Nanonails: a simple geometrical approach to electrically tunable superlyophobic surfaces.

Nanonails: A Simple Geometrical Approach to Electrically Tunable
Superlyophobic Surfaces
A. Ahuja,† J. A. Taylor,‡ V. Lifton,§ A. A. Sidorenko,| T. R. Salamon,† E. J. Lobaton,†
P. Kolodner,† and T. N. Krupenkin*,‡
Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
ReceiVed July 31, 2007. In Final Form: September 14, 2007
In this work, dynamically tunable, superlyophobic surfaces capable of undergoing a transition from profound
superlyophobic behavior to almost complete wetting have been demonstrated for the first time. In the initial state,
with no voltage applied, these surfaces exhibit contact angles as high as 150° for a wide variety of liquids with surface
tensions ranging from 21.8 mN/m (ethanol) to 72.0 mN/m (water). Upon application of an electrical voltage, a
transition from the superlyophobic state to wetting is observed. We have examined experimentally and theoretically
the nature of these transitions. The reported results provide novel methods of manipulating liquids on the microscale.
Introduction
It has been known for a long time that a high degree of surface
roughness enhances the wetting properties of solids.1-4 Modern
nanofabrication techniques allow the creation of a wide range
of sophisticated surface topographies,5-8 such as fractal sur-
faces,7,8 that strongly magnify this effect. Such surfaces serve
as a basis for so-called superhydrophilic (water contact angle
approaching 0°) and superhydrophobic (water contact angle
approaching 180°) materials that demonstrate a range of
remarkable properties.9,10
More recently, this approach was extended to demonstrate
more advanced types of surfaces that allow dynamic tuning of
their wettablity all the way from superhydrophobic behavior to
almost complete wetting.11-14 Although these surfaces show
substantial promise for potential applications, their applicability
is mostly restricted to high-surface-tension liquids that have a
minimum-free-energy state achieved by minimizing the area of
contact between the liquid and a solid.11,14
In this work, we propose an approach that allows the creation
of a new type of tunable nanostructured surface that operates by
“locking” the liquid in an electrically controllable metastable
nonwetting state. This opens up the possibility of creating tunable
superlyophobic surfaces (i.e., surfaces capable of demonstrating
wetting behavior ranging from complete wetting to active
“repelling” of virtually any liquid, independent of its surface
tension). The approach relies on 3D nanoscale surface topography,
which controllably “pins” the contact line and thus provides a
tunable energy barrier capable of holding the liquid in a desirable
metastable state. In terms of geometry, this approach further
extends the idea of contact line pinning on “mushroom” structures
previously proposed in the literature.15-17
Using this approach, we have experimentally demonstrated
tunable nanostructured surfaces that are capable of undergoing
a transition from profound superlyophobic behavior to strong
wetting. In the initial state, with no voltage applied, these surfaces
exhibit contact angles as high as 150° for a wide variety of
liquids with surface tensions ranging from 21.8 mN/m (ethanol)
to 72.0 mN/m (water). Upon application of an electrical voltage,
contact angles of the liquids on these surfaces can be reduced
to below 30°.
The proposed approach opens a pathway to a simple, material-
independent method for creating electrically tunable superlyo-
phobic surfaces and thereby extends many of the potentially
attractive properties (high liquid droplet mobility, controllable
chemical reaction initiation, tunable drag reduction, etc.11-14)
usually associated with tunable superhydrophobic surfaces to a
much broader range of materials and applications. To extend
previously-demonstrated, dynamically tunable superhydropho-
bic nanostructured surfaces11,14 into the superlyophobic domain,
one needs to understand the reasons that prevent traditional
superhydrophobic surfaces from being able to “repel” low-surface-
tension liquids. When a droplet of a liquid is placed on a very
rough surface, such as a fractallike surface7,8 or a surface covered
with a carpet of high-aspect-ratio micro- or nanoscale spikes,
bumps, or posts,11,14,18 it assumes a state that minimizes its free
energy. This can be achieved by the droplet either wetting the
rough surface and thus substantially increasing the total area of
the liquid-solid interface or by minimizing the liquid-solid
contact area by dewetting most of the surface. In the latter case,
* Corresponding author. E-mail: tnk@engr.wisc.edu.
† Lucent Technologies.
‡ Current address: Department of Mechanical Engineering, University of
WisconsinsMadison, 1513 University Avenue, Madison, Wisconsin 53706.
§ Current address: mPhase Technologies, 150 Clove Road, Little Falls,
New Jersey 07424.
| Current address: Department of Chemistry, University of the Sciences
in Philadelphia, South 43rd Street, Philadelphia, Pennsylvania 19104.
(1) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466.
(2) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
(3) Callies, M.; Quere´, D. Soft Matter 2005, 1, 55 and references therein.
(4) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1.
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Proc. Natl Acad. Sci. U.S.A. 2005, 102, 1848.
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4917.
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X. Chem. Mater. 2005, 17, 6177.
(8) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125.
(9) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D.
W.; Riehle, M. O. Nano Lett. 2005, 5, 2097.
(10) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O‹ ner, D.; Youngblood, J.;
McCarthy, T. J. Langmuir 1999, 15, 3395.
(11) Krupenkin, T.; Taylor, J.; Schneider, T.; Yang, S. Langmuir 2004, 20,
3824.
(12) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew.
Chem. 2004, 43, 357.
(13) Krupenkin, T.; Taylor, J. A.; Kolodner, P.; Hodes, M. Bell Labs Tech.
J. 2005, 10, 161.
(14) Krupenkin, T. N.; Taylor, J. A.; Wang, E. N.; Kolodner, P.; Hodes, M.;
Salamon, T. R. Langmuir 2007, 23, 9128.
(15) Bico, J.; Thiele, U.; Quere´, D. Colloids Surf., A 2002, 206, 41.
(16) Herminghaus, S. Europhys. Lett. 2000, 52, 164.
(17) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C.
AdV. Mater. 2005, 16, 1929.
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T.; Dailey, J. W.; Picraux, S. T. J. Phys. Chem. B 2004, 108, 12640.
9Langmuir 2008, 24, 9-14
10.1021/la702327z CCC: $40.75 © 2008 American Chemical Society
Published on Web 10/12/2007

the droplet remains attached to the substrate at a relatively small
number of points on top of the structured layer, such as at spike
tips or nanopost tops.11,14,18 In the first case, the state of the
droplet is described by the Wenzel equation,1 and in the second
case, by the Cassie-Baxter equation,2 often referred to as the
Wenzel and Cassie-Baxter states, respectively.
The Cassie-Baxter state forms the basis of superhydropho-
bicity. In this state, the liquid-solid contact area and thus the
liquid-solid interaction are greatly reduced, and the liquid
behaves to a large extent as if supported by a thin air layer rather
than by a solid substrate. This behavior leads to a plethora of
unusual properties, including extremely high contact angles, high
contact-line mobility, and substantial slip at the liquid-substrate
interface.2-19
The free energy of a liquid droplet in the Wenzel and Cassie-
Baxter states depends strongly on the equilibrium contact angle
that the liquid exhibits on the same substrate in its unstructured,
flat topographical form. For high-surface-tension liquids with
flat-substrate contact angles well above 90°, the Cassie-Baxter
state typically provides the minimum-energy state and gives rise
to superhydrophobicity. For very low surface tension liquids,
according to the Fox and Zisman equation,21 the flat-substrate
contact angle goes well below 90°. In this case, the Cassie-
Baxter state typically becomes unstable, and the liquid tends to
assume the minimum-energy Wenzel state in which the substrate
is fully wetted. For a detailed discussion of the conditions under
which the transition between Cassie-Baxter and Wenzel states
occurs, see ref 15. The behavior described above is observed for
all types of superhydrophobic surfaces and prevents them from
exhibiting superlyophobic behavior (i.e., being able to “repel”
liquids with very low surface tension).
One obvious way to extend the applicability of traditional
superhydrophobic surfaces into the superlyophobic regime is to
ensure that the Cassie-Baxter state always remains the minimum-
free-energy state. This can be achieved by developing coatings
with extremely low surface energies so that the flat-substrate
contact angles are well above 90° even for low-surface-tension
liquids. Some limited success using this approach has been
achieved recently, with several researchers reporting self-
assembled-monolayer-coated nanostructured surfaces that support
contact angles on the order of 120° for a few liquids with
moderately low surface tension, such as organic oils.6,20,22
However, the applicability of this approach is rather limited. Its
further extension to a broader range of important low-surface-
tension liquids such as organic solvents and oils that are ubiquitous
in the human environment and have surface tensions of around
20 mN/m and below requires the development of stable coatings
(19) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G.
A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701.
(20) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed.
1997, 36, 1011.
(21) Fox, H. W.; Zismann, W. A. J. Colloid Sci. 1950, 5, 514.
(22) Hikita, M.; Tanaka, K.; Nakamura, T.; Kajiyama, T.; Takahara, A.
Langmuir 2005, 21, 7299.
Figure 1. Nanostructured substrate topography. (a) Schematic representation of the overhang geometry. The solid structure is shown in red,
and the liquid is shown in blue. (b) Scanning electron microscopy (SEM) image of 2-ím-pitch nanonails. The nail head diameter D is about
405 nm, the nail head thickness h is about 125 nm, and the nanonail stem diameter d is about 280 nm. (c) SEM image of 0.9-ím-pitch nanonails.
The nail head diameter D is about 383 nm, the nail head thickness h is about 187 nm, and the nanonail stem diameter d is about 260 nm.
(d) Side view of the honeycomb substrate. The cells are arranged on a hexagonal lattice with a period of 30 ím. The inset shows a close-up
view of the overhang on the honeycomb wall. The overhang extends approximately 0.4 ím on each side of the wall.
10 Langmuir, Vol. 24, No. 1, 2008 Letters

with surface energies of just a few mN/m, well below any values
reported in the literature to date.10,20,23,24 Moreover, even in its
present form, this approach relies on the quality of a delicate,
very-low-energy coating, which makes the system highly
vulnerable to surface contamination and degradation.
Experimental Results and Discussion
In this work, we propose and experimentally demonstrate a
new approach that relies almost exclusively on surface topography
to achieve the desired substrate wetting properties. Instead of
attempting to guarantee that the Cassie-Baxter state always
remains the minimum-energy state, we create a system in which
the strength of the energy barrier that separates the metastable
Cassie-Baxter state from the stable Wenzel state is designed to
be high enough to achieve effective locking of the liquid in the
desired nonwetting state. The idea is to create a special type of
3D surface topography that normally inhibits transitions from
the Cassie-Baxter state to the Wenzel state but would allow
such transitions under the influence of external stimuli, such as
electrical fields. A particular example of such topography is the
“nanonail” structure shown in Figure 1a-c. Each nanonail consists
of a conductive silicon stem and a dielectric silicon oxide nail
head. The resulting structure is covered with a thin conformal
layer of low-surface-energy fluoropolymer. During electrically
induced transitions from the nonwetting to the wetting state, the
presence of the conductive nail stem allows one to generate high
electrical field values in the immediate vicinity of the liquid-air
interface, as will be discussed below.
The reason that nanonails are so effective at preventing
spontaneous transitions from the nonwetting Cassie-Baxter state
to the wetted Wenzel state becomes obvious as one considers
the advancing contact angle ı0 that the liquid has to exceed in
order to wet the bottom part of each nail head, as shown in Figure
1a. To achieve even modest values of ı0, well below 90°, the
liquid surface would have to deform considerably or “sag” in
between neighboring nanonails, leading to a liquid-air interface
with curvature on the order of several inverse micrometers; see
Figure 1b,c. However, according to the Laplace law,25 such a
large curvature is possible only in the presence of high hydrostatic
pressure, on the order of 104 Pa, which is orders of magnitude
greater than the capillary pressure in a macroscopic liquid droplet.
Thus, unless high pressure is applied, the liquid has to stay on
top of the nanonails and is unable to penetrate inside and wet
the nanostructured layer. As is evident in Figure 2a,b, the nanonail
topography indeed exhibits superlyophobic behavior even for
strongly wetting liquids such as ethanol (21.8 mN/m) and
methanol (22.5 mN/m), which readily wet a flat fluoropolymer
surface and have corresponding contact angles on the order of
40°.
Obviously, the nanonail geometry is not the only structure
that allows one to lock the liquid in the nonwetting Cassie-
Baxter state. Essentially any geometry that features “overhang”
analogues to a nail head would produce similar results. An
example of an alternative geometryshoneycombs with overhangs
is shown in Figure 1d. The behavior of liquids on this topography
is quite similar to the case of nanonail-covered substrates and
will be discussed below.
In this work, we experimentally investigated two types of
nanonail-covered substrates (Figure 1b,c) and one type of
honeycomb with an overhang (Figure 1d). The structures were
etched in silicon using reactive-ion etching.28 In the case of
honeycombs, a layer of thermal oxide approximately 50 nm thick
(23) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111,
7155.
(24) Vezenov, D. V.; Zhuk, A. V.; Whitesides, G. M.; Lieber, C. M. J. Am.
Chem. Soc. 2002, 124, 10578.
(25) Adamson, A. W., Gast, A. P., Eds. Physical Chemistry of Surfaces, 6th
ed.; Wiley: New York, 1997.
Figure 2. Contact angles of various liquids on nanonail-covered
substrates. (a) A nanonail-covered substrate in action. Droplets of
two liquids with the very different surface tensions, 72 mN/m (water)
and 21.8 mN/m (ethanol), sit next to each other on the 2-ím-pitch
nanonail substrate. (b) Contact angles on a flat substrate and on a
nanonail-covered substrate vs liquid surface tension. Data for alcohols
(from ethanol to cyclopentanol) are represented by circles, and data
for water-ethanol mixtures (from 100 to 32% ethanol) are represented
by squares. Empty squares and empty circles represent flat-substrate
contact angles. Black and yellow squares represent water-ethanol
mixtures on nanonails with 0.9 and 2 ím pitch, respectively. Red
and green circles represent alcohols on nanonails with 0.9 and 2 ím
pitch, respectively. (c) Contact angle cosines on a nanonail-covered
substrate vs contact angle cosines on a flat substrate. Squares and
triangles represent water-ethanol mixtures (ranging from pure
ethanol to pure water) on nanonails with 2 and 0.9 ím pitch,
respectively.
Letters Langmuir, Vol. 24, No. 1, 2008 11

was grown on the structures using a thermal oxidation process.
In all three cases, the structures were conformally coated with
an �20-nm-thick smooth layer of a fluoropolymer11,13,14 (F/C
ratio of 1.55 and about 18 mN/m critical surface tension) using
a chemical vapor deposition process.28 An identical coating was
also deposited on flat silicon wafers to allow a comparison with
flat-substrate contact-angle measurements. Nanonails with a
height of 7 ím were arranged on a square lattice with a pitch
of 2 ím, and honeycombs were arranged on a hexagonal lattice
with a period of 30 ím. Specific measurements of the nail head
and honeycomb geometries are given in Figure 1b-d. A detailed
description of the method used to manufacture the structures is
available in Supporting Information.
A wide range of low-surface-tension liquids having different
chemical natures were used to characterize the superlyophobic
behavior of the nanostructured substrates. Among those tested
were alcohols (methanol, ethanol, 1-propanol, 1-butanol, 1-oc-
tanol, 1-decanol, and cyclopentanol), water-alcohol mixtures,
aromatic hydrocarbons (o-xylene and styrene), ethers (1,4-dioxane
and anisole), esters (methyl methacrylate), and silicone oils
(polyphenylmethylsiloxane and tetraphenyltetramethyltrisi-
loxane). All of the above-mentioned liquids formed highly mobile
droplets on these surfaces, with associated contact angles
exceeding 131°.
Typical results for the observed advancing contact angle as
a function of the liquid surface tension are shown in Figure 2
using alcohols and water-ethanol mixtures on nanonails as
examples. As one can see in Figure 2a, all tested alcohols readily
wet flat, fluoropolymer-coated substrates, exhibiting contact
angles well below 90°. Yet these liquids form droplets with very
high contact angles, approaching 150°, on both 0.9- and 2-ím-
pitch nanonail substrates. In addition to demonstrating high contact
angles, the droplets on nanonail substrates exhibited very high
mobility (i.e., low contact-angle hysteresis), approaching the
behavior of “rolling ball” droplets on previously studied nanograss
substrates.11,14 Water-ethanol mixtures with analogous surface
tensions demonstrate similar results and are also shown in Figure
2b. Data for the other liquids on nanonails and the results obtained
for honeycombs are available in Supporting Information.
In the Cassie-Baxter state, the cosine of the observed contact
angle ı depends linearly on the cosine of the flat-substrate contact
angle ı0. The experimentally measured dependence of cos ı is
plotted as a function of cos ı0 in Figure 2b for the case of water-
ethanol mixtures with surface tensions ranging from pure water
(72 mN/m) to pure ethanol (22 mN/m). The observed linear
dependence supports our assumption that liquid droplets are in
the Cassie-Baxter state on a nanonail substrate.
The position of the liquid-air interface can be directly observed
using a polymerization technique.11 In this case, a UV poly-
merizable monomeric liquid (NOA72, Norland Inc., surface
tension 40 mN/m) was employed. After dispensing on the
substrate, the NOA72 droplet was polymerized in situ by UV
irradiation. The solidified droplet was then carefully delaminated
(26) Brakke, K. A. The Motion of a Surface by Its Mean CurVature; Princeton
University Press: Princeton, NJ, 1977.
(27) Sethian, J. E. LeVel Set Methods: EVolVing Interfaces in Geometry, Fluid
Mechanics, Computer Vision and Materials Sciences; Cambridge University
Press: Cambridge, U.K., 1996.
(28) Madou, J. Fundamentals of Microfabrication, 2nd ed.; CRC Press: Boca
Raton, FL, 2002.
Figure 3. Atomic force microscopy (AFM) images of the solidified
liquid-substrate interface. (a) Liquid-substrate interface on a 0.9-
ím-pitch nanonail substrate. Color intensity represents surface
topography, with the darker colors corresponding to lower elevations.
The green line indicates the surface profile scan shown in panel c.
(b) Liquid-substrate interface on a 2-ím-pitch nanonail substrate.
The green line indicates the surface profile scan shown in panel d.
(c) Surface scan through a single nail head imprint shown by the
green line in panel a, 0.9-ím-pitch nanonail substrate. The apparent
slope of the imprint walls is due to the shape of the AFM scanning
tip. (d) Surface scan through a single nail head imprint shown by
the green line in panel b, a 2-ím-pitch nanonail substrate.
Figure 4. Liquid-air interface modeling results for a 0.9-ím-pitch
nanonail substrate. (a) Typical shape of the constant-curvature liquid-
air interface. Colors represent surface elevation, with red corre-
sponding to the highest elevation and blue corresponding to the
lowest. The hole in the center represents the bottom edge of the nail
head. In this example, the interface curvature is equal to 0.55 ím-1.
(b) Interface curvature (blue curve) and additional interfacial area
with respect to the flat interface (green curve) as a function of the
angle ı0 between the interface and the nail head bottom (Figure 1a).
12 Langmuir, Vol. 24, No. 1, 2008 Letters

from the substrate, and its bottom surface was investigated using
atomic force microscopy (AFM). The AFM results are presented
in Figure 3 a-d and show that the liquid-air interface between
the posts is substantially flat, as expected from the negligibly
small hydrostatic pressure within the droplet. Both the 2 and
0.9-ím-pitch nanonails leave clear imprints along the bottom
surface of the droplet, with the imprint depth consistent with the
nail head shape as measured from SEM images in Figure 1b,c.
Because the nail head shapes are different for the 0.9- and 2-ím-
pitch nanonails, the level at which the liquid-air interface resides
is also different. For the 2-ím-pitch nail heads, the interface
resides approximately halfway between the top and the bottom
of the nail head, which is consistent with the �75° flat-substrate
contact angle exhibited by NOA72. The conelike shape of the
0.9-ím-pitch nail head precludes the liquid from ever attaining
a 75° contact angle with the nail head sidewalls, leading to an
interface that is aligned with the bottom of the nail head.
As was discussed above, the shape of the liquid-air interface
is determined by the Laplace law, which requires the liquid-air
interface to have constant curvature, the value of which is
directly proportional to the hydrostatic pressure in the liquid and
inversely proportional to the liquid surface tension. The resulting
shape of such a constant-curvature interface can be readily
calculated using numerical modeling.26,27 Typical results of such
calculations are shown in Figure 4a. A more detailed discussion
of the computational approach can be found in Supporting
Information.
Knowledge of the interface shape provides important infor-
mation about the interface stability because it relates the flat-
substrate contact angle to the maximum hydrostatic pressure
that the interface can sustain. The transition to the Wenzel state
occurs when the liquid-air interface forms an angle with the
bottom of the nail head that exceeds the flat-substrate advancing
contact angle. In this case, the liquid can advance by wetting the
bottom of the nail head and then propagating further down the
nanonail. The predicted interfacial curvature is plotted as a
function of the local contact angle in Figure 4b. The magnitude
of the predicted curvature indicates that even strongly wetting
liquids with flat-substrate contact angles as low as 30-40° can
sustain substantial pressures on the order of 104 Pa. This further
supports the notion that the nail heads are capable of strongly
inhibiting transitions to the fully wetted Wenzel state.
Similar calculations allow one to estimate a lower bound on
the height of the energy barrier that separates the metastable
Cassie-Baxter state from the minimum-free-energy Wenzel state.
The energy of this barrier is proportional to the liquid surface
tension multiplied by the excess liquid-air interfacial area that
has to be created to deflect the interface strongly enough to
initiate the transition, as discussed above. The results of these
calculations are shown in Figure 4b and suggest that the required
energy is on the order of 10-8 erg, which is orders of magnitude
above the characteristic thermal fluctuation energy at room
temperature.
A more detailed analysis of the interface stability goes beyond
the scope of this letter and will be given elsewhere. Here we
simply note that further calculations show that the nail head
structures by themselves appear to provide an interface that is
rather stable to many common perturbations such as thermal
fluctuations, sound, vibration, and so forth. However, the presence
of defects such as missing or damaged nanonails, nail heads or
delaminated coating or the presence of a hydrophilic particle
stuck in between nanonails seem to be capable of substantially
reducing the interface stability. Our experimental observations
are consistent with this notion.
Switching of the surfaces from the superlyophobic state to the
hydrophilic state is achieved by applying a voltage between the
liquid and conductive core of the nanonail or honeycomb structure.
This procedure is very similar to the one employed to control
previously demonstrated tunable superhydrophobic nanostruc-
tured surfaces.11,14 Because the liquid is electrically isolated from
the conductive core of the structures by the silicon oxide overhang,
no electrical current develops upon voltage application. However,
because of the close spatial proximity, a substantial electrical
field can develop in the space between the conductive cores and
the liquid. The resulting electrostatic attraction can force the
liquid to wet the underside of the overhang, thus inducing a
wetting transition. The exact details of this mechanism require
further investigation. However, the order of magnitude of the
field-induced stress p acting on the liquid-air interface can be
estimated by dividing the electrostatic energy stored in the space
between the liquid and the tip of each conductive core by their
characteristic separation distance s. Assuming that the capacitance
of the core-liquid interface scales as 1/s, we arrive at the following
estimate: p � (1/2)��0s-2V-2, where V is the applied voltage, �0
is the vacuum permittivity, and � is the effective permittivity of
the medium in the gap. By substituting characteristic value of
separation s � 102 nm and V � 101 V, we obtain p � 104 Pa,
which is comparable to the pressure-stability threshold estimated
previously from the shape of the liquid-air interface. Thus, at
least in principle, direct electrostatic attraction can be sufficient
to induce superlyophobic-to-hydrophilic transitions in our
structures. The above estimate also indicates that the switching
voltage is growing rapidly as the characteristic size of the overhang
increases. In particular, it is desirable to have the overhang be
a submicrometer size in order to keep the switching voltage
within the 101-102 V range.
To investigate electrically induced transitions quantitatively,
the apparent contact angle of various liquids was measured as
a function of applied voltage. Typical results are shown in Figure
5a-c. As one can see from Figure 5a,b, there is a sharp transition
from the superlyophobic state to the hydrophilic state for both
nanonail and honeycomb substrates, which is achieved at voltages
between approximately 40 and 90 V, depending on the liquid
and the substrate. The dependence of the cosine of the contact
angle on the square of the applied voltage is shown in Figure
5c using a honeycomb substrate as an example. As expected,
liquids with lower surface tension demonstrate smaller contact
angles both before and after the transition. Transition voltages,
however, do not appear to follow a simple trend. They seem to
differ little for butanol, octanol, and cyclopentanol, despite
substantial differences in the surface tensions of those liquids.
At the same time, the transition voltage for ethanol appears to
be substantially higher, despite the fact that it has the lowest
surface tension in the group. Additional investigation of the
dependence of the transition voltages on the surface tension,
hydrostatic pressure, temeperature, and chemical nature of the
liquids is currently underway and will be published elsewhere.
Conclusions
The dynamically tunable surfaces that were produced require
further improvement before they become capable of covering
the whole possible range from truly superlyophobic behavior to
completely superhydrophilic behavior. However, even in their
present form, the exceptional tunablity range exhibited by these
surfaces makes them unique. We believe that the results
demonstrated in this work strongly support the idea of using 3D
topography to lock the liquid in a dynamically controllable
metastable state. This approach dramatically extends the options
Letters Langmuir, Vol. 24, No. 1, 2008 13