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Atmospheric Water Harvesting: Role of Surface Wettability and Edge Effect



Atmospheric water is emerging as an important potable water source. The present work experimentally and theoretically investigates water condensation and collection on flat surfaces with contrasting contact angles and contact angle hysteresis (CAH) to elucidate their roles on water mass collection efficiency. The experimental results indicate that a hydrophilic surface promotes nucleation and individual droplets growth, and a surface with a low CAH tends to let a smaller droplet to slide down, but the overall water mass collection efficiency is independent of both surface contact angle and CAH. The experimental results agree well with our theoretical calculations. During water condensation, a balance has to be struck between single droplet growth and droplet density on a surface so as to maintain a constant water droplet surface coverage ratio, which renders the role of both surface wettability and hysteresis insignificant to the ultimate water mass collection. Moreover, water droplets on the edges of a surface grow much faster than those on the non-edge areas and thus dominate the contribution to the water mass collection by the entire surface, directly pointing out the very important role of edge effect on water condensation and collection.
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© 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1700019 (1 of 7)
Atmospheric Water Harvesting: Role of Surface Wettability
and Edge Effect
Yong Jin, Lianbin Zhang,* and Peng Wang*
Y. Jin, Prof. P. Wang
Water Desalination and Reuse Center
Division of Biological and Environmental Science and Engineering
King Abdullah University of Science and Technology
Thuwal 23955-6900, Saudi Arabia
Prof. L. Zhang
Key Laboratory of Materials Chemistry for Energy Conversion
and Storage (HUST) of Ministry of Education
School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan 430074, China
DOI: 10.1002/gch2.201700019
In a typical water condensation, there
are three sequential steps: nucleation,
growth of individual water droplet, and
droplet coalescence.[2] Nucleation is an ini-
tial step in forming water liquid from water
vapor. There are two types of water nuclea-
tion: homogeneous and heterogeneous
nucleation, with the former occurring in
the absence of any foreign substrate while
the latter occurring on foreign substrate
with a temperature lower than vapor satu-
ration temperature. Heterogeneous nuclea-
tion is a dominant mechanism in forming
atmospheric water liquid under ambient
conditions, and it is affected by parameters
such as substrate temperature, vapor pres-
sure, vapor temperature, and wettability
of substrate. Generally, when the state
of substrate and vapor is set, water vapor
nucleates faster on a hydrophilic substrate
than on a hydrophobic one due to its lower
energy barrier.[3] Following the nucleation
is growth of an individual water droplet,
and in this step, water vapor condenses on the preformed
droplet surfaces and the mass of individual liquid droplet grows
with time. When individual droplets grow larger, they tend to
coalesce to minimize the total surface energy. Two important
characteristics of the droplet coalescence stage are linear growth
of droplet size on flat surfaces and constant water droplet sur-
face coverage ratio, which is defined as ratio of the projected
area of the droplets to the total substrate surface area.[2a,4] Both
the growth of individual water droplet and droplet coalescence
can be greatly influenced by aerodynamic parameters[2a,5] such
as ambient air velocity.
Typically observed in our daily life is an edge effect that leads
to faster water condensation and visually bigger water droplets
on the substrate surface region near its edges than its central
surface region.[4b] The edge effect is majorly due to special
local aerodynamic conditions at the edges where higher air
velocity can be expected, which facilitates growth rate of water
Once a coalesced droplet reaches a certain critical size,
its gravity overcomes its retention force, the droplet would
start to move downward along the surface, and the substrate
surface is thus renewed for another water condensation cycle,
a process known as substrate renewal or water droplet removal.
Equation (1) determines the critical size of a water droplet that
would initiate droplet downward movement on a flat substrate
Wsin(coscos )
αγ θθ
Atmospheric water is emerging as an important potable water source. The
present work experimentally and theoretically investigates water conden-
sation and collection on flat surfaces with contrasting contact angles and
contact angle hysteresis (CAH) to elucidate their roles on water mass collec-
tion efficiency. The experimental results indicate that a hydrophilic surface
promotes nucleation and individual droplets growth, and a surface with a low
CAH tends to let a smaller droplet to slide down, but the overall water mass
collection efficiency is independent of both surface contact angle and CAH.
The experimental results agree well with our theoretical calculations. During
water condensation, a balance has to be struck between single droplet growth
and droplet density on a surface so as to maintain a constant water droplet
surface coverage ratio, which renders the role of both surface wettability
and hysteresis insignificant to the ultimate water mass collection. Moreover,
water droplets on the edges of a surface grow much faster than those on the
non-edge areas and thus dominate the contribution to the water mass collec-
tion by the entire surface, directly pointing out the very important role of edge
effect on water condensation and collection.
Water Harvesting
1. Introduction
Atmospheric moisture is abundantly present in our ambient
air and is emerging as an important source of potable water,
especially in areas with little rain but relatively high humidity.[1]
A good understanding of water condensation, a phase change
process in which water vapor is transformed to liquid water
and which involves both heat and mass transfers, is a key to an
effective atmospheric water harvesting.
© 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA,
Weinheim. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.
Global Challenges 2017, 1, 1700019
1700019 (2 of 7) © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
where mg is the gravitational force from water mass,
is the
tilting angle of substrate,
water is the water surface tension,
R and
A are water receding and advancing contact angles on
the substrate, respectively, and W is the droplet width.[7] Thus,
a smaller contact angle hysteresis (CAH), which is defined as
the difference between advancing and receding contact angles
R), means a smaller critical droplet size. In order
to have a high substrate renewal rate, a substrate with a low
CAH is preferred, assuming other conditions being constant.
A higher renewal rate in turn surely leads to a higher availa-
bility of substrate surface for water condensation and a quicker
turnover of the surface. The above discussions are seemingly
leading us to believe that a lower CAH would, in the end, result
in a higher water mass collection rate, which is the ultimate
goal of any practical atmospheric water harvesting. One major
objective of this work is to prove that this seemingly reasonable
argument is actually false.
In parallel with mass transfer, heat transfer is another
important factor in water condensation as considerable amount
of latent heat is released therein and the heat is transferred
from the condensing water droplets to the substrate. Given
the fact that water is a poor heat conductor with a heat con-
ductivity of 0.6 W m1 K1 at 20 °C, heat transfer is generally
the limiting factor in promoting water condensation and collec-
tion efficiency. It has been reported that dropwise condensation
tends to have a higher heat transfer efficiency than filmwise
condensation due to the heat barrier effect of water.[8] Given the
inherent complexity of heat transfer process, the research pro-
gress in heat transfer is lagging behind that of mass transfer
in the general field of water condensation.[9] However, recent
and ongoing advancement in special wettability/morphology/
composition surface preparation is providing scientists with
unprecedented opportunities to better understand the heat
transfer in the water condensation process.[9,10] Connecting
mass transfer with heat transfer, it is expected that a fast water
droplet removal surface guarantees a high heat transfer effi-
ciency. For example, it has been reported that slippery surfaces
with extremely low CAH enable fast removal of water droplet
once condensed, which in turn leads to enhanced heat transfer
With a clear aim to enhance water mass collection efficiency,
a great amount of research efforts have been made by scientists
to investigate the water condensation and subsequent collection
processes,[1,5] but there are certain fundamental aspects yet to be
fully understood. First, fast water removal surface can be some-
times intuitively and wrongly taken for granted to lead to high
water mass collection. Second, detailed studies on dynamics of
water droplet growth and the kinetics of water mass collection
are rare, which, if present, would provide valuable insights and
thus considerably advance the current understanding of these
processes. The aim of this work is, by carefully designed and
prepared surfaces with different wettability and CAH, real-time
recorded droplet growth dynamics, continuously monitored
water mass collection, and thoroughly conducted theoretical
calculations, to provide convincing evidences to prove the insig-
nificant role of surface wettability and more importantly CAH
on water mass connection efficiency. The results of this work
highlight the often overlooked but critical effect of the surface
edge regions of a substrate on the growth of individual water
droplets and thus overall water mass collection efficiency and
thus show light on meaningful means of promoting atmos-
pheric water harvesting efficiency for practical applications.
2. Results and Discussion
The present work focuses on water condensation on flat sur-
faces that are in contrast to surfaces with roughness. It is known
that rough surfaces with proper surface chemistry assisted
with micro/nanostructure geometry could exhibit unique and
in many cases extreme wettability,[12] but water condensation
on such surfaces is always considerably more complicated
than on their flat surface counterparts,[10b,d,13] with too many
parameters coming to play. The scientific community is still at
an early stage of its learning curve in fully grasping water con-
densation on rough surfaces. In contrast, water condensation
on flat surfaces grows droplets in well-known patterns referred
to as “breath figure.”[14] Thus, flat surfaces were rationally
selected over rough ones in light of the goal of the project. We
also believe that by choosing flat surfaces, the uncertainty in
explaining the experimental results is considerably reduced and
the manageability of the project is greatly raised. It is believed
that the conclusions based on flat surfaces provide fundamental
aspects to the same processes on rough surfaces and thus have
trustworthy applicability to rough surfaces.
In the present work, three flat surfaces (namely, polydi-
methylsiloxane (PDMS)), octadecyltrichlorosilane (ODTS), and
hydrophilic respectively) with contrasting contact angle and
CAH were prepared. Table 1 presents the static contact angles,
advancing contact angle, and receding contact angles of water
on three different surfaces, while Figure 1 shows digital photos
of water droplets during the contact angle measurements. As
can be seen, the dimethyldimethoxysilane (DMS)-modified
surface presents a moderate hydrophilicity (
= 72.0°) and a
relatively high CAH (16.2°). The hydrophobic surfaces modi-
fied with PDMS and OTDS show similar static contact angles
(101.6° and 107.0°, respectively), but CAH for the two surfaces
differs from each other significantly (4.8° vs 19.8°). The low
Global Challenges 2017, 1, 1700019
Table 1. Summary of static and dynamic contact angle measurements, droplet growth rates, predicted droplet sliding diameter, and predicted and
measured sliding time of all three surfaces.
Static contact
angle [
contact angle [
contact angle [
Predicted sliding
diameter [mm]
Edge growth
rate [μm s1]
Nonedge area
growth rate [μm s1]
Predicted sliding
time [s]
sliding time [s]
PDMS 101.6 ± 1.0 103.0 ± 0.3 98.2 ± 0.4 4.8 1.34 0. 868 0.316 1544 1508 ± 251
Hydrophilic 72.0 ± 2.5 79.0 ± 0.9 62.8 ± 3.0 16.2 3.17 1.331 0.525 2382 2292 ± 429
ODTS 107.0 ± 1.1 113.3 ± 0.6 93.5 ± 1.4 19.8 2.34 0.719 0.270 3255 3314 ± 672
1700019 (3 of 7) © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
CAH of the PDMS-modified surface arises from the liquid-like
property of the surface PDMS groups as the silicon–oxygen
bond can easily rotate, while the high CAH of the ODTS-
modified surface is from the amorphous or crystal-like prop-
erty of the surface ODTS groups.[15] Moreover, the atomic
force microscopy (AFM) images show that roughness of all
the surfaces prepared in this study is in sub-nanometer range,
indicating the flatness of the surfaces (Figure S1, Supporting
There exist a few reports on low CAH flat surfaces, one of
which is so-called SLIPS, which is inspired by the pitcher plant
with lubricating oil infused into its surface nanostructures.[16]
There have been reports utilizing the SLIPS surfaces for water
condensation and collection,[5,11,17] but a disappointing result is
that the water collected by such a surface was found to be a mix-
ture of water and lubricating oil,[1b] which challenges the appli-
cation stability of the SILPS. Water condensation experiments
were conducted in a homemade, temperature- and humidity-
controlled chamber. In the chamber was a vertically placed,
constant temperate cooling stage to which the modified silicon
wafer substrate was attached. There is noteworthy characteristic
of the substrate attachment onto the cooling stage: the left
and right sides of the silicon wafer substrate were positioned
within the cooling stage while the upper and lower sides being
extended beyond the cooling stage. The special positioning of
the silicon wafer substrate on the cooling stage helps reduce
water condensation on the side cross-sections of the left and
right edge sides of the substrate, and the condensed water
droplets therein are not collected on purpose by design. More-
over, the side cross-sectional surface of the substrate’s upper
edge side is intentionally cut so that condensed water there
would not fall off from the front face of the substrate (Figure S2,
Supporting Information) and thus will not be counted and
the water condensed on the bottom edge side’s cross-section
would be drained and diverted away from the water collection
vessel. This way, the edge effect from the bottom edge, left and
right edge sides are all delicately eliminated so to fully focus
our investigation on the upper edge of the substrate. All these
purposeful designs are to make sure that only water condensed
on the front surface of the modified silicon wafer are collected
for the purpose of quantifying water collection efficiency by
these special wettability surfaces. The growth dynamics of
water droplets on the substrate surfaces are monitored by a
Figure 2 shows the images of water droplets growing on the
different surfaces at the end of the first 100, 200, 300, 400, and
500 s. To facilitate discussion, the term “edge” thereafter refers
to the linear edges of the front surface of modified silicon sub-
strates. Some direct and major observations are as follows: (1)
The droplets on the upper edges are always larger than those
in the nonedge and central surface regions, regardless of sur-
face wettability, which is a good proof of the edge effect on
water condensation. (2) The droplets grow larger with time
regardless of the locations on the surfaces (edge vs nonedge
areas), the wettability (hydrophilic vs hydrophobic), and CAH
Global Challenges 2017, 1, 1700019
Figure 1. Digital images of the droplets on three surfaces during contact
angle measurement (
A, and
R are on the left, in the middle, and on
the right, respectively).
Figure 2. Images showing the growth of droplets on the edge and nonedge areas of the PDMS- and ODTS-modified hydrophobic surfaces, along with
the hydrophilic surface at the end of first 100, 200, 300, 400, and 500 s.
1700019 (4 of 7) © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
of the surfaces. (3) The droplet size on the hydrophilic surface
is always bigger than those on the hydrophobic surfaces (i.e.,
PDMS- and ODTS-modified surfaces) at any time point. (4) For
all of the surfaces, condensation follows dropwise condensa-
tion, indicated by well-separated individual droplets on the sur-
faces at any time (100 to 500 s).
In order to quantitatively describe the evolution of conden-
sation on the substrates, the growth dynamics on the edges
and in the nonedge areas are carefully examined. Figure 3a,b
presents the observed droplet sizes as a function of time
on both the edge and nonedge areas, respectively, based
on which the following three major observations are made.
(1) It is clear that the droplet growth on all surfaces follows
a linear relationship with time, which is in a good agreement
with theory prediction and implies that the observation period
(100–500 s) is within the droplet coalescence stage.[2a,4b] The
linearly fitted growth rates are presented in Table 1. (2) It is
also clear that the droplets grow much faster in size on the
hydrophilic than on the PDMS- and ODTS-modified hydro-
phobic surfaces on both edge and nonedge areas, while they
grow similarly in size on the two hydrophobic surfaces,
suggesting that hydrophilicity facilitates individual droplet
growth. (3) More interestingly, the droplet growth rates on
the edges for PDMS, ODTS, and hydrophilic are 2.7, 2.7,
and 2.5 times faster than those on the nonedge areas of the
same surfaces, which is a direct and very significant proof of
the effect of edge on significantly facilitated water condensa-
tion and also indirectly points to the reliability and stability of
our measurement system.
The water droplet surface coverage ratio (including edge and
nonedge areas), which was defined previously, on the surfaces
is then obtained based on the images from the recorded droplet
growth dynamics following the literature method [2a,4b] and is
presented as a function of time in Figure 3c.
The water coverage ratios remain relatively constant after
a period of time (i.e., 200 s and beyond) and are 58%, 50%,
and 50% for the hydrophilic, the PDMS-, and POTS-modified
surfaces, respectively, indicating that water droplets cover larger
area on a hydrophilic surface than on hydrophobic ones and
that the water cover ratio is irrelevant to CAH.
As droplets continuously grow in size, there would be a
droplet-sliding process at one point. As the droplets on the
edges grow much faster than those in the nonedge areas,
the droplets always slide down from the edge to the bottom.
The average sliding time for the hydrophilic, the PDMS-, and
ODTS-modified surfaces is determined to be 2291, 1507, and
3314 s, respectively (Figure 3d).
After addressing two very important parameters in water
condensation, droplet growth rate and droplet surface coverage
ratio, we then turn to address some theoretical aspects of the
process mathematically. Even though droplets condensing on
the substrate are never uniform in size in reality, an assumed
average size predicted from the droplet growth rate can still be
used with reasonable accuracy in estimation when the variation
in droplet size is not very wide, which is the case in this work
(Figure 3a,b). Since the average droplet size could be predicted
based on droplet growth rate, and droplet density (N, number
of droplets per unit surface area) could be calculated by droplet
Global Challenges 2017, 1, 1700019
Figure 3. The observed droplet size as a function of time on a) the edge and b) the nonedge areas of the three surfaces; c) water droplet surface cov-
erage ratios; d) measured average sliding time for the three surfaces. (Note: measurement of nonedge droplets started from 200 s because nonedge
droplets in 100 s are too small).
1700019 (5 of 7) © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
surface coverage and droplet size, the total volume or mass of
water condensed per unit surface area may thus be predicted
based on the droplet growth rate and droplet surface coverage,
both of which are known to us by now (Figure 3a–c).
In more details, given the difference in surface wettability
and thus in the geometry of the droplets on the surface, the
hydrophobic and hydrophilic surfaces are treated differently.
Assuming that a single water droplet is a partial sphere, the
droplet sizes observed by the camera images for the hydro-
phobic surfaces are indeed the real droplet diameter (Ro of
the left droplet in Figure 4a), while the observed droplet sizes
by the camera images for the hydrophilic surface are actually
the width of the droplets (Ro of the right droplet in Figure 4a).
Moreover, since the droplets are continuously growing in size
during the condensation, advancing contact angle, along with
the average droplet size (Ro), is then used to calculate the
volume of single droplet (Vsingle) (Figure 4a). With Vsingle known,
droplet density (N) could be calculated as the droplet surface
coverage (Figure 3c) divided by the average droplet size (Ro). At
last, the total water volume per unit substrate surface area (
is calculated by Equation (2)
The detailed mathematical calculations can be found in the
Supporting Information.
Interestingly, the calculated growth rate of condensed water
volume V
per unit area of both the edge and nonedge areas
shows relatively independence on the surface wettability and
CAH of these surfaces. However, V
d for the edges of PDMS-,
ODTS-modified, and the hydrophilic surfaces is constantly 2.7,
2.7, and 2.5 times than that for the nonedge areas of the same
surfaces, consistent with the water droplet growth rates as
previously reported (Figure 3a,b). This discovery worrisomely
implies that water collection rate might be the same for sur-
faces with different contact angles and thus wettability.
As presented previously, hysteresis affects droplet-sliding
time on these surfaces. The critical size (D) of a droplet, beyond
which the droplet moves on a vertically placed surface under
an influence by gravity (as shown in Figure 4b), is theoretically
determined by Equation (3)
26sin (cos cos)
(1 cos)(2 co
×××− ×+ (3)
is the density of droplet and g is the gravity constant.
The predicted critical droplet sizes, determined from the
camera images, are 3.17, 1.34, and 2.34 mm for the hydro-
philic, PDMS-, and OTDS-modified hydrophobic surfaces,
respectively (Table 1), indicating that smaller CAH allows for
smaller water droplets to move on surface. As the water drop-
lets grow faster and bigger on the edges than on the nonedge
areas and the critical droplet size for sliding (D) is uniformly
the same across the entire surface irrespective of edge and
nonedge areas, it is thus always the droplets grown on the
edge area that slide down first from the top edge of the sub-
strate surface and sweep away the droplets present in their
paths of downward movement on the surface. Based on this,
the sliding time, the time point when the droplets initiate their
movement, of the droplets at the edge area is then calculated
to be 2260, 1540, and 2840 s for the hydrophilic, PDMS-, and
ODTS-modified surfaces, respectively. The calculated sliding
time matches well with the measured ones for all three sur-
faces, which again indirectly confirms the reliability and sta-
bility of our condensation system. The results seem to suggest
that the droplets grow only by the rate measured at the first
500 s and the high hysteresis does not slow down the droplet
growth rate in a later stage. In other words, our results con-
clude that the droplet growth rates are independent of the sur-
face CAH in this work.
However, this does not lead to a general conclusion that hys-
teresis would not affect droplet growth rate at all. As a matter
of fact, with a considerably high hysteresis of a surface, we are
uncertain whether a different scenario would be happening
given the possibility that heat transfer may play a bigger role
there as the high hysteresis would facilitate heat transfer to
substrate. Nevertheless, our results clearly indicate that the
heat transfer factor, which is related to surface CAH, is not a
significant player in water condensation under the conditions
employed in this work.
Figure 5 presents the time course of the measured water
mass collected. There is an initial period where the rate is
not stable while the first sliding cycle is taking place, and as
time goes on, stable rates are achieved on all surfaces. Inter-
estingly and somewhat as expected, after
an initial stabilization period, water mass
collected increases linearly with time with
the slopes for all three surfaces being sta-
tistically the same (the inset in Figure 5).
Thus, our result sadly indicates that sur-
faces with different surface wettability and
hysteresis tend to have the same water
mass collection rate, which agrees well with
the theoretical calculations in this work.
However, it is worth pointing out that, for
a surface with a lower hysteresis, it allows
for a smaller droplet to slide down and thus
leads to a faster surface turnover, and the
water mass collected at any time point is
always greater than that on a surface with a
higher hysteresis.
Global Challenges 2017, 1, 1700019
Figure 4. a) Schematic showing a single water droplet condensing on the hydrophobic (left)
and hydrophilic (right) substrates. b) Schematic showing sliding of a water droplet on a
vertically place substrate surface.
1700019 (6 of 7) © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3. Conclusion
In conclusion, surfaces with contrasting CA and CAH were
prepared and the condensation experiments were conducted on
these surfaces. It turned out that water droplets on the edges
always grew faster than those in the nonedge area. However,
both the theoretical calculations and experimental observations
conducted in this work showed that the total water mass collec-
tion rate by the different surfaces was comparable, independent
of surface wettability and hysteresis. During water condensa-
tion, a balance has to be struck between single droplet growth
and droplet density on a surface so as to maintain a constant
water droplet surface coverage ratio, which, we believe, renders
the role of both wettability and hysteresis insignificant. The
results of this work unequivocally point out the importance of
increasing fraction of edge or pseudo-edge structures which
would promote locally favorable aerodynamics and rough struc-
tures that support removal of condensed droplets to increase
effective condensation area on water condensation surfaces.
For example, surfaces mimicking the cactus would be highly
4. Experimental Section
Materials: DMS (95%), sulfuric acid (95–98%), and ODTS (98%) were
purchased from Sigma-Aldrich. 2-Isopropanol (electrochemical grade) was
purchased from Fisher Scientific. Ethanol absolute was purchased from
VWR International. Deionized (DI) water was used in all of the experiments.
Preparation of Surfaces with Different Contact Angles and CAH: Polished
silicon wafer of 54 mm × 50 mm × 0.625 mm was degreased with
ethanol by sonication and then treated by oxygen plasma. Hydrophobic
surface with low CAH, denoted as PDMS-modified surface, was prepared
on the polished front side of the plasma-cleaned silicon wafer following
a literature method.[18] Briefly, an aliquot of 2.85 mL DMS, 31.80 mL
isopropanol, and 135 μL sulfuric acid were mixed together gently, and
the mixture solution was let to sit still for 30 min before use. The silicon
wafer was then immersed in the mixture solution for 10 s before being
taken out for drying in ambient air with a relative humidity of 60% and
a constant temperature at 21 °C for 30 min. After drying, the substrate
was washed with copious ethanol and DI water.
Hydrophobic surface with high CAH, denoted as ODTS-modified
surface, was prepared by immersing the plasma-cleaned silicon wafer in
5 × 103 m ODTS toluene solution for 10 min, followed by washing with
ethanol and water repeatedly.
Moderately hydrophilic surface, denoted as hydrophilic surface,
was prepared by chemical vapor deposition of DMS on the precleaned
silicon wafer under 70 °C for 1.5 h in a 100 mL container, followed by
heating at 80 °C for 1 h to evaporate loosely bound DMS.
Characterization of Surfaces: Static contact angles were measured with
a commercial contact angle system (OCA 35, DataPhysics, Filderstadt,
Germany) at ambient temperature using a 4 μL water droplet as a probe.
Advancing and receding contact angle measurements were conducted
by adding and withdrawing the probe water with a speed of 0.5 μL s1,
respectively. Each contact angle value reported was an average of four
individual measurements at different locations on the same surfaces.
AFM images were taken by Agilent 5500 SPM using tapping mode.
Water Condensation Experiments: Figure 6a is a schematic
showing the condensation experimental setup (images also shown
in Figure S3a in the Supporting Information). Briefly, condensation
experiment was conducted in a homemade humidity chamber with a
stable environmental temperature at 21 °C. The relative humidity inside
the chamber was maintained at 100% by continuous moisture supply
from a commercial ultrasonic humidifier.
In the humidity chamber was a vertically placed cooling stage to which
the modified silicon wafer substrate was attached. The cooling stage was
maintained at a constant temperature at 4 °C by circulating chilly water.
As presented in Figure 6b and Figure S3b (Supporting Information), the
left and right sides of the substrate were kept within the cooling stage
while the upper and down sides being extended out of the cooling stage.
Growth dynamics of water droplets on the substrate surfaces was real
time recorded by a microcamera (Dinocapture 2.0) connected with a PC,
Global Challenges 2017, 1, 1700019
Figure 5. Condensation water mass collection kinetics of the PDMS- and
ODTS-modified hydrophobic surfaces and the hydrophilic one. Note: the
box on the figure delineates the initial unstable zone, while the inset figure
shows the kinetics of the stable stages on the three surfaces.
Figure 6. Schematic of a) homemade water condensation and collection experimental setup, and b) special positioning of the modified silicon wafer
substrates attached on the cooling stage.
1700019 (7 of 7) © 2017 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Global Challenges 2017, 1, 1700019
and the mass of water collected by the substrate surfaces was monitored
by an electronic balance connected with a PC. The water droplets pinned
at the bottom of the substrate surfaces were drained away from the
condensed water collection container by a copper mesh covered with
cotton that was placed very close to the bottom of the substrate surface
but falling short of touching it.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
The authors are grateful to KAUST for very generous financial support.
Conflict of Interest
The authors declare no conflict of interest.
condensation, contact angle hysteresis, edge effect, water collection,
Received: March 13, 2017
Revised: May 14, 2017
Published online: June 23, 2017
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... The methods currently available such as conventional fog capture and dewing require either large energy inputs or high relative humidity (RH) which make their application in arid regions unfeasible. [4][5][6] Therefore, an ideal atmospheric water harvesting system should be able to collect water from dry air (low RH) with a low energy supply. ...
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The UiO-66 metal-organic framework (MOF) has been identified as a promising hydrophilic material for water harvesting. Recent studies show that its water uptake ability at low relative humidity (RH) can be improved by incorporating hydrophilic functional groups into the framework. In this work, we provide computational insights into the adsorption of water in UiO-66 and its functionalized derivatives to reveal the role played by different adsorption sites and functional groups in the adsorption mechanism. We started by developing molecular models for UiO-66, UiO-66-NH2, UiO-66-OH, and UiO-66-(OH)2 compatible with the MB-pol data-driven many-body potential of water. We then benchmarked these models against ab initio data. We used these models to perform molecular dynamics simulations and calculate radial distribution functions, IR spectra, and two-dimensional density distribution maps for water in the MOFs. These results consistently show that the μ3-OH sites are the preferential interaction sites for water in UiO-66 and all its variants, and the formation of localised water clusters inside the octahedral pores is responsible for the abrupt step in the experimental adsorption isotherms. Furthermore, the presence of functional groups in the framework allows water to cluster in the octahedral pores at lower RH, thus making the MOF a more efficient water harvester. Overall, this study provides molecular-level insights into the pore filling process of UiO-66 and its functionalized derivatives, which are needed for the design of efficient water harvesting materials based on MOFs.
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Humans face a severe shortage of fresh water due to economic growth, climate change, overpopulation, and overutilization. Atmospheric water harvesting (AWH) is a promising solution where clean water is collected from the air through various approaches, including dropwise condensation. However, designing surfaces that balance rapid condensation with efficient water removal is challenging. To address this issue, inspired by the efficient water collection mechanisms in the skin of cold-blooded tree frogs, we propose an eco-friendly approach to collect fresh water from cooled window glass. We fabricated various planar and TiO2 nanostructured surfaces including surfaces mimicking a lotus leaf and a hybrid surface mimicking a desert beetle and a cactus, with different wettability levels such as superhydrophilic, hydrophilic, hydrophobic, superhydrophobic, and biphilic. Sub-cooling of glass substrates between 5 and 15 °C using a Peltier device significantly enhanced the condensation process for all surfaces, with modest dependency on surface properties. This cooling temperature regime could be achieved by geothermal cooling methods that consume little energy. To improve visibility for window applications, we developed hydrophobic polymer nanofilm-modified glass substrates using a simple spin-coating technique, and achieved comparable water harvesting efficiency to that of nanostructured substrates. Our study provides insight into the optimal surface structures and cooling temperature for window glass AWH systems that could be used with an underground cooling system.
The UiO-66 metal-organic framework (MOF) has been identified as a promising hydrophilic material for water harvesting. Recent studies show that its water uptake ability at low relative humidity (RH) can...
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Harvesting water from air in sorption‐based devices is a promising solution to decentralized water production, aiming for providing potable water anywhere, anytime. This technology involves a series of coupled processes occurring at distinct length scales, ranging from nanometer to meter and even larger, including water sorption/desorption at the nanoscale, condensation at the mesoscale, device development at the macroscale and water scarcity assessment at the global scale. Comprehensive understanding and bespoke designs at every scale are thus needed to improve the water‐harvesting performance. For this purpose, a brief introduction of the global water crisis and its key characteristics is provided to clarify the impact potential and design criteria of water harvesters. Next the latest molecular‐level optimizations of sorbents for efficient moisture capture and release are discussed. Then, novel microstructuring of surfaces to enhance dropwise condensation, which is favorable for atmospheric water generation, is shown. After that, system‐level optimizations of sorbent‐assisted water harvesters to achieve high‐yield, energy‐efficient, and low‐cost water harvesting are highlighted. Finally, future directions toward practical sorption‐based atmospheric water harvesting are outlined.
Over the past decade, a new class of slippery, anti-adhesive surfaces known as slippery covalently-attached liquid surfaces (SCALS) has emerged, characterized by low values of contact angle hysteresis (CAH, less than 5°) with water and most solvents. Despite their nanoscale thickness (1 to 5 nm), SCALS exhibit behavior similar to lubricant-infused surfaces, including high droplet mobility and the ability to prevent icing, scaling, and fouling. To date, SCALS have primarily been obtained using grafted polydimethylsiloxane (PDMS), though there are also examples of polyethylene oxide (PEO), perfluorinated polyether (PFPE), and short-chain alkane SCALS. Importantly, the precise physico-chemical characteristics that enable ultra-low CAH are unknown, making rational design of these systems impossible. In this review, we conduct a quantitative and comparative analysis of reported values of CAH, molecular weight, grafting density, and layer thickness for a range of SCALS. We find that CAH does not scale monotonically with any reported parameter; instead, the CAH minimum is found at intermediate values. For PDMS, optimal behavior is observed at advancing contact angle of 106°, molecular weight between 2 and 10 kg mol-1, and grafting density of around 0.5 nm-2. CAH on SCALS is lowest for layers created from end-grafted chains and increases with the number of binding sites, and can generally be improved by increasing the chemical homogeneity of the surface through the capping of residual silanols. We review the existing literature on SCALS, including both synthetic and functional aspects of current preparative methods. The properties of reported SCALS are quantitatively analyzed, revealing trends in the existing data and highlighting areas for future experimental study.
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Life is dependent on water. However, in terms of the potential effects, water scarcity is quickly emerging as one of the most critical problems in the world. To access more fresh water for drinking, sanitation, and irrigation, water can be harvested from different forms of water on earth. Atmospheric harvesting is the best alternative for producing fresh water for everyday life and reducing global water shortages. To date, many modern technologies have been introduced for this application, with several prototypes being demonstrated. Thus, this study explores the potential benefits of the current atmospheric water harvesting systems in terms of their modes, atmospheric conditions, and production rate and examines the key factors that affect the efficiency of atmospheric water harvesting, such as temperature and humidity. According to the studies, there has been a significant advancement in energy harvesting and conversion technology, along with atmospheric water harvesting, over the past few years, including new mechanisms and technical paths. However, there are still many obstacles; in particular, most of the technologies depend on outdoor conditions. In order to overcome this issue, new directions need to be investigated. Here, we discuss the principles, advantages, limitations, and potential applications of these technologies.
The rapid increase in the global population and the reckless behavior of depleting potable drinking water have accumulated into a colossal crisis in previous decades. Some four billion people (i.e., two-thirds of the global population) worldwide face low-to-high water stress. Ensuring access to safe drinking water remains a universal challenge and is now formally recognized as an international development priority by the United Nations framework. In this connection, along with other parallel technologies, Atmospheric Water Harvesting (AWH) is emerging as an effective methods means to overcome the water scarcity in arid regions, especially inland areas lacking liquid water sources. And, Beyond the conventional system engineering that improves the water yield, novel moisture-harvesting materials provide new aspects to promote the AWH technology—benefiting from their high tunability and processability. Innovative material and structural designs at micro/and nanoscale facilitate the water harvesters with desirable features; such as high-water uptake, facile water collection, and long-term recyclability, thus, boosting the rapid development of next-generation atmospheric water generators. In principle, AWH technologies could be classified into three categories; condensation, sorption, and hybrid. This chapter summarizes the water harvesting technologies from perspectives of surface design, material choice, test setups, performance analysis, and significant findings.
Water treatment consumes lots of energy from fossil fuels nowadays, and the emission of CO2 enhances the temperature on earth, resulting in more and more hazards. Thus, clean water production enabled by green energy without CO2 emission is attracting more and more attention. Herein, we propose a novel solar evaporation system achieving both solar evaporation and water storage with two different unique hydrogels based on a three-dimensional (3D) printing technique. The hydrogel absorber demonstrates an ultrahigh absorptance (98.2%) of solar light, while the water-storage hydrogel absorbs more than 100 times its own weight of water, demonstrating super water-storage performance with strong self-adhesiveness. The solar vapor generation rate can be as high as 3.14 kg·m-2·h-1, with a solar evaporation efficiency up to 91.2% irradiated by 1.43 sun. Furthermore, our environmentally friendly solar evaporation system achieves ultrahigh water purification efficiency of 99.99% for salt, heavy ions, and acid/alkaline with remarkable stability and durability. Our solar evaporation system promises long-lasting applications for the hydrological cycle enabled by solar energy, such as seawater desalination, sterilization, wastewater purification, and so on.
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Dropwise condensation can be enhanced by superhydrophobic surfaces on which the condensate drops spontaneously jump upon coalescence. However, the self-propelled jumping in prior reports is mostly perpendicular to the substrate. Here, we propose a substrate design with regularly spaced micropillars. Coalescence on the sidewalls of the micropillars leads to self-propelled jumping in a direction nearly orthogonal to the pillars and therefore parallel to the substrate. This in-plane motion in turn produces sweeping removal of multiple neighboring drops. The spontaneous sweeping mechanism may greatly enhance dropwise condensation in a self-sustained manner.
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Water vapor condensation is commonly observed in nature and routinely used as an effective means of transferring heat, with dropwise condensation on non-wetting surfaces exhibiting heat transfer improvement compared to filmwise condensation on wetting surfaces. However, state-of-the-art techniques to promote dropwise condensation rely on functional hydrophobic coatings which either have challenges with chemical stability or are so thick that any potential heat transfer improvement is negated due to the added thermal resistance of the coating. In this work, we show the effectiveness of ultra-thin scalable chemical vapor deposited (CVD) graphene coatings to promote dropwise condensation while offering robust chemical stability and maintaining low thermal resistance. Heat transfer enhancements of 4x were demonstrated compared to filmwise condensation, and the robustness of these CVD coatings was superior to typical hydrophobic monolayer coatings. Our results indicate that graphene is a promising surface coating to promote dropwise condensation of water in industrial conditions, with the potential for scalable application via CVD.
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Bumps are omnipresent from human skin to the geological structures on planets, which offer distinct advantages in numerous phenomena including structural color, drag reduction, and extreme wettability. Although the topographical parameters of bumps such as radius of curvature of convex regions significantly influence various phenomena including anti-reflective structures and contact time of impacting droplets, the effect of the detailed bump topography on growth and transport of condensates have not been clearly understood. Inspired by the millimetric bumps of the Namib Desert beetle, here we report the identified role of radius of curvature and width of bumps with homogeneous surface wettability in growth rate, coalescence and transport of water droplets. Further rational design of asymmetric convex topography and synergetic combination with slippery coating simultaneously enable self-transport, leading to unseen five-fold higher growth rate and an order of magnitude faster shedding time of droplets compared to superhydrophobic surfaces. We envision that our fundamental understanding and innovative design of bumps can be applied to lead enhanced performance in various phase change applications including water harvesting.
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In this study we investigate the effect of geometrical or thermal discontinuities on the growth of water droplets condensing on a cooled substrate. Edges, corners, and cooled and noncooled boundaries can have a strong effect on the vapor concentration profile and mass diffusion around the drops. In comparison to growth in a pattern where droplets have to compete to catch vapor, which results in a linear water concentration profile directed perpendicularly to the substrate, droplets near discontinuities can get more vapor (outer edges, corners), resulting in faster growth or less vapor (inner edges), giving lower growth.When the cooling heat flux limits growth instead of mass diffusion (substrate with low thermal conductivity, strong heat exchange with air), edge effects can be canceled. In certain cases, growth enhancement can reach nearly 500% on edges or corners.
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Vapor condensation plays a key role in a wide range of industrial applications including power generation, thermal management, water harvesting and desalination. Fast droplet nucleation, efficient droplet departure as well as low interfacial thermal resistance are important factors that determine the thermal performances of condensation; however, these properties have conflicting requirements on structural roughness and surface chemistry of the condensing surface or condensation modes (e.g., filmwise vs. dropwise). Despite intensive efforts over the past few decades, almost all studies focused on the dropwise condensation enabled by superhydrophobic surfaces. In this work, we report the development of a bio-inspired hybrid surface with high wetting contrast that allows for a seamless integration of filmwise and dropwise condensation modes. We show that the synergistic cooperation in the observed recurrent condensation modes leads to the improvements in all aspects of heat transfer properties including droplet nucleation density, growth rate, self-removal, as well as overall heat transfer coefficient. Moreover, we propose an analytical model to optimize the surface morphological features for dramatic heat transfer enhancement.
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When condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump due to the conversion of excess surface energy into kinetic energy. This phenomenon has been shown to enhance condensation heat transfer by up to 30% compared to state-of-the-art dropwise condensing surfaces. However, after the droplets jump away from the surface, the existence of the vapor flow towards the condensing surface increases the drag on the jumping droplets, which can lead to complete droplet reversal and return to the surface. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface that impede heat transfer until they can be either removed by jumping again or finally shedding via gravity. By characterizing individual droplet trajectories during condensation on superhydrophobic nanostructured copper oxide surfaces, we show that this vapor flow entrainment dominates droplet motion for droplets smaller than R ≈ 30 µm at moderate heat fluxes (q" > 2 W/cm^2). Subsequently, we demonstrate electric-field-enhanced (EFE) condensation, whereby an externally applied electric field prevents jumping droplet return. This concept leverages our recent insight that these droplets gain a net positive charge due to charge separation of the electric double layer at the hydrophobic coating. As a result, with scalable superhydrophobic CuO surfaces, we experimentally demonstrated a 50% higher overall condensation heat transfer coefficient compared to that on a jumping-droplet surface with no applied field for low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement, but also offers avenues for improving the performance of self-cleaning and anti-icing surfaces as well as thermal diodes.
Liquid–vapour phase change is a useful and efficient process to transfer energy in nature, as well as in numerous domestic and industrial applications. Relatively recent advances in altering surface chemistry, and in the formation of micro- and nanoscale features on surfaces, have led to exciting improvements in liquid–vapour phase-change performance and better understanding of the underlying science. In this Review, we present an overview of the surface, thermal and material science to illustrate how new materials and designs can improve boiling and condensation. There are many parallels between boiling and condensation, such as nucleation of a phase and its departure from a surface; however, the particular set of challenges associated with each phenomenon results in different material designs used in different manners. We also discuss alternative techniques, such as introducing heterogeneous surface chemistry or direct real-time manipulation of the phase-change process, which can offer further control of heat-transfer processes. Finally, long-term robustness is essential to ensure reliability and feasibility but remains a key challenge.
On a hierarchical polymer surface consisting of microposts and nanopillar arrays, water droplets are nucleated and grown selectively in the grooves between the microposts as the vapor pressure increases, whereas water droplets are randomly nucleated on a flat surface and surfaces consisting of microposts or nanopillars only.
Recent strategies to prepare "omniphobic" surfaces have demonstrated that minimizing contact angle hysteresis (CAH) is the key criterion for effectiveness. CAH is affected by chemistry and topography defects at the molecular and higher levels, thus most surfaces exhibit significant CAH. Preparative methods for stable coatings on smooth substrates with negligible CAH (<2°) for a broad range of liquids have not been reported. In this work, we describe a simple and rapid procedure to prepare omniphobic surfaces that are stable under pressure and durable at elevated temperatures. Consistent with theory, they exhibit sliding angles that decrease with liquid surface tension. Slippery omniphobic covalently attached liquid (SOCAL) surfaces are obtained through acid-catalyzed graft polycondensation of dimethyldimethoxysilane. The smooth, stable, and temperature-resistant coatings show extremely low CAH (≤1°) and low sliding angles for liquids that span surface tensions from 78.2 to 18.4 mN m(-1) .
Hydrophobic PVDF-HFP nano-webs were fabricated by a facile electrospinning method and proposed for harvesting fog from the atmosphere. A strong adhesive force between the surface and water droplet has been observed which resist the water shedding from the surface. The water droplets on the inhomogeneous nanomats showed high contact angle hysteresis. Impregnation of nanomats with lubricants (Total Quartz Oil and Krytox 1506) decreased the contact angle hysteresis and hence improved roll off of water droplets on the nanomats surface. It was found that water droplets of 5 µl size (dia.=2.1 mm) and larger roll down on an oil impregnated surface, held vertically, compared to 38 µl (dia.=4.2 mm) on plain nano-web. Contact angle hysteresis reduced from ~95° to ~23° with the Krytox 1506 impregnation.