both the entropy ΔS and enthalpy ΔH changes in the
system, where ΔG = ΔH – TΔS. The entropy decreases because water molecules loose
translational and rotational mobility around the apolar (hydrophobic) molecules (solvation shell).
At room temperatures these entropy changes are driving-force for the hydrophobic effect. On the
other hand, the enthalpy changes are important in water-water hydrogen bond formation around
the apolar molecules. Moreover, the hydrophobic effect does not depend much on the
temperature increase because both the entalphy and entropy increase compensate their changes.
24
The term lypohilicity also originates from Greek (fat-liking) and expresses ability of a substrate
to dissolve in a lypophylic solvent,that is in an nonpolar solvent like alkane. The term lypophylic
signifies also property of a solid or liquid opposite to lypophobic one. By the same, the lypophilic
substances are either insoluble or poorly soluble in water.
2.1. Solubility Criterion
Historically substances (including molecules and ions), therefore, have been called hydrophilic if
they are readily soluble in water, in contrast to hydrophobic substances that are poorly soluble in
aqueous environment.
25
Hydrophilic solids are often hygroscopic and pick up water from the
air.
26
Taking simple examples from the kitchen, both salt (sodium chloride; electrolyte) and
sugar (sucrose; nonelectrolyte) easily dissolve in water, in large quantities, and both of these
substances are therefore hydrophilic, as per this general definition. Since surfaces of salt and
sugar crystals are chemically identical to the composition of bulk of the crystals, they must be
hydrophilic as well. In fact, mining and mineral processing community has recognized
hydrophilicity of natural salts such as halite (NaCl) and potash (KCl) for a long time. These
minerals are naturally not floatable and air bubbles will not stick to their surfaces in water.
27
According to new studies finite contact angles for saturated salt solutions were observed for
some of the soluble salt crystals such as KI, KCl, NaHCO
3
.
28,29
Other natural inorganic salts,
3

majority of organic pharmaceutics, and various artificial and natural organics including many
polymers dissolve enthusiastically in water as well, releasing individual molecules and/or ions
from a solid matrix, which then become surrounded by water molecules. This means that we
already identified a large group of materials that are hydrophilic and have hydrophilic surfaces.
A dissolution test could be misleading however, in identification of many solids having
hydrophilic surfaces. The solubility process is governed by the balance of intermolecular forces
between molecules of liquid and solid, together with an entropy change that accompanies the
dissolution and solvation.
26
For example, detergents, although soluble in water are classified
under the group of amphipathic substances with dissolution in aqueous phase controlled by their
hydrophilic-lyophilic balance, presence of type and amount of polar functional groups.
30
Complete spreading of water drops placed on compressed discs of the detergents is prevented by
a hydrophobic portion of the surfactant molecules.
31
In fact, alignment of surfactant molecules
can produce either hydrophilic or hydrophobic moieties or when crystalized anisotropic crystals
with planes of different wetting characteristics.
30
Arrangement and directionality of surface
atoms and functional groups have therefore, serious consequences in wettability of surfaces
exposed to wetting liquid. Further, strong covalent and ionic bonding in ceramics or metallic
bonding in metals and alloys or large conformational entropy of long polymeric molecules
prevent these solids from dissolving into water, though their surfaces usually prefer water
environment over nonpolar air.
2.2. Polar Spreads on Polar
“Like dissolves like” is a widespread useful rule of thumb for predicting solubility of solids in
water. This simplistic approach predicts that any solid with a similar chemical structure to water
will dissolve in it; in other words, in polar water the polar solids will dissolve best. Similar
concept has been adopted to surfaces and hydrophilic surfaces are those having polarity, where
surface molecules or their chemical groups have an electric dipole or multipole moment. It leads
us to the simple but still qualitative definition of hydrophilic surfaces: “like spreads on like” or
“polar spreads on polar.”What appears to be a rule of thumb cannot however, predict
hydrophilicity of metal surfaces. Metal surfaces, if not covered with an oxide layer, have nothing
common with a structure or polarity of water. On contrary, water is known to spread out
completely or nearly completely on noble metals such as gold, silver, copper and other (see next
section). In these systems, dispersion forces alone are adequate to render water spreading on
clean surfaces of noble metals.
32
2.4. Fine Particle Partition
Finely-divided solids with hydrophilic surfaces on which water spreads completely tend to sink
in water when placed on the surface of bulk water. Most of fine particles however, are not so
well wetted by water and they float on the water surface. The relative hydrophilicity/
hydrophobicity nature of such fine particles can also be determined qualitatively by analyzing
formation of Pickering emulsions.
33
Powder tend to collect at water/oil interface and act as
stabilizers of emulsions if blended with similar volumes of oil and water.
34
The interface
becomes concave with respect to the liquid which better wets particles; i.e., an oil-in-water
emulsion is formed with hydrophilic (90
o
>θ>0
o
) particles and water-in-oil emulsion when
particles are oleophilic (hydrophobic; θ>90
o
).
35
4

2.5. Contact Angle Value Criterion
Water (or other polar liquid) is preferred on hydrophilic surface over a nonpolar phase such as air
or oil. It is therefore no surprise, as already mentioned earlier, that 90 degrees of water contact
angle measured in air environment is traditionally a popular cut between hydrophilic and
hydrophobic surfaces; hydrophobic surface when water contact angles is larger than 90 degrees
(θ>90
o
) and hydrophilic one with contact angles of θ<90
o
.
9
This ninety-degrees cut has not been
adopted by the mining and mineral processing community. Instead, naturally hydrophobic
minerals, also called naturally floatable minerals, are those to which air bubble attach in water;
water contact angle is therefore larger than zero.
27,36
A serious practical problem can emerge however, when using the contact angle value in defining
hydrophilic surfaces. It is related to the means with which the contact angle is measured. For
example, solid state can dictate the measuring technique and measurements of contact angles on
powder differ from that for the bulk specimen with a flat surface.
37
Further, the measured
contact angle can be of different value depending whether it is measured for water that advances
(or recently advanced) over a dry surface of the solid or recedes (or recently retreated) from the
wet solid surface.
38
The difference between advancing contact angle and receding contact angle,
known as contact angle hysteresis,
39
is common to heterogeneous and rough surfaces,
40
often
depended on the volume of liquid used in measurements.
41
Contact angle hysteresis value also
depends on whether the measurements are done under static or dynamic conditions and the rate
of liquid movement.
42
Discussion of all the measuring technique and obstacles with the
measurements are beyond this review. Any discussion in this paper refers to static contact angles,
including advancing and receding contact angles, measured on specimens rather than powder.
It is not always recognized that even surfaces that are smooth and homogeneous can demonstrate
the contact angle hysteresis.
43,44
Formation of stable thin water films of different thickness on
hydrophilic surfaces is the reason behind this phenomenon and was explained using the concept
of disjoining pressure
†
introduced by Derjaguin in 1936,
45
which operates in thin layer near the
three-phase contact line. It was reported that on surfaces of quartz, glass, metal surfaces,
45
two
different water films, α-(adsorption) film and β-(wetting) film (both of different thickness) can
coexist in equilibrium with the bulk water sitting on the solid surface.
43
α-films are stable films
and can be obtained in the course of the adsorption process, during, for example, contact angle
measurements in air saturated with water vapors. β-films, on the other hand, are metastable
films and can only be obtained by decreasing the thickness of thicker films. As the consequence,
contact angle measuring technique and methodology of deposition of liquid on a solid surface
can influence type of film that is formed on the hydrophilic surface and surrounds vicinity of
liquid meniscus and therefore, affect the measured contact angles.
46
We will ignore these
problems in this sub-section and eventually return to some of them later.
As per our own practical experience, and many others, sessile-drop and captive-bubble
techniques are often the methods of choice in static contact angle measurements for bulk
materials with smooth surfaces. Contact angles are more reproducible if measured for the water
drops/air bubble having a base diameter of a few millimeters,
41
which size is enlarged/reduced
†
Disjoining pressure is defined as a difference in a thin liquid film adjacent to surfaces confining
it and in the bulk of this liquid phase.
5

over the “dry” solid area before advancing contact angle measurements or reduced/enlarged over
the “wet” solid area before receding contact angle measurements. In both cases, the shape of
water drop/air bubble must be stabilized, typically several seconds before contact angle
reading.
46
The sessile-drop technique is more commonly used, outside mining and mineral
processing laboratories, due to its simplicity. In the captive-bubble method, attachment of the gas
bubble to the sample immersed in water or other liquid is required, not always possible if thick
water film remains stable on a solid surface. However, benefit of the captive-bubble method is
that both solid and gas phases are already saturated with water or water vapor and measurements
of contact angle are carried out under more stable and reproducible conditions. Additionally, this
technique more closely reflects flotation conditions of solid particles in processing of materials.
36
Contact angles measured with either sessile-drop or captive-bubble techniques although often
well reproducible, should be repeated several times and statistically valid average values,
together with a standard deviation, should be reported. Representative contact angle values can
be used for not only identification but also a classification of hydrophilic and hydrophobic
surfaces. In fact, most of the contact angle values (advancing contact angles) published in the
past were measured with these techniques, and the values are equal or close to what could be
measured using the above-mentioned experimental protocol.
‡
Now returning to our latest definition of the hydrophilic surface, defined by the water
(advancing) contact angle smaller than 90
o
, it can be easily found that most of the natural and
man-produced materials could be grouped under this category, including biological membranes,
majority of inorganic minerals such as silicates, hydroxylated oxides, ionic crystals, metallic
surfaces, and even majority of polymers. In fact, it is easier to identify all hydrophobic materials
and surfaces since hydrophilic ones are more abundant in nature. Only saturated hydrocarbon-
based products such as wax, polyethylene, polypropylene, self-assembled monolayers with
hydrocarbon functional group as well as fluorine-based polymers, hydrocarbons, and monolayers
are hydrophobic. Any inclusion of heteroatoms other than fluorine (particularly oxygen) into the
structure of hydrocarbons, or even presence of double or triple bonding, add a polarity to the
polymer and molecule reducing its hydrophobicity and introducing or enhancing hydrophilicity
of the surface. There is a number of minerals that are called naturally hydrophobic minerals
including graphite, coal, sulfur, molybdenite, stibnite, pyrophyllites, and talc but the water
contact angles on these minerals
§
were reported to vary from 20 to 88 degrees
47
and therefore,
their surfaces do not fit to the above definition of hydrophobic one with contact angle >90
degrees. Further, there is not known ceramic with hydrophobic surface. Also water contact
angles on metals and alloys are smaller than 90 degrees. Metals (other than noble metals) and
alloys, however, as the result of oxidation, are typically covered with a thin film of an oxide
‡
In many publications published in recent few years, if not in majority, good protocols of contact
angle measurements developed in the past are validated and such issues like a minimum size of
the drop necessary in measurements, multiple measurements, and sometimes the need for
saturated environment, are ignored. Also the contact angles are measured for just deposited small
drops, without paying attention to advancing and receding contact angles and stabilization of the
drop shape.
§
Static contact angles often measured after attachment of a air bubble to the mineral immersed in
water.
6

layer, often hydroxylated, and the contact angles measured on these materials represent wetting
properties of this layer and not bare metal/alloy.
2.6. Recent Definitions
Van Oss proposed to use the free energy of hydration (∆G
sl
) as the way of the absolute measure
of hydrophilicity and hydrophobicity of both molecules and condensed phases.
48
Based on
analysis of the free energy of hydration for a number of different compounds, he found that
hydrophobic compounds attract each other in water when ∆G
sl
> -113 mJ/m
2
, whereas they repel
each other when ∆G
sl
< -113 mJ/m
2
.
48
He then used this (approximate) value as a cut between
hydrophilic and hydrophobic material.
Vogler
49
on the other hand proposed a cut between hydrophilic and hydrophobic surfaces based
on the appearance of long-range attractive hydrophobic forces. Using the data on experimentally
measured hydrophobic forces, together with reported wetting characteristics of substrates used in
force measurements, he concluded that hydrophilic surfaces are those with water contact angle of
θ < 65
o
and water adhesion tension of τ >30 mN/m.
49
We will return to the models proposed by
van Oss and Vogler in the next section.
2.7. Summary
Table 1 summarizes all definitions of hydrophilic surfaces discussed in this section, and lists
major problems with these definitions. Since almost all of the solids, with the exception of
several saturated and fluorinated hydrocarbons, have affinity to water beyond (always existing)
London dispersion interactions, a large spectrum of hydrophilic surfaces surrounds our daily
activities. Hydrophilic surfaces are not the same, however, and differences in wetting
characteristics among them are expected. It would be important therefore, to classify hydrophilic
surfaces into sub-groups based on contact angle values, degree of hydrophilicity, strength of
interactions with water, etc.
3. Measure of Hydrophiliciy and Hydrophobicity
As for hydrophilic surface it is a surface that “attracts water” and the water contact angle should
be smaller than 90°.
23
In many papers, as discussed earlier, zero contact angle is expected for
water on a hydrophilic surface. For example in the recent paper Sendner et al.
50
wrote: “one
experimentally easily accessible parameter characterizing the surface hydrophobicity is the
contact angle which ranges from 180
o
(for a hypothetical substrate with the same water affinity
as vapor) down to 0
o
for a hydrophilic surface”.
True zero contact angle (in algebraic sense) has very serious implication for the energy balance
expressed by the Young equation:
50,51
cos
s sl l
γγ γ θ
−=
(1)
where γ
s
is the solid surface free energy, γ
l
is the liquid surface free energy (the liquid surface
tension) ,γ
sl
is the solid/liquid interfacial free energy, and θ is the equilibrium contact angle.
7

Now, if the contact angle is equal zero indeed, θ = 0, then cosθ = 1 and Eq.(1) reduces to:
s sl l
γγ γ
−=
(2)
This case occurs rarely, if ever, in practical systems and we will discuss this issue more
extensively in the next section. The zero contact angle is the limit of applicability of the Young
equation. Visually observed “zero contact angle” does not mean that Eq (2) applies to this
situation. Such systems are better characterized by the work of liquid spreading W
s
(also known
as spreading coefficient) defined as the work performed to spread a liquid over a unit surface
area of a clean and non-reactive solid (or another liquid) at constant temperature and pressure
and in equilibrium with liquid vapor:
( )
s s l sl
W
γ γγ
=−+
(3)
In case of two liquids, all components of Eq.(3) are either liquid surface tension or liquid-liquid
interfacial tension and are therefore, measureable. In the case of solids, neither solid surface free
energy nor solid-liquid interfacial free energy are easy measurable. However, if the liquid does
not spread completely but forms a definite contact angle, then applying Young equation, the
work of spreading can be easily calculated from measured contact angles and surface tension of
liquid as long as θ >0:
( )
cos 1
sl
W
γθ
= −
(4)
It is difficult however, to determine W
s
for surfaces on which water spreads completely. Zero
contact angle would imply zero work of spreading as well, W
s
= 0. On a contrary, W
s
> 0 (no
measureable contact angle) for a complete spreading and W
s
< 0 for liquids that retreats to lenses
with finite contact angle. Therefore the work of spreading could be used as a measure of a solid
surface hydrophilicity. The concept is not entirely new as similar approach was proposed by van
Oss.
48
Van Oss proposed to use the free energy of hydration (∆G
sl
) as the way of the absolute measure
of hydrophilicity and hydrophobicity of both molecules and condensed phases.
48
The free energy
of hydration (solvation) can be defined by means of the Dupre equation:
sl sl s l a
ΔG = γ - γ - γ = - W
(5)
The absolute value of free energy of hydration equals to the work of adhesion (Wa). Instead of
coping with not measureable solid surface free energy and solid-liquid interfacial free energy,
van Oss et al.
52-54
proposed to split the surface free energy into components representing
Lifshitz-van der Waals and acid-base interactions. Components of solid surface free energy or
liquid surface tension are determined from contact angle measurements using at least three
different probing liquids of varying surface tension and polarity. This model however, is beyond
the scope of this review and will not be discussed here.
Van Oss also analyzed the free energy of hydration for a number of different molecules and
found that hydrophobic molecules which attract each other in water have ∆G
sl
> -113 mJ/m
2
,
8

whereas hydrophilic molecules have this value more negative ∆G
sl
< -113 mJ/m
2
.
48
He then used
this (approximate) value as a cut between hydrophilic and hydrophobic material.
Eq.(5) can be further modified by substituting the Young’s equation:
( )
cos 1
sl l
G
γθ
∆=− +
(6)
Taking the cut between hydrophilic and hydrophobic surfaces proposed by van Oss, we can
calculate the value of the equilibrium contact angle from Eq.(6) which describes transition
between hydrophilic and hydrophobic surface. The value is θ ≈56
o
for ∆G
sl
= -113 mJ/m
2
, and as
the result indicates zero water contact angle is not needed for the solid surface to be called
hydrophilic. Additionally, this value suggests that hydrophobic surfaces are already those with
56
o
<θ<90
o
. It is interesting to notice that similar cut between hydrophilic and hydrophobic
surfaces was concluded by Vogler in 1998.
49
Based on analysis of experimental long-range
attractive (hydrophobic) forces he came to the conclusion that hydrophilic surfaces are those
with water contact angle of θ < 65
o
and water adhesion tension of τ >30 mN/m. The adhesion
tension is defined as:
cos
l
τγ θ
=
(7)
Taking into account previous recommendations, we propose the classification of hydrophilic and
hydrophobic surfaces based on contact angle, work of spreading, free energy of hydration and
water adhesion tension as shown in Table 2. Hydrophilic surfaces are those on which water
spreads completely, visually “zero contact angle.” Partially hydrophilic and hydrophobic solids,
vast majority of materials, called here weakly hydrophilic and weakly hydrophobic, are those on
which water films are unstable and beads up to lenses with contact angle smaller than 90
degrees. Hydrophobic surfaces are those commonly recognized with water contact angles at least
90 degrees. We also include superhydrophilic and superhydrophobic surfaces in Table 2 but they
will be discussed later.
Now the question which we address in the next section is: can water spread completely on flat
hydrophilic materials?
4. The Case of Complete Spreading
As per discussion in the previous section, the first group of hydrophilic solids that we identified
is the group of soluble salts. Are these solids perfectly wetted by water? This question has only
partially been answered in the technical literature. Past research clearly showed that air bubbles
do not attach to both soluble and semi-soluble minerals in water (saturated with these solids).
27
This suggests that water films remain stable on surfaces of these minerals. Whether water will
spread out on dried surfaces of these minerals is not so obvious, however. For water-soluble
solids such as salt or sugar, the measurements of advancing contact angles are either impossible
or experimentally difficult with challenging interpretation of the results. The advancing water
contact angle for such “reactive solids”
55,56
cannot be determined and the angles measured
represent values for water with dissolved solid, measured for either partially saturated (under not
equilibrated conditions) or saturated aqueous solutions with surface tension that differs from
9

surface tension for pure water.
28,29
The substance dissolution also changes the surface
topography of the solid, adding roughness component to the complexity of the three phase
system examined. It does not mean that they could not reveal infinite advancing water contact
angle values. For example, Miller et al.
28,29
determined contact angles on a number of soluble
salt crystals. Saturated solutions spread completely on NaCl, NaF, and Na
2
CO
3
whereas as high
as 8, 20 and 25
o
contact angles were reported for KCl, NaHCO
3
, and KI, respectively.
Further, commonly used pharmaceutical products such as many pills are made of hydrophilic
drug powder coated with protective layer to reduce the kinetics of drug dissolution. Although the
drug without protective coating dissolves in water, drops placed on compressed discs of this
powder (or on single crystals of drugs if such available) of these anisotropic organic solids will
typically not spread out completely. Water contact angles on insulin and lactose as high as 36-42
degrees and 22-28 degrees, respectively (E. Chibowski and J. Drelich, unpublished), were
estimated in our research using a thin layer wicking technique,
57,58
these angles are probably far
from equilibrium ones since neither powder or water could be equilibrated in such tests.
It was reported in the literature that water can spread out completely or nearly completely on just
a few nonporous and smooth materials (Table 3) including glass,
1
gold,
59
copper,
60
silver,
60
chromium,
26
selected oxides (having OH groups on the surface)
61,62
including quartz
63
and
amorphous silica surface,
64
and biological specimens (such as biological membranes and lipid
layers),
48
cleaved mica,
65
and then only if these materials are freshly prepared and/or their
surfaces are carefully cleaned.
1,66,67
The surfaces of these solids have strong affinity towards
water molecules and have been commonly recognized as hydrophilic, sometimes called solids
with strongly hydrophilic surfaces to differentiate them from other hydrophilic surfaces on which
water contact angle is larger than 5-10 degrees but smaller than 90 degrees.
**
At the first glance, zero contact angles should be fairly common. According to the Young
equation, when θ = 0:
S L SL
γ γ + γ ≥
.
††
If the water-solid interfacial free energy approaches a
near zero value, which probably is the case for solids capable of interacting with water molecules
through hydrogen bonding such as oxides with hydroxyl groups on the surface, then all solids
with
2
S
γ 72.8 mJ/m≥
at ~22
o
C could satisfy the conditions of perfect water spreading on them.
In fact, metals, alloys, ceramics, ionic salts
68
have surface free energy higher than 72.8 mJ/m
2
and surface free energy value of only organic polymers (currently known) is less than that for
water.
69
If such variety of high-surface energy materials is available to us, why water spreading and
developing thick films are not commonly observed on them? Why don’t these materials remain
covered with a water film all the time? The formation of water films on many inorganic
materials, including natural minerals, could probably be observed if competitors of water such as
oxygen and volatile organics are eliminated from the material’s environment. High energy of
material surfaces is a short-lasting effect because constituents of surrounding phase either
chemically react with material or adsorb on surfaces or both in attempt to reduce the tensions on
**
It is also quite common to divide surfaces for hydrophilic (θ
W
<5-10
o
) and partially hydrophilic
(~10
o
<θ
W
<90
o
).
††
It should be recognized that the Young’s equation does not apply to the cases of zero apparent
contact angle.
10

the surface and produce more stable system. Example is an oxide layer, which covers majority of
metals as well as many other single-elemental materials and ceramics. It is the result of chemical
reaction of elements with oxygen from air or aqueous phase during either material’s production
or service. Mercaptans and many other organic compounds that humans, and other living species,
breathe out, diffuse and adsorb, often through strong chemical bonding, on solid surfaces. Any
changes on surfaces of high-energy materials reduce its surface tensions, changing also surface
affinity towards water. We will return to the issue of surface contamination in a separate section,
whereas corrosion of materials is ignored in this paper and we only discuss the surfaces that
remain stable during the time of examination of their surface wetting properties resulting solely
from physical interactions.
Is this possible however, to attain zero value for the apparent (water) contact angles
‡‡
on the
smooth, homogeneous and inert surfaces of the above hydrophilic materials? The question is not
easy to answer as determination of the contact angles smaller than 5-10 degrees with commercial
contact angle measuring instruments, which typically rely on an image analysis of the shape of
either liquid drop or meniscus, is rather impossible.
Since most of scientific research requires that measurements of contact angles are conducted on
clean surfaces, we concentrate our attention on such systems. But even if the surface of
hydrophilic mineral, metal, ceramic is well prepared for measurements, the macroscopic water
contact angle of zero value is rare, if measured accurately on any solid surface ever, as discussed
earlier. It is usually the contact angle that is near zero value. Why zero water contact angle is
difficult to observe on smooth surfaces of hydrophilic materials has been partially answered by
Russian scientists through concept of disjoining pressure and formation of stable thin water
films, in fact with microscopic contact angle
§§
that differ from macroscopic contact angle.
70-74
Autophobic properties of thin film often prevent formation of thick water films. Qualitatively,
this can be explained based on changes in surface free energy of solid surface modified with
water film and properties of water film that differ from the bulk water. Strong hydrophilic
surfaces affect diffusion, rotation, and orientation of water molecules located near the
hydrophilic solid surface. As the result, the interfacial water molecules, usually from one to three
layers of molecules, are more organized than in the bulk.
70-74
Also an interface, and therefore
tension, is expected between ordered thin film of water and “amorphous” water bulk.
75
The
tension at the surface of an organized water layer, if could be measured, should be smaller than
surface tension of water.
76
Indeed, several measurements showed finite contact angles for water
placed on ice, ice representing frozen structure of water.
76-78
For example, Knight reported
(receding) contact angle of 12 degrees for water on somewhat rough surface of ice at a
temperature below 0
o
C.
77
At similar temperature, Ketcham and Hobbs found water contact angle
of about 20 degrees.
78
More recently, surface free energy of ice was estimated through contact
angle measurements with different liquids by van Oss et al.
76
and found to be 69.2 mJ/m
2
as
‡‡
Apparent contact angle also known as macroscopic contact angle (sometimes also called
geometric contact angle) is that observed with the optical means on any type of surface: smooth,
rough, and/or heterogeneous.
§§
Microscopic contact angle is that observed at a junction of the three phases at a scale of several
micrometers or smaller.
11

compared to 75.8 mJ/m
2
for water at 0
o
C. This low value of the surface free energy of ice
explains relatively large water contact angles measured experimentally.
The presence of molecular or nanometer-sized thin water films on hydrophilic materials is
probably more widespread than commonly recognized. Although the measurements of disjoining
pressure of water films are still not popular, stable thin water films, including adsorption α- films
and wetting β- films, were recorded on a few hydrophilic surfaces of such materials as quartz,
glass, and metals.
45
α-films are stable films and can be obtained in the course of the adsorption
process, during, for example, contact angle measurements in air saturated with water vapors. β-
films, on the other hand, are metastable films and can only be obtained by decreasing the
thickness of thicker films. It cannot result from water spreading.
In summary, the existence of true zero contact angle is still a question worth further studies. In
practice many researchers use 5-10 degrees as an arbitrary cut for complete spreading of water
on hydrophilic surfaces, as well as superhydrophilic surfaces discussed later.
5. Common Methods to Produce Hydrophilic Surfaces
The enhancement of hydrophilicity of surfaces can be approached through either deposition of a
molecular or microscopic film of a new material, more hydrophilic than the substrate, or by
modification of chemistry of the substrate surface. Molecular modification or deposition of
coating is more common choice for inorganic substrates whereas modification of surface
chemistry is broadly used in the case of polymeric materials. In this section, the most commonly
used methods for making surface hydrophilic are briefly reviewed. Examples of applications in
fabrication of superhydrophilic coatings will be discussed later.
5.1. Deposited Molecular Structures
A number of organic molecules adsorb from either solution or a vapor phase on selected solids,
spontaneously organize into self-assembled monolayers, changing wetting characteristics of the
substrate.
79
The most commonly studied densely packed molecular structures include
alkanethiols on gold,
80,81
silver,
82-84
copper,
82-84
platinum,
85,86
and palladium,
87
chlorosilanes on
silicon oxide,
88-91
aluminum,
92,93
titanium
94
and other oxides,
94
phosphonic acids on titanium
95,96
aluminum,
97,98
and other oxides.
96
Both mono- and multi-layers can also be deposited
mechanically through a Langmuir-Blodgett film technique, although physically deposited
multilayers suffer from poor stability when contacted with liquids.
79
Deposited organic layers
make the surface hydrophilic if the end group is polar, other than saturated hydrocarbon-based
group or fluorinated group. The highest hydrophilicity are probably the groups capable of
interacting with water molecules through hydrogen bonding such as -OH, -COOH, POOH.
80,81,99
On any of such layers however, zero water contact angle was never recorded.
Beside arranging self-assembled monolayers of chemically-bonded short functional molecules to
inorganic surfaces, a great deal of research has focused on coating of materials with
macromolecules and biomacromolecules, especially popular in modification of polymers
contacting biofluids, including blood.
100
Albumin
101-103
and heparin
104-106
have been widely used
12

as biomacromolecules. Among synthetic polymers, poly(ethylene glycol)
100,107,108
and
phospholipid-like
108-112
macromolecules have been studied extensively. In the typical
bioengineering applications of such coatings, however, the hydrophilicity of grafted or
physically adsorbed dense structures of biomacromolecules or synthetic macromolecules is
usually of secondary importance and both biocompatible and non-fouling are more important.
These protecting coatings intent to prevent protein adsorption when materials come into contact
with biological fluids.
113
5.2. Modification of Surface Chemistry
Over the last few decades, many advances have been made in developing surface treatments by
plasma, corona, flame, photons, electrons, ions, X-rays, gamma-rays, and ozone to alter the
chemistry of polymer surfaces without affecting their bulk properties.
114,115
Plasma treatment, in
air or oxygen environment,
116,117
corona
118,119
and flame
118,120
treatments are the most designated
techniques in oxidation of surfaces of polymers.
121
In both plasma and corona treatments, the
accelerated electrons bombard the polymer with energies 2-3 times that necessary to break the
molecular bonds, producing free radicals which generate cross-linking and react with
surrounding oxygen to produce oxygen-based functionalities.
116
Polar groups being typically
created on the surface are hydroxyl, peroxy, carbonyl, carbonate, ether, ester, and carboxylic acid
groups.
119
In flame treatment, surface combustion of polymer takes place with formation of
hydroperoxide and hydroxyl radicals.
120,122
Oxidation depth through flame treatment is around 5-
10 nm, and over 10 nm for air plasma treatment.
123
Plasma, corona and flame treatments end in
extensive surface oxidation and results in highly wettable surfaces. Polar groups produced
during surface oxidation have tendency to be buried away in the bulk when in contact with air
for extended period of time, but they remain on the surface when in contact with water or any
other polar environment.
124
Polymers also oxidize and degrade under a UV (ultraviolet) light, and, for example, polymeric
outdoor consumer products need addition of UV absorbers when exposed to the sunlight to
inhibit discoloration, cracking, and fading.
125,126
UV light has a wavelength in the range 10 nm to
400 nm (energy of 3 eV to 124 eV), the incident photons of which have enough energy for
breaking intermolecular bonds of most of the polymers, promoting structural and chemical
changes of the macromolecules.
127
The exposure of the polymer to UV radiation causes chain
scission, crosslinking, and increases the density of oxygen-based polar groups at the substrate
surface, making the surface more hydrophilic.
128-131
Recently, UV light is used to control
polymerization reaction and pattern microstructures of different wettability for a variety of
applications of microfluidic devices.
132
Alkali treatment of polymers, especially at elevated temperature, can also enhance surface
hydrophilicity of polymers.
133-135
Hydroxyl and carboxyl groups are among the hydrophilic
groups formed on the surface of polymers such as polyolefins and polyethylene terephthalate
during their etching with concentrated bases.
136,137
Finally, anodic potential was used to control electrochemically treat conductive oxide surface
and control its wetting characteristics.
138,139
13

6. Contamination of Hydrophilic Surfaces and Their Cleaning
The hydrophilic surface must be kept free of contaminants such as airborne organics, moisture
and dust particles to preserve its wetting characteristics. A freshly prepared hydrophilic surface
when exposed to the laboratory environment tends to achieve its lowest energy (most stable
state) by instantaneous changes at the surface, e.g., adsorption of water molecules or organic
contaminants. In this way, contamination of hydrophilic surface and consequently a reduction of
surface energy occur naturally for many materials.
The problem of contamination of high-energy surfaces with organics is not always well
recognized in many laboratories. For example, there had been a long controversy in both the
mining and mineral processing and surface chemistry communities about hydrophobicity of
metals such as native gold and silver.
140,141
Water contact angles as high as 55-85 degrees were
reported in the literature for gold surfaces.
141,142
After the work of Bewig and Zisman
143
and then
of Schrader,
60,144
as well as others,
59,141
it became clear that pure water can spread out completely
over the surface of a freshly prepared clean metal such as gold,
59,141,143,144
platinum,
143
copper,
60
and silver.
60
Physical interactions at the metal-water interface are strong and consist solely of
dispersion forces.
32
The Hamaker constant for metals is an order of magnitude higher than
Hamaker constant for water.
26
Unfortunately, reports on stability of hydrophilic surfaces in the
laboratory environment as well as type organics attracted by hydrophilic surfaces and kinetics of
their adsorption are rare. Among those, Bewig and Zisman
145
showed that even nonpolar vapors
of hexane and benzene adsorb on clean surfaces of metals and the temperature of a contaminated
metal must be raised by at least 100 degrees to remove the last monolayer of these hydrocarbons.
One of the first systematic studies reported on the phenomenon of contaminant adsorption at
high-energy surfaces was presented by Bartell and Bristol in 1940,
146
although the protocols and
precautions to prevent contamination of specimens in contact angle measurements were
recognized decades earlier (see, for example, a brief review in the book by Sutherland and
Wark
36
). Bartell and Bristol showed that the wetting characteristics of quartz and glass depend
not only on the state of the solid surface but also on particular day of contact angle
measurements. They also found that the measured water contact angles were closely related to
the degree of humidity in the atmosphere. White
147
reported the kinetics of contact angle change
for water drops placed at the surfaces of mica and oxidized surfaces of nickel, aluminum, and
nichrome when these materials were exposed to laboratory air. He observed a fast increase in
contact angle values in the first 10-20 hours and only a few degrees after that in the next two
days. The water contact angle increased from nearly zero value to 15-20 degrees for mica and
nickel and to 32-37 degrees for aluminum and nichrome.
White
147
also showed that vapor of the mineral oil adsorb less on glass and mica than aluminum
and magnesium and the transition metals showing the greatest increase. Similar observations
were made earlier for the adsorption of fatty acids from solutions and vapor phase which showed
that there is less adsorption on mica, gold, platinum and chromium than on nickel, iron and
copper.
148
Both studies revealed that surfaces become contaminated at different rates and to
different levels, result of adsorption driven by molecule-surface interactions. White also
proposed that organics can be gettered from the air by adsorption onto oxidized metal surfaces
and therefore used as filling media in storage compartments to maintain surface cleanness of
lower energy specimens such as glass or mica.
14

Even small quantities of organic contaminants make a large difference in wettability of
hydrophilic surfaces.
36,59,141
Typical experiment relies on storage a sample in a laboratory air and
monitoring periodically the changes in contact angles. Since air quality in each lab is ill-defined
and composition can vary substantially from lab to lab,
149
the results can be poorly reproducible.
They are however, very useful to understand the problem of airborne contamination that
researchers can deal with in regular laboratory activities.
Recent studies suggest that even a few tens of degrees of water contact angles can be observed
on glasses and metal oxides as the result of surface contamination with airborne
hydrocarbons.
66,67,150
When cleaned, metal oxides
66,150
and commercial glasses
67
demonstrate
water contact angle at a level of a few degrees. Strong hydrophilicity of these materials was
reported to degrade, however, during storage in laboratory air at ambient conditions. In 3 to 4
days of storage, the water contact angle increased to 50-60 degrees for aluminium oxide
150
and
tin oxide,
66
35-38 degrees for silica, 80-90 degrees for titanium oxide and chromium oxide, and
to above 100 degrees for zirconium oxide.
66
The water contact angle increase from 20 to over 50
degrees for glasses exposed to ambient air for the same time.
67
Interestingly, in the case of both
glasses and metal oxides, Takeda et al.
66,67
found that the surface OH groups attract organic
contaminants and OH group density correlates with the adsorption of organics from the
atmosphere.
Hydrophilic surfaces adsorb water even from the laboratory environment and the amount of
water sitting on the hydrophilic surface depends on relative humidity. Although the phenomenon
of formation and stability of water films at hydrophilic surfaces is important in many areas of
science and technology, such as mineral processing, electronic industry, microtechnology, and
many others, not enough research has been done to study the properties of adsorbed water films,
including monolayers. It is generally accepted that under ordinary atmospheric conditions,
hydrophilic surfaces adsorb at least a monolayer of water. For example, a clean glass surface is
covered with a monlayer of adsorbed water at relative humidities of around 30-50 % at 20
o
C.
151
Formation of a water film composed of as many as twenty molecular layers, or more, may occur
at the clean surface of high-energy solids, especially at high relative humidities, >90-95%.
152
For
example, Rykerd et al.
153
measured ellipsometrically the thickness of the adsorbed water film on
a fused silica surface and found it ranging from 2.4 to 9.0 nm, depending on the water vapor
pressure. Staszczuk
154
used gas chromatography to determine the water adsorption isotherm on
quartz at 20
o
C and found that about 16 statistical water layers adsorbed from a gas phase
saturated with water vapor. Also similar experiments using the chromatographic technique
showed that about 15 statistical water layers may adsorb onto a marble surface.
155
Water films
with thickness from 1.0 to 8.0 nm were also reported for muscovite mica.
156
Water if already present on the hydrophilic surface can probably prevent or at least slow down
the adsorption of the organic contaminants. Unfortunately water surface also attracts organics,
surface-active contaminants, when open to the laboratory air. Volatile organics are in exhaled
breath
157
and therefore, always contaminate laboratory air. After adsorption on a layer of water at
sufficient quantities, they probably could destabilize the water film, exposing solid surface to
them; something that was probably never studied in details. Good practice in many surface
chemistry labs is, therefore, to keep clean hydrophilic samples immersed in water before using
them for experimentation and testing. Such storage is obviously acceptable if sample’s integrity
and surface chemistry remain intact in water.
15

Many experimental contact angles are unreliable because the failures to work with clean solid
surfaces. There should be no justification for work with surfaces that have not been prevented
from systematic and accidental contamination, and properly tested for contamination. All
instrumentation used in preparation of specimens (cutting, polishing, sputtering) should be freed
from grease and any other organics, e.g. by washing it with appropriate (nonionic) detergent
solutions, organic solvents (benzene, ethanol, chloroform), and/or acids (sulphuric acid-
dichromate mixture). Annealing of samples at high temperatures, >500
o
C,
147
allows to oxidize
organics to carbon dioxide and water, this approach can significantly alter the chemistry of the
surface and therefore, is only acceptable to certain inorganic materials. Surfaces of oxides such
as quartz, for example, undergo dehydration at such high temperatures which results in increase
of nearly zero water contact angle to 30-40 degrees.
63,158
Thus, the oxide surfaces are not
necessarily well wettable by water when clean, and the water contact angle is closely related to
the density of OH groups on the oxide surfaces.
63,158
Oxides surfaces can be cleaned by
degreasing and boiling in 30% hydrogen peroxide.
147
Specimens should be always handled in
latex gloves and never kept close to a mouth as breath contains tens, if not hundreds, of volatile
organics.
157
Majority of surface treatments that are commonly used for modification of surface chemistry of
polymers such as plasma, corona, flame, photons, electrons, ions, X-rays, gamma-rays, and
ozone treatments, briefly reviewed in previous section are also effectively used in cleaning
substrates. The use of particular technique is rather dictated by its availability and applicability to
particular type of solids.
7. Defining Superhydrophilic (Superwetting) Surfaces
In our previous paper,
159
we proposed a definition of superhydrophilic (superwetting) surfaces.
We also briefly discussed meanings to facilitate superhydrophilicity. Here we repeat our
definition and then discuss issues related to manipulation of such surfaces through the control of
surface roughness. Before doing that however, we start with a definition of the superhydrophobic
surface since the term “superhydrophobic surface” appeared in the literature earlier then the term
“superhydrophilic surface”. Both terms are opposite to each other in respect to solid surface
wetting properties. During the last several years, the superhydrophobic materials and coatings
have attracted attention of a large number of research laboratories, all over the world, as
evidenced by the explosion of published papers (see several reviews
1-15
on this topic and
references therein). The term of suprhydrophobicity was introduced in 1996 by Onda et al.
20,21
to
describe unusually high water contact angles, not observed on flat and smooth hydrophobic
materials. Commonly accepted now meaning of superhydrophobic surface is a surface on which
water (advancing) contact angle is at least 150
o
, and the contact angle hysteresis as well as the
sliding (or rolling off) angle
***
do not exceed 5-10
o
. Superhydrophobic surfaces were inspired by
biological specimens,
160-179
and their artificial substitutes were manufactured by chemical,
physical and/or mechanical modifications of both organic and inorganic materials.
1-15
A
common feature (not always necessary) of superhydrophobic surfaces is their proper two-level
topography, with micro- and nano-sized asperities/posts, similar to what was first observed on
***
Sliding/rolling angle is the minimum angle of sloped solid at which water (liquid) drop rolls
off the surface.
16

the lotus leaves and 200 others water-repellent plant species.
160-178
Because the scope of this
article is focused on superhydrophilic (superwetting) surfaces, the superhydrophobic ones will
not be described in detail.
Since surface roughness is necessary feature of superhydrophobicity and superhydrophilicity, it
can be said that the principles of these phenomena was actually founded several decades ago by
Wenzel
180
and Cassie and Baxter
181
who described contact angles and different mechanisms of
wetting on rough surfaces. Validity of their equations in description of liquid wetting at
superhydrophobic or superhydrophilic surfaces will be discussed later.
As mentioned above, an opposite to superhydrophobic is superhydrophilic surface. This type of
surfaces are also of a great interest now,
139,182-208
although still some questions regarding their
definition remain open.
13
The superhydrophilic surfaces may have many practical applications
like antifogging, antifouling or self-cleaning, and others.
139,209-216
Superwetting is also important
in biological systems, like cell activity, proliferation, signaling activity, etc.
217
It is generally
accepted that first prerequisite for a surface to be superhydrophilic (superwetting) is that water
(liquid) apparent contact angle is less than 5
o
. In our previously published note
159
we suggested
to refer superhydrophilic (or superwetting) surface only to a textured and/or structured surface
(rough and/or porous) possessing roughness factor (r = ratio of real surface area to projected
surface area) defined by Wenzel equation
180
larger than r >1, on which water (liquid) spreads
completely. In the light of the above, clean glass or freshly cleaved mica surfaces (as well as
other example of hydrophilic surfaces discussed earlier) are not superhydrophilic ones, although
water can spread over them completely. Such surfaces are simply naturally hydrophilic. In other
words, superhydrophilic (superwetting) surfaces cannot be achieved without manipulation of
roughness of hydrophilic materials (materials), on flat surfaces of which water (liquid) droplets
do not spread completely and remain as lenses with contact angle smaller than 90
o
. In term of a
wicking parameter, Ws:
cos 0
s sv sl l
W
γγγ θ
=−= >
(8)
A minimum roughness of the surface necessary to initiate liquid wicking that results in zero
apparent contact angle is commonly predictable through the Wenzel equation (discussed in the
next section):
1
cos
r
θ
≥
(9)
Figure 3 shows the correlation between the contact angle on a smooth surface of the material
(Young’s contact angle; θ) and the minimum value of the roughness factor (r) that is necessary
for the rough surface of this material to promote complete spreading of liquid. It shows that with
a moderate roughening of the substrate surface, r = 1.2- 2, superhydrophilicity or in general,
superwetting, should be possible on any material having an intrinsic contact angle less than 60
degrees. For materials with θ>65-70
o
, the roughening might not be a practical approach due to
the extremely high values needed for r, although theoretically liquid on any rough material
should spread to zero (or nearly zero) apparent contact angle. In practice however, it is also
observed that liquid penetration into rough structure of the substrate might be difficult. For
17

example, the results presented by Onda et al.
20,21
revealed the limitation of liquids to spread
completely on extremely rough substrates.
Liquid drops can remain suspended on many rough and textured surfaces even if condition by
equation (9) is fulfilled. It corresponds to the three-phase system trapped in a meta-stable
state,
218
and such surfaces should be treated more like porous or solid-air composite
materials.
219,220
The invasion of liquid can be inhibited on materials of particular design,
geometry, size and contour of surface features and protrusions, and an energetic barrier
associated with unfavorable geometry of the substrate for liquid wicking must be overcome.
9,221-
225
This energetic barrier if larger than thermal energy
7
needs to be overcome by mechanical
means such as vibrations,
226,227
impact,
228,229
or load imposed on the drop.
225,230
By manipulating
liquid reentrant profiles on rough features, opposite effects are often desired in which lack of
liquid penetration into protrusions of the rough and textured surface, with liquid drops
remaining suspended, is beneficial for the design of superhydrophobic and supeoleophilic
surfaces.
231,232
In fact special designs are not necessary and using structures of nanotubes
233
and
nanofibers
8,234
as coating can often provide similar results.
8. Surface Topography Effects on Wetting: Common Models and Their Limitations
(Wenzel and Cassie-Baxter Models)
It is now well accepted that surface topography plays a crucial role in liquid spreading on a solid
surface. The surface topography may either enhance or reduce wetting, depending on the
contours and size of the protrusions. There are two possible cases of solid surface wetting that
may occur, which were actually outlined long time ago by Wenzel
180
and Cassie-Baxter.
181
If the
liquid fills in the ‘valleys’ in the rough surface then the apparent (observed) contact angle θ
rough
on such surface is described by Wenzel’s equation:
cos cos
rough
r
θθ
=
(10)
Where r is roughness parameter which is larger than 1, r > 1, which expresses the ratio of the true
the solid surface to its horizontal projection, and θ is the equilibrium contact angle that would be
measured on a flat surface of the same solid. It can be said that ‘chemistry’ of the surface is
reflected in θ while the effect of the roughness involves the r parameter.
235
McHale et al.
235
stated that Wenzel’s equation predicts also changes in the apparent contact angle θ
rough
caused by
changes in the equilibrium contact angle ∆θ induced by surface chemistry, which is given as
follows:
sin
sin
rough
rough
r
θ
θθ
θ

∆= ∆



(11)
They concluded that the change in surface chemistry “is amplified by the rough surface into a
large change in the observed contact angle”. According to Eq.(11) for θ = 90
o
the amplification
factor is equal exactly to the roughness factor r in Eq.(10) and approximately for the angles
around 90
o
.
235
18

Wenzel’s equation (10) indicates that for suitably large roughness the apparent contact angle
drops to zero degrees, θ
rough
= 0, or increases up to 180 degrees, θ = 180
o
, (“roll-up of the
liquid”). The boundary between these two cases is determined by cosθ = ± 1/r,
235
see Eq.(9).
In case of narrow valleys between surface protrusions it may happen that liquid penetration is
inhibited and the liquid remains on tops of the protrusions. As the result, the air is trapped
beneath the liquid and liquid is sitting on what is commonly referred to as a composite surface;
i.e., on asperities of the solid separated by air gaps. In such a case the liquid contact with the
solid surface is much reduced and the system is described by Cassie-Baxter equation:
181
( )
cos cos 1
CB S S
θ ϕθ ϕ
−
= −−
(12)
where ϕ
s
is the fraction of the liquid base in contact with solid surface, ϕ
s
< 1, and (1- ϕ
s
) is the
fraction of the liquid base in contact with air pockets. Air is not wetted by water and therefore
the water/air contact angle equals to 180
o
. Hence the cos180
o
= -1, this leads to the minus sign in
the second term of Eq.(12). A complete roll-up of droplet cannot takes place on a flat solid
surface since there is no natural or man-made hydrophobic material with a water contact angle
larger than 118-120 degrees (only fluorinated materials/surfaces such as PTFE can exhibit such
hydrophobicity). Nevertheless, the Cassie-Baxter equation (12) predicts that an enhancement of
the contact angle up to its super-hydrophobic value (> 150
o
) can be obtained by roughening of
solid surface and by manipulating its texture.
Both Wenzel and Cassie-Baxter equations suggest that increasing surface roughness (or
texturing) leads towards superhydrophobic state, where by changing the surface chemistry and
making the solid more hydrophobic we can observe a transition from Wenzel to Cassie-Baxter
states.
235
Metastability of liquid configuration is the common problem for liquid in contact with
rough and/or textured surfaces, promoting the Cassie-Baxter state. Extra mechanical energy
through for example, vibrations or pressure loads on the liquid, sometimes is necessary to
reinforce switch from metastable to stable state. The Cassie-Baxter state is usually easy to
recognize as liquid droplet will roll-off the rough surface at a low tilting angle. In the case of
Wenzel’s state, on the contrary, the droplet sticks to the surface and large tilting angle is required
to roll it off. Low tilting angle corresponds to low contact angle hysteresis, i.e. the difference
between advancing and receding contact angles.
However, Gao and McCarthy
236
in 2007 published a paper "How Wenzel and Cassie were
wrong", questioning correctness both of Wenzel and Cassie-Baxter approaches. They argued that
in wetting process important is contact line and not contact area and the advancing, receding and
the contact angle hysteresis are determined by solid/liquid interactions at the three phase contact
line. These contact angles are governed by an activation energy, which must be overcome to
move the three-phase contact line from one to another metastable (or stable) state. The
significance of analyzing the three-phase contact line region in which surface forces operate
instead of total surface area under the liquid was well recognized in the past.
237-239
According to Gao and McCarthy,
236
the contact area is valid as reflected by "ground-state energy
of contact line and the transition states between" the subsequent contact lines. Actually similar
conclusion was drawn earlier by Extrand
240
for chemically heterogeneous surfaces. Also work by
19

Drelich
238
on chemically heterogeneous surfaces and by Moulinet et al.
239
on rough surfaces
pointed to the same need of analyzing the shape and contortion of the three-phase contact line.
The statement of Gao and McCarthy was based on some experimental results obtained on three
differently prepared two-component (hydrophilic-hydrophobic) surfaces. It was a stimulus to a
hot discussion that rolled over Langmuir journal putting forward pro and con arguments.
241-245
Nosonovsky
241
derived generalized forms of Wenzel and Cassie-Baxter equations concluding
that Wenzel equation is valid if for a rough surface r = const. However, for nonuniformly rough
surface, generalized Wenzel equation should be applied, where r is a function of x,y coordinates:
( )
cos , cos
rough
rxy
θθ
=
(13)
where:
( )
2
2
,1
dz dz
rxy
dx dy




=++






Then, the generalized Cassie-Baxter equation for a composite surface can be expressed in similar
way:
(
) ( )
1 12 2
cos , cos , cos
CB
f xy f xy
θ θθ
−
= +
(14)
Here f
1
+ f
2
= 1 and θ
1
and θ
2
are contact angles corresponding to the two components, i.e. air
and solid. According to Nosonovsky the generalized forms of Wenzel and Cassie-Baxter
equations apply to the surfaces whose protrusions and/or heterogeneities are small in comparison
to the size of liquid/vapor interface. Because most of superhydrophobic or superhydrophilic
surfaces possess multiscale protrusions and valleys, the use of classical Wenzel or Cassie-Baxter
equations is not straightforward as the solid area wetted by liquid is difficult to determine. If the
surface roughness is present under the droplet but is absent in the triple contact line, like it
probably happened in the work of Gao and McCarthy,
236
then Young equation applies instead of
classical Wenzel or Cassie-Baxter, stated Nosonovsky.
241
Then Panchagnula and Vedantam
242
concluded that Cassie-Baxter equation is correct if
appropriate surface area fraction is taken into account, i.e., the fraction that contact line
experiences during its advancing. Gao and McCarthy
243
replied that Wenzel and Cassie
equations “should be used with knowledge of their faults” and that they had considered contact
line instead of the area fractions in earlier published papers, which helped to understand the
contact angle hysteresis, the lotus effect, and hydrophobic surfaces.
244,245
McHale
246
put forward the question whether “Cassie and Wenzel: were they really wrong?” and
gave the answer that these equations can be used if the surface fraction and the roughness
parameter appearing therein are taken as global parameters of the surface and not as those
defined for the contact area of the droplet. According to him the local form of these equations
“allows patterning of the surface free energy”. In case of superhydrophobic surface the apparent
contact angle results from minimization of the surface free energy by small displacements of the
contacting line. If the droplet penetrates the valleys then Wenzel wetting mechanism occurs.
246
Later Whyman et al.
247
has published “rigorous derivation of Young, Cassie–Baxter and Wenzel
20

equations”. They presume free displacement of the triple contact line and related the potential
energy barrier to advancing and receding contact angles. This energy barrier is defined by the
liquid adhesion and the solid roughness. Hence, a larger the energy barrier causes larger contact
angle hysteresis (the Wenzel and Cassie states, respectively). Moreover, the derivation predicts
also low contact angle hysteresis for low contact angle values. However, in a broad range of the
contact angles (50
◦
–140
◦
) the contact angle hysteresis does not depend on the equilibrium contact
angle, which is not the case for superhydrophobic surfaces. Also, except for very small droplets,
the droplet volume does not determine the contact angle hysteresis. However, larger contact
angle hysteresis can be expected for liquid whose surface tension is lower.
247
Further, Marmur and Bittoun
248
demonstrated theoretically that both Wenzel and Cassie
equations are good approximations of contact angles on imperfect surfaces but it should be
recognized that they are valid when the size ratio of the liquid drop to the wavelength of
roughness or chemical heterogeneity is sufficiently large. They also showed that local
considerations of the shape and length of the contact line and global considerations involving
interfacial area within the contact line do not contradict but complement each other.
248
Recently, also Erbil and Cansoy
249
tested validity of Cassie-Baxter and Wenzel equations to
evaluate contact angles on 166 samples having patterned superhydrophobic surfaces (square and
cylindrical pillars). They have used literature data recently published in eight papers. It was
possible to calculate roughness parameter from Wenzel equation and fraction of the water/solid
contact surface under the droplet to the total projected of the droplet basement. Then they
compared the calculated values with the experimental ones obtained from the contact angles
measured on flat and rough surfaces, respectively. They found that Wenzel equation was wrong
for most of the tested samples, i.e. 74% in the case of cylindrical and 58% of the square pillars.
Moreover, for rest of the samples significant deviation from the prediction of Wenzel equation
was also high (68%) and it was not caused by contact angle measurement errors. In the case of
Cassie-Baxter equation the authors have found 65% wrong results for cylindrical-pillar patterned
surfaces and 44% wrong results in the case of square-pillar patterned surfaces. Also deviations
from theoretical Cassie-Baxter contact angles were large for most of the samples. These results
show that both Wenzel and Cassie-Baxter equations give more qualitative than quantitative
evaluation of the relationship between the contact angles on rough and flat surfaces and still the
exact mechanisms of rough surfaces wetting is open for further studies. Also molecular dynamics
simulation results obtained by Leroy and Muller-Plathe
250
for a nanometer-scaled rough graphite
showed that Wenzel’s theory fails “to predict even qualitatively the variation of the solid-liquid
surface free energy with respect to the roughness pattern.” However, for the Cassie wetting state
the solid-liquid surface free energy could be well predicted from the Cassie-Baxter equation.
Similar testing on real randomly-coarse surfaces has not been carried out yet and results could
shed more light on applicability of Wenzel and Cassie Baxter models to many surfaces of
practical significance.
Interpretation of the experimental contact angles on rough substrates is always difficult because
of the apparent pinning of the contact line on defects such as edges of asperities, causing
departure from the Wenzel assumptions whether in term of surface area or contact line
length.
251,252
Both shape and sharpness of roughness features and their edges affect pinning of the
contact line as is concluded from a diligent experiment with posts of different shapes performed
by Oner and McCarthy.
253
21

Lately Chibowski
254
suggested to use water (and other probe liquids as well) contact angle
hysteresis for characterization of solid surface wetting properties via calculation of its apparent
surface free energy,
†††
γ
s
tot
.
51,255-257
The energy can be calculated from the advancing θ
adv
and
receding θ
rec
contact angles of one liquid only whose surface tension is γ
l
. The equation reads:
( )
(
)
2
1 cos
2 cos cos
l adv
tot
S
rec adv
x
γθ
θθ
+
=
++
(15)
The general feature of the apparent surface free energy as a function of contact angle hysteresis
(CAH) relationship is the energy decrease with increasing hysteresis. The relative decrease of the
apparent surface free energy is strongly sensitive to the advancing contact angle value. With
increasing its value the apparent surface free energy drastically decreases even if the contact
angle hysteresis is the same. For example, for θ
adv
= 120
◦
and CAH = 10
◦
the decrease in the
apparent surface free energy amounts 13.6% in comparison to its value at zero hysteresis.
However, if θ
adv
amounts to 170
◦
, with the same hysteresis, the energy decreases as much as
nearly 60%. Of course, the absolute value of the apparent surface free energy decrease is large in
the former case, i.e., from 18.2 to15.7 mJ/m
2
, in comparison to the decrease in the latter case,
i.e., from 0.55 to 0.22 mJ/m
2
.
254
These results also show differences between the two
mechanisms of wetting process, i.e., suspended or collapsed drops, for hydrophobic and
superhydrophobic surfaces.
9. Methods of Preparation Superhydrophilic and Superwetting Surfaces
Most solids are naturally rough; however, their roughness is usually insufficient to reinforce a
superhydrophilic state of the material surface. Although any natural or synthetic material could
be converted to one with superhydrophilic surface by chemical treatment and mechanical
roughening or converted to sub-microscopic particles and then deposited to form a
superhydrophilic coating, only a few materials have been explored for such applications. Among
inorganic materials, titanium oxide (TiO
2
)
188-191,193,194,200
and zinc oxide (ZnO)
192,194,258,259
are
frequently studied because of their photoinduced self-cleaning capability, and silica (SiO
2
)
189,260-
266
due to its hydrophilicity and availability at a low price. Films of nanoparticles are often
deposited on substrates from solutions/suspension,
189
ink-jet printing,
200,201
by a sol-gel
technique,
188,191
spin coating
190,191
or through sputtering.
258
Sub-microscopic structures grown
from solutions,
259,267
through lithographic
196
and electrochemical
199
techniques are also used.
Polymers are also attractive materials for superhydrophilic coatings but they surfaces typically
require oxidation. Improvement in hydrophilicity of polymer surfaces, as discussed earlier, can
be obtained with a help of many techniques that change surface chemistry such as the surface
irradiation using gamma rays
114
or ion irradiation,
187
electron beam,
114
plasma
268
and corona
treatment.
117,269,270
In order to make the polymer superhydrophilic the treatment must also have
effect on surface roughness or the chemical treatment must be proceeded by surface roughening.
†††
Apparent surface free energy is imaginary energy calculated based on apparent contact
angles.
22

In recent years, coatings with switchable wetting properties attract interest of many research
groups.
271
Several coatings showing a transition from syperhydrophobic to superhydrophilic or
reverse were demonstrated.
185,192,203,206,272
This has been accomplished for films obtained by sol-
gel process, for example upon heating,
188,272
as well by electrochemical method (aluminum
oxidation)
199
or coatings.
186,190,191,193,195,198,200,205,273
For example, transformation or even
reversible transformation, depending on the treatment, of carbon nanotubes or buckypaper from
superhydrophilic to superhydrophobic can be achieved by heating in vacuum, UV radiation or
ozone treatment.
206
Zhang et al.
207
obtained micro-nano structured nylon 6,6 whose as-formed
surface was suprewetting but after treatment with formic acid and ethanol and then dipping in
paraffin wax solution in ethyl ether and drying, reversed to superhydrophobic . A reversible
superhydrophilic to superhydrophobic WO
3
nanostructured films on alumina or tungsten
substrates were produced by Gu et al.
203
The superhydrophobic film was obtained by covering
the surface with n-dodecanethiol from its solution in ethanol, while the superhydrophilic surface
was obtained by etching it with sodium dodecylbenzene sulfonate in concentrated HNO
3
solution.
10. Applications of Superhydrophilic and Superwetting Surfaces
10.1. Anti-fogging Surfaces
The need for anti-fogging surfaces arises in response to the challenge of visualization under high
humidity. Swimming goggle is a most obvious example for such a scenario. Since the relative
humidity is a strong function of temperature, the vapor can easily reach its saturation limit due to
the temperature fluctuation or at a relatively cold solid surface, such as, the lenses or transparent
walls to see through. As a result, significant condensation in form of tiny droplets can be induced.
The originally transparent solid surfaces will then fog and lose their optical clarity. In recently
years, the necessity of anti-fogging surfaces has been highlighted by micro- and nanofluidic
applications such as visualization of two phase flow in the cathode microchannels of proton
electrolyte membrane fuel cells.
274
Similar challenges will also be encountered when stagnant
multiphase environment in microreactors (e.g., for cell cultivation
275
) needs to be visualized. Anti-
fogging surface can also found applications in our daily life. When a food item is packaged and
displayed in a refrigerated cabinet, the relative humidity inside the package increases due to the
decrease of temperature. Consequently, water tends to condense on the inner surface of packages.
A superhydrophilic surface can be anti-fog because water spreads on the rough hydrophilic
surface to form a thin film instead of droplets. Such an effect can be easily illustrated by placing
a piece of superhydrophilic polyester film on top of a cup filled with hot water.
276
As Figure 4
shows, the plasma-treated superhydrophilic polyester film (right side) remained clear due to the
formation of a continuous water film. As a comparison, the untreated polyester film (left side)
was covered by water droplets and fogged after several minutes. Recent results
277
also revealed
that similar plasma treatment can also generate superhydrophilic "nanoturf" surface with anti-
reflection property. It is reported that optical transmittance of a nanoturf surface is enhanced up
to 92.5% as compared to a flat PUA surface (89.5%).
277
It is noted that the superhydrophilic treatment is different from traditional anti-fogging coating
widely used for swimming goggles and eyeglasses. The later usually employs various surface
coatings to treat the surface hydrophobic, which tends to have low adhesion with the tiny water
droplet formed on it. Such hydrophobic anti-fog surfaces are usually more durable than the
23

superhydrophilic surfaces that can be obtained by existing technology. However, a coating
approach might be undesirable in many conditions, such as inside a microchannel. The safety of
those chemical agencies for biomedical sample and food is questionable especially when the
surface is subjected to environments of high temperature and high humidity (e.g., pasteurization
process). Other concerns of hydrophobic anti-fog coating its efficacy when polymer film is
extruded (process temperature: 200-300
o
C), the cost of the agencies and the relatively small area
it can be uniformly applied on.
10.2. Bio-fouling and its Prevention/Release
The continuous water thin film formed on a hydrophilic or superhydophilic surface has a
profound impact on their interactions with molecules and microorganisms, including biofouling
and biocompatibility (detailed in section 10.3).
In marine engineering, fouling has mainly been used to describe the growth of miroorganisms,
algae, plant etc. on a surface (e.g., of a ship) immersed in sea water. Biomedical devices can also
be subject to fouling as a deposit of cells and biomolecues (e.g., proteins and DNAs). Fouling
usually changes the original property of the surface negatively and significantly impacts the
performance of the device or equipment. It is preferable to avoid (at least slow down) or reverse
biofuling, with strategies known as anti-fouling and fouling-release, respectively.
278
Boicides,
such as tributyltin moiety (TBT), have been widely used in the anti-fouling coating of marine
vessels.
279
The concerns on environmental impact, as well as the need for biomedical
applications, are driving the development of no-toxic anti-fouling and fouling-release methods,
such as microtopography to mimic the surfaces of shells and scales of marine life.
280-282
Surface chemistry has also been known as a strong factor to affect fouling and its
prevention/release. Extensive works by Bier and coworkers since 1960s have led to the
establishment of a predictive curve, as Figure 5 shows, to show the relationships between critical
surface tension of solid surface and the degree of biological fouling retention.
283
It is understood
that fouling is such a complex issue that can not be sufficiently determined solely by surface
energy or contact angle. However, the Bier curve has been proved to be an effective means to
indicate the relative tendency of fouling in many cases, including blood fouling of biomedical
devices or implants and bio-fouling of marine vessels.
283
Of particular interest has been a region
with relatively low surface energy of 22-24 mN/m, known as theta surfaces, which require
minimal energy to detach biofilms. Strictly speaking, as theta surfaces are fouling-release instead
of anti-fouling surfaces, which means external forces (e.g., flow) and intervention are required to
periodically remove the already fouled surfaces. It is interesting to look at the end of very high
surface energy, or the hydrophilic part of the cure. A trend is clearly seen that for high-surface-
energy materials, the degree of fouling actually decreases with surface energy. It can be
explained by the strong affinity between surface and water molecule, which establishes a barrier
to prevent interaction between fouling agent and surface and thus delayed the fouling. Indeed,
recent work by Meng’s group has shown significant reduction of fouling by fluorescein and
fluorescent proteins after the surfaces are treated to be superhydrophilic.
284
It should be noted
that such results have been obtained in a relatively short period (30 min incubation time) with
static liquid. They are thus mainly indicative for applications such as micro total analysis
systems (µTAS) and not necessary for long-term prevention and release of biofouling.
284
The
difference in short-term and longer-term
285
fouling behaviors of superhydrophilic and
24

hydrophilic surfaces can be attributed to the quick degradation of hydrophilicity, which tends to
be unstable.
10.3. Other Applications in Biomedical Field
Hydrophilic coatings have been used in the medical field for the last few decades, for example in
catheters, guide wires, and other vascular access devices for fertility, contraception, endoscopy,
and respiratory care. Polyvinylpyrolidone, polyurethanes, polyacrylic acid, polyethylene oxide,
and polysaccharides were the main polymeric components in hydrophilic coatings. Reduction in
friction was the key need in design of hydrophilic coatings. Recently, these coatings are also
moving toward anti-fouling, antimicrobial and/or biologically active surfaces that perform tasks
other than imparting lubricity. Also superhydrophilic coatings attracted interest among
biomedical engineering research teams. Unfortunately, many claims of superhydrophilic surfaces
or coatings do not comply with our definition presented earlier in this paper, as well as in our
previous note.
159
For this reason, we remind our readers that flat surfaces with strong affinity to
water should be simply called hydrophilic and we follow this definition in reviewing recent
research activities in improving biocompatibility and affinity to water of implant materials.
Improving Hydrophilicity of Polymeric Bio-Implants. Biomedical applications of polymers
include vascular grafts, heart valves, artificial hearts, catheters, breast implants, contact lenses,
intraocular lenses, components of extracorporeal oxygenators, dialyzers and plasmapheresis
units, coatings for pharmaceutical tablets and capsules, sutures, adhesives, and blood
substitutes.
286
Stents, lenses, catheters, and implants require biologically non-fouling surfaces to
which proteins, lipids and cells do not adhere. Both catheters and lenses are made hydrophilic,
although for different purposes. Catheters and quidewires require low friction (coefficient of
friction of 0.3 or less) so they are easily maneuvered within patient’s vasculature.
287,288
Hydrophilic coatings were found to provide better lubricity compared to hydrophobic
coatings.
288,289
Lenses must be wetted by tear fluid to move relatively freely on the eye,
providing wearer comfort.
290,291
The applied research on surface modification of contact lenses is
substantial
289,292-296
and mostly deals with making surface of polymer hydrophilic.
Contact lenses were introduced into the field of vision correction after discovery of highly
oxygen permeable silicone hydrogels that satisfy the metabolic needs of the cornea, maintain its
physiological health, and can be worn continuously for several days.
297,298
However, due to
hydrophobicity of silicone hygrogels they require hydrophilic coatings for improved wettability
with tear fluid, wearing comfort and biocompatibility. Contact lenses, when inserted into the eye,
accumulate proteins and other tear film components to which bacteria can adhere threatening
adverse clinical events.
299
Advanced contact lens coatings are not only hydrophilic but also have
low biofouling characteristics. Chemical modifications that create low-fouling surfaces have
been the area of intensive research not only in the field of vision correction but in biomedical
applications in general. Surface coatings included neutral hydrophilic polymers such as
polyacrylamide and poly(ethylene oxide) (PEO),
300
phospholipids,
301
dextran,
302
pullulan,
303
and
others.
304,305
PEO has been the most popular polymer.
305,306
Recently, Shimizu et al.
307
synthesized hydrophilic silicone hydrogels from 2-methacryloyloxyethyl phosphorylcholine
(MPC) and bis(trimethylsilyloxy)-methylsilylpropyl glycerol methacrylate (SiMA) by
25

controlling the surface enrichment of MPC units. New silicone-based hydrogel maintains high
oxygen permeability and the MPC units at the surface are responsible for low protein adsorption.
Titanium-Based Biomaterials. Due to their high biocompatibility, elastic modulus that closely
matches human bone, good ductility, fatigue and tensile strength, titanium (Ti) and Ti-based
alloys are very popular for orthopedic implants.
308,309
High biocompatibility of Ti-based
biomaterials is attributed to surface oxide layer. In fact, almost all Ti-based implants undergo
some sort of anodization, electropolishing, passivation and/or other treatment, used to control
type of oxide layer, its thickness and surface topography.
310
It is just only in the last couple of
years when photoinduced hydrophilic and photocatalytic cleaning properties of titanium oxides
22
have been explored for applications in the area of biomaterial implants. There is sufficient
evidence in support the removal of organic contaminants
311
and bacteria
312
adsorbed on TiO
2
surface by the photo-oxidization process. Such self-cleaning is believed to occur particularly in
the case of TiO
2
films that exhibit hydrophilicity.
311
Self-sterilization capability of TiO
2
surfaces,
ignored in the past, will likely be explored by biomedical industry sector in the nearest future.
Changes in the bioactivity of titanium and chromium-cobalt alloy surfaces during their aging and
exposure to the ultraviolet (UV) light treatment were recently studied.
313,314
The study conducted
uncovered a time dependent biological degradation of biomaterials, which was restored by UV
phototreatment. The restoration was more closely linked to hydrocarbon contaminants removal
than the hydrophilicity induced during UV treatment. These two effects are inter-related because
surface of implant materials has enhanced affinity to water when it is free of organic
contaminants. However, surface OH groups are needed to make the interaction strong through
hydrogen bonding.
310
More recently, Ogawa et al.
315,316
demonstrated that UV light treatment of TiO
2
is effective in
converting implant material surfaces to hydrophilic ones, and this conversion enhanced
osteogenic environment. They found that the number of rat bone marrow-derived osteoblasts
cultured and attached to hydrophilic surfaces was substantially greater than on untreated TiO
2
surfaces. Adhesion of a single osteoblast was also enhanced on UV-treated TiO
2
with virtually
no surface roughness or topographical features. Osteoblasts on UV-treated TiO
2
surfaces were
larger and with increased levels of vinculin expression and focal contact formation, although the
density of vinculin or focal contact was not influenced by hydrophilicity.
The same research group also found that TiO
2
with restored hydrophilicity has higher albumin
and fibronectin protein adsorption, human osteoblast migration, attachment, differentiation, and
mineralization than untreated TiO
2
surfaces even if untreated surfaces are freshly prepared.
317
Time-related degradation of TiO
2
bioactivity was found to be significant in regular storage
conditions, what affected recruitment and function of human osteoblasts. However, UV
treatment restored and often enhanced TiO
2
surface bioactivity.
Ogawa et al.
315-317
also demonstrated that photofunctionalization of materials can be
accomplished through a coating process. Non-Ti biomaterials can be coated with TiO
2
particles
which are effective in developing functional biomaterials and improve their bioactivity.
Superhydrophilicity for Growing Bone-Like Structures. The new generation of orthopedic
implants and tissue engineering scaffolds is explored through accurately designed 3D structures
of materials.
318
Efforts underway concentrate on improving the bioactivity and biocompatibility
26

for the core materials used in orthopedic applications such as Ti-based alloys
319-322
and
polymers.
320,322-324
Surface treatments include coating with biomimetic calcium phosphate (CaP)
bioactive layers or chemical modifications to enhance hydroxyapatite formation on the
biomaterial surface when in contact with the living bone. Figure 6 shows examples of porous
superhydrophilic and biocompatible coatings of calcium phosphate produced at Michigan Tech.
Biological properties of the coated implants and scaffolds depend not only on the chemical
composition of the coating but also its structures. The ideal coating should resemble the structure
of natural bone, favorable for cell anchoring and cell culture, and should be run-through 3D
structure. Hydroxyapatite and tricalcium phosphate coatings accelerate osteoblast cell attachment
and proliferation, reducing inhalation process and enhancing hard tissue integration.
319-322
Hydrophilicity was found to favor deposition of Ca-based bioactive coatings on biomaterials.
Recently, Lai et al.
318
used hydrophilic– hydrophobic patterned template work to fabricate
structured octacalcium phosphate film on bioactive TiO
2
nanotube surfaces. By controlling
wettability pattern desired hierarchically structured OCP films were manufactured.
Wu et al.
325
produced the 3D complex-shaped microporous titanium-based scaffolds with
superhydrophilic surface characteristic via a facile low-temperature alkaline-based hydrothermal
process. They achieved a hierarchical structure on the nano- and micro-scale that closely
resemble the structural organization of a human bone, and these submicroscopic structures are
primarily responsible for the superhydrophilicity of the scaffold. Due to good wettability of
material surfaces by alkaline solution used in hydrothermal process, it can penetrate the entire
exposed scaffold surface despite the complex topographies of 3D porous scaffold.
Biomimetically grown structures favor the formation of a smooth junction between the bone
tissue and scaffold and benefit the long-term fixation of the scaffold. The enhancement in
hydrophilicity of TiO
2
is closely related to the formation of highly crystallized anatase
TiO
2
,
312,326
which can be promoted by increasing the conversion voltage during anodic oxidation
or the subsequent annealing.
326
Although rutile is more stable titanium oxide, anatase is
considered to be more advantages for medical applications. Anatase adhere more strongly to Ti
metal and absorbs more PO
4
3-
and OH
-
ions in the body fluid, which ions favor formation of a
bone-like apatite structure.
327,328
A bioactive and superhydrophilic TiO
2
coatings were prepared on PET film substrates using dip
coating method and subsequent glow discharge plasma treatment by Pandiyaraj et al.
329
The
chemical and morphological characteristics of the cleaned and rough TiO
2
coatings induced the
growth of bone like apatite layers from simulated body fluid solution.
10.4. Enhanced Boiling Heat Transfer
Known as a most efficient mode of heat transfer, boiling has been employed in a broad range of
power generation and thermal management devices, such as nuclear power plants,
330
refrigeration,
331
cooling of electronics
332
and chemical reactors.
333
Boiling heat transfer can also
be significantly affected by surface wettability. Figure 7 shows a boiling curve which correlate
the heat flux with wall superheat. Nucleate boiling starts from point A, with vapor bubbles
forming on the overheated surface. The nucleate boiling continues to fully develop from B to C.
At point C, the het flux eventually reaches its maximum value, known as critical heat flux
(CHF), because a continuous vapor film is formed as an effective thermal insulation layer.
Further heating beyond point C will lead to dramatic increase of wall temperature and device
27

failure. Therefore, CHF marks the maximum heat flux that can be provided by a boiling-based
cooler.
It is intuitive that the continuous water film formed on a hydrophilic or superhydrophilic surface
can delay the formation of vapor film in boiling and thus improve CHF. Experimentally,
vertically aligned nanoforests of hydrophilic/superhydrophilic nanorods,
334
nanowires
335,336
and
CNTs
337
have shown the potential to significantly improve boiling heat transfer. For example,
both CHF and heat transfer coefficient (HTC) have been improved by more than 100% in Ref.
335
Such improvements have been attributed to the dramatically increased density of nucleation
sites, high surface tension force of superhydrophilic nanostructures to pump liquid and the cavity
stability provided by the nanopores.
334,335
At the same time, it is also shown that a surface with
mixed hydrophilic and hydrophobic micro patterns can enhance pool boiling to almost the same
degree. For example, 65% and 100% improvements on CHF and HTC respectively
338
has been
achieved with a hydrophilic network decorated by hydrophobic islands of ~100µm. In spite of
the relatively simple configuration of the surface, the results has been convincingly explained by
the fact that the hydrophilic network can prevent formation of vapor film by attracting liquid
while the hydrophobic region can promote nucleation and help to remove gas bubble
efficiently.
338
10.5. Other Applications
Many other applications of hydrophilic and superhydrophilic surfaces are not included in the
above discussions. For example, hydrophilic modification has been long known as an effective
way to improve adhesion.
339,340
It has also been explored recently to decrease the impedance of
neural microelectrode arrays.
341
Switchable wettability may find applications in reconfigurable
microfluidic devices, such as droplet-based lab-on-a-chips by electrowetting-based
actuation,
342,343
liquid microlens
344
and arrayed optics.
345
The wettability switching mechanism
has been comprehensively reviewed recently.
346
More examples, as well as their preparation
methods can be found in section 9 of this paper. Surfaces may exhibit tunable wettability from
superhydrophilic to superhydrophobic, especially those coated with conductive polymers
347
or
nanomaterials, such as ZnO nanorods,
348
carbon nanotubes
349
and graphene.
350
The research on
extreme wettability is a highly dynamic field. It can be expected that more applications of
superhydrophilic surface will be developed in the foreseeable future.
11. Conclusion
We define superhydophilic surfaces, and coatings, as rough (and sometimes porous) surfaces
(coatings) of materials having affinity to water stronger than to nonpolar air in the air/water/solid
system, and when on these rough surfaces water spreads completely. Flat and smooth surfaces
of hydrophilic materials, on which water spreads completely (even if hydrophilicity results from
photoinduced or other cleaning), do not belong to this category. Vast majority of materials could
be considered hydrophilic due to a polar-type contribution to the solid-water interactions and
therefore, there is a need to group them under different categories, with different degree of
hydrophilicity. The literature lacks of such classification, posing challenges for researchers to fill
this gap of science. In this review paper, using the values of (advancing) water contact angles (θ)
we have proposed to classify smooth solid surfaces for hydrophilic (θ≅0
o
), weakly hydrophilic
28

(0<θ<(56-65
o
)), weakly hydrophobic ((56-65
o
)<θ<90
o
)and hydrophobic (90
o
≤θ<120
o
). The
exact cut in contact angle value separating weakly hydrophilic from weakly hydrophobic
materials needs to be determined in a future research. Another challenge ahead relates to
meaning and interpretation of water contact angle with zero value, if such contact angle can be
measured experimentally.
The research on superhydrophilicity has emerged in the last few years, with noticeable increase
in number of publication since 2000, and will certainly attract attention of many research groups
in the nearest years to come. The progress on fabrication and characterization of
superhydrophilic surfaces and coatings, along with understanding of a liquid spreading on such
materials, is driven by a broad application of superhydrophilic surfaces in products with anti-
fogging screens, windows and lenses, anti-fouling coatings, microfluidic devices, biocompatible
implant devices, coatings for enhanced boiling heat transfer, foils for food packaging, and many
others. There is already a wide spectrum of products available on market which design was
inspired by superhydrophilic phenomenon. These products include anti-fogging mirrors for
bathrooms and cars, shields of helmets for motorcycles, swimming goggles, lenses of eyeglasses,
and safety eyeglasses and shields.
In spite of young age of superhydrophilicity term, many research activities from the past could
be considered as a solid foundation for this new sub-discipline. For example, surfaces of
hydrophilic materials were roughened in the past to improve adhesion in composites,
biocompatibility in implant devices, or simply to enhance spreading of liquid, even so these
activities were not linked yet to superhydrophilicity.
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