© 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
In a typical water condensation, there
are three sequential steps: nucleation,
growth of individual water droplet, and
droplet coalescence. 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. 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
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.
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.
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,
tilting angle of substrate,
water is the water surface tension,
A are water receding and advancing contact angles on
the substrate, respectively, and W is the droplet width. 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 m−1 K−1 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. 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. 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, 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.” 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.
contact angle [
contact angle [
rate [μm s−1]
growth rate [μm s−1]
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. 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.
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 (
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
Interestingly, the calculated growth rate of condensed water
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
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
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. 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 × 10−3 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 s−1,
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 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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