Tunable Water Harvesting Surfaces Consisting of Biphilic Nanoscale

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Tunable Water Harvesting Surfaces Consisting of Biphilic Nanoscale Topography Youmin Hou, YUHE SHANG, Miao Yu, Chenxi Feng, Hongyu Yu, and Shuhuai Yao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05163 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Tunable Water Harvesting Surfaces Consisting of Biphilic Nanoscale Topography Youmin Hou, ‡a Yuhe Shang, ‡a Miao Yu, a Chenxi Feng, a Hongyu Yu a and Shuhuai Yao *a a

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science

and Technology, Hong Kong, China * Corresponding author. E-mail: [email protected] ‡ These authors contributed equally to this work

KEYWORDS: water harvesting, condensation, wetting contrast, nanoscale topography, superhydrophobic

ABSTRACT

Water scarcity has become a global issue of severe concern. Great efforts have been undertaken to develop low-cost and highly efficient condensation strategies to relieve water shortages in arid regions. However, the rationale for design of an ideal condensing surface remains lacking due to the conflicting requirements for water nucleation and transport. In this work, we demonstrate that a biphilic nanoscale topography created by a scalable surface engineering method can achieve an ultra-efficient water harvesting performance. With hydrophilic nano-bumps on top of a

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superhydrophobic substrate, this biphilic topography combines the merits of biological surfaces with distinct wetting features (e.g., fog-basking beetles and water-repellent lotus), which enables a tunable water nucleation phenomenon, in contrast to the random condensation mode on their counterparts. By adjusting the contrasting wetting features, the characteristic water nucleation spacing can be tuned to balance the nucleation enhancement and water transport to cope with various environments. Guided by our nucleation density model, we show an optimal biphilic topography by tuning the nanoscale hydrophilic structure density, which allows a ~349% water collection rate and ~184% heat transfer coefficient as compared to the state-of-the-art superhydrophobic surface in a moisture-lacking atmosphere, offering a very promising strategy for improving the efficiency of water harvesting in drought areas.

Water crises have consistently featured among the top ranked global risks over the past years. Owing to climate change and competition from energy generation, water availability will decline by two thirds by 2050, initiating a critical economic impact in arid regions. For current water harvesting technologies, water condensation and collection via surface engineering is of great importance to advance the technological development.1-3 In recent decades, the research of natural organisms featuring superhydrophobicity, e.g., lotus,4 pitcher plants,5-7 insect wings,8, 9 etc., has not only generated viable approaches for manufacturing non-wetting surfaces for efficient fog and water collection,10-17 but has also provided fascinating science of gravity-independent liquid transport.7,

18-25

Nevertheless, the hydrophobic surfaces normally generate a huge nucleation

barrier during water condensation,26,

27

which is unfavourable for capturing moisture in a dry

atmosphere. Distinct from these superhydrophobic creatures, the fog-basking beetle that inhabits


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desert lands offers an alternative method of water collection.28, 29 Even in arid climates, the beetle’s surface that possesses complementary wetting properties can efficiently trap thin moisture by the hydrophilic bumps. The major downside of the natural surfaces of desert beetles, however, is that the benefit of efficient water nucleation is outweighed by slow liquid transport on the microscale hydrophilic structures (see Supplementary Figure S4).30-32 The droplets pinned on the microscale hydrophilic structures reduce the area available for rapid nucleation, and thus impede the continuous water harvesting on such surfaces. To date, nucleation enhancement without compromising liquid transport remains a challenge for water harvesting in arid regions. To address the issue mentioned above, here we develop a facile, scalable method for creating biphilic structured surfaces that explore both merits of desert beetles and the lotus effect.33, 34 By integrating the nanoscale hydrophilic bumps on top of superhydrophobic nanostructures (Figure 1a-b), the resulting structural topography retains superior moisture capture efficiency and avoids liquid pinning that exists on natural beetles. The bumpy structures studded on the biphilic surface can be well controlled in nanoscale dimensions and produced on various substrates (e.g., aluminum, copper, zinc, silicon, etc.) by employing a mask-free electrospraying technique.35, 36 Compared with the conventional approaches using lithography, mask shielding, inkjet printing, or replica molding,31-33,

37-42

the electrospraying technique can not only rapidly generate a large

number of nanoscale structures on surfaces of various shapes. but also conveniently adjust the structural geometry and distribution of the hydrophilic patterns. Through use of multi-nozzle arrays, the electrospraying method enables a scalable fabrication of the biphilic topography. By changing the surface topography and wettability, we can deliberately tune the trade-off between water nucleation and transport on the resulted biphilic surface. In this work, we experimentally demonstrate that, in a moisture-lacking atmosphere, our biphilic surface achieves a twofold higher


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water collection rate as compared to untreated or superhydrophobic surfaces. This study not only offers a practical strategy for the water-stressed community to ease the growing shortage of clean water, but also provides scalable manufacturing techniques for tailoring surface engineering with patterned micro/nanostructures and heterogeneous wettability. RESULTS AND DISCUSSION Figure 1a shows a schematic illustration of forming a biphilic surface with nanoscale hydrophilic bumpy structures. We first fabricated the nanostructures (Al2O3·H2O(s)) on a polished aluminum surface through a boehmitage process. The superhydrophobic functionalization on the alumina interface was obtained by depositing a fluorinated monolayer (FDTS) using chemical vapor deposition. The nanoscale hydrophilic bumps were subsequently created on top of the nanostructures by dispersing a nylon-CNT composite solution through an electrospraying emitter. An electric field was applied between the emitter and aluminum substrate to induce the liquid polarization of the nylon-CNT mixture, generating a mist of highly charged droplets that were driven towards the alumina surface by the electrostatic force (detailed explanation on the sample preparations and experimental setup can be found in the Supplementary). During the ejecting process, the size of the emitted nylon-CNT droplets shrank to nanoscale by solvent evaporation and Coulomb fission (i.e., droplet subdivision due to high charge density). The electrical repulsive interaction among the charged droplets allowed a uniformly distributed pattern on the sprayed interface. Owing to the excellent adhesion properties of nylon, the sprayed nylon-CNT particles adhered firmly on the alumina nanostructures after the solvent completely evaporated. After heating at 80 °C for 30 minutes, the sprayed particles gradually transitioned to dome-shaped hydrophilic bumps atop the superhydrophobic foundation. Figure 1B shows a representative example of the as-fabricated biphilic surface with nanoscale hydrophilic bumps.


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Cyclical water capture on biphilic nanoscale structures The condensation dynamics on the biphilic surface with hydrophilic nano-bumps is in striking contrast to the conventional perspective, as demonstrated by the experimental snapshots via optical and environmental scanning electron microscopes (Figure 1c and d). Different from water adhesion on macro/microscale hydrophilic structures (see Supplementary Figure S4), we found that the nanoscale hydrophilic bumps could cyclically activate the droplet nucleation without liquid pinning (see Video 1). When the droplets grew and interacted with each other, the merging droplet could be timely ejected from the biphilic surface despite of the hydrophilic nano-bumps. The results suggest that the droplet adhesion force on the nanoscale hydrophilic areas is far below the critical threshold for droplet self-departure. The ESEM snapshots (see Figure 1d and Video 2) highlight the continuous water nucleation cycle on the nanoscale hydrophilic patterns, during which the new droplets (A’-E’ indicated in Figure 1d) repeatedly condensed at the same positions where the departed droplets were previously located (A-E indicated in Figure 1d). The cyclical water nucleation implies that, in long-term water harvesting, the nanoscale hydrophilic structures continually clear out the condensed water and are exposed to the vapor phase. Accordingly, even in arid climates, the biphilic surface with hydrophilic nano-bumps can keep trapping moisture from air in a continuous manner. During the condensation, the droplet density and spacing show a significant dependence on the distribution of the scattered nano-bumps. This allows us to control the water phase change by adjusting the structural topography of the biphilic surfaces. The electrospray-assisted surface engineering studied here offers a convenient approach to tailor the nanoscale hydrophilic geometry. Table 1 presents the detailed parameters for obtaining different structural topographies. The morphology, mean diameter, and density of nano-bumps can be regulated by tuning the


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applied voltage, the feeding flow rate of sprayed solution, and the spraying time. For the prepared biphilic (B-1, B-2, and B-3) and superhydrophobic surfaces, we compared the condensation dynamics in three representative stages–initial nucleation, coalescence, and steady state–in the same environment (ambient temperature was 23 °C and relative humidity was 50%), as shown in Figure 2a-d. Different distributions of the hydrophilic nano-bumps led to a substantial variation of water capture at all three stages. The snapshots reveal that the droplet number grows incrementally with the hydrophilic area fraction in the initial nucleation period (t ~ 10s). The increased droplet number density on the biphilic surfaces results in closer droplet spacing and apparently shorter timescale for activating the droplet coalescence (t = 25 ~ 50s), compared to the superhydrophobic surface (t = 110s).

However, the steady state water trapping rate does not increase monotonically

with the droplet coalescence frequency or the hydrophilic area fraction. The durable water harvesting necessitates the tuning of contrasting wetting features to balance the nucleation enchantment and water transport. Figure 2e plots the time evolution of overall droplet number density during condensation on different surfaces. We observed that, by allocating the hydrophilic bumps with a moderate area fraction of 0.58%, the biphilic surface B-2 achieved almost twofold increase in droplet density (~1.53 × 109 m-2) as compared to the superhydrophobic surface (~6.30 × 108 m-2). The appropriate increase of the droplet density on the surface B-2 accelerated droplet interaction and departure at steady state (see Video 3), thus maintaining the nucleation enhancement without degradation for more than 10 minutes. By contrast, with dense hydrophilic patterns (φ ~ 1.56 %), the droplet number density on the surface B-3 sharply dropped from ~2.36 × 109 to ~1.03 × 109 m-2 during the 10-minute condensation period. Despite the fivefold enhancement in the droplet density in the first 30 seconds, the narrowing droplet spacing apparently degraded the water transport frequency and


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anchored more droplets after coalescence (see Video 4). The immobile droplets progressively covered the hydrophilic nano-bumps on the surface, and therefore disrupted the cyclical water capture at steady state. On the other side, for the surface B-1, the droplet density and size distribution were comparable with that on the superhydrophobic surface, that is, the water nucleation and transport rate were also limited because of the sparse hydrophilic structures (φ ~ 0.18%). The comparisons in three condensation stages show that the initial droplet nucleation density dominates the water harvesting rate at steady state, indicating that the cyclical water capture shall be predominately maintained via control of contrasting wetting patterns. Tuning droplet nucleation spacing To better reveal the underlying physics of droplet nucleation dynamics on a surface with contrasting wettability, we investigated the correlation between nucleated droplet spacing (LN) and the nanoscale biphilic geometry by extending the classical theory of heterogeneous nucleation.4347

Based on the experimental observations, the nucleation spacing LN is greater than the mean

distance of the hydrophilic nano-bumps when water vapor condenses at low supersaturations.48-50 That is, the individual hydrophilic bump only activates single embryo droplet formation during the nucleation process. Furthermore, the critical nucleation radius of water embryo rcr < 12 nm was one order of magnitude smaller than the length scale of hydrophilic bumps, indicating that the initial water nucleation behaviours on superhydrophobic and hydrophilic areas were independent of each other. Thus, the heterogeneous nucleation probability for forming a single nucleus on a biphilic surface, PBP, can be determined for distinct materials, 𝑃𝐵𝑃 = 𝑃𝐻𝑃𝐿 + 𝑃𝐻𝑃𝐵 ― 𝑃𝐻𝑃𝐿 ⋅ 𝑃𝐻𝑃𝐵

(1)


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where PHPL and PHPB denote the water nucleation probability on the involved hydrophilic nanobumps and superhydrophobic areas, respectively. By assuming arbitrarily formed water embryo, the droplet nucleation process on a surface with uniform wettability is quantitatively described by considering the heterogeneous water nucleation rate on a specific surface area A, which gives P (T, A) = 1- exp (-A·∫Tsat Ts

J(T)/C(T) dT). Here J(T) is the heterogeneous nucleation rate for

forming one water nucleus, C(T) represents the cooling rate of the solid-vapor interface, Tsat is the vapor saturation temperature, and Ts is the surface temperature (see Supplementary Section 3). Hence, by substituting P(T, A) for structures with different wettability into Eq. (1), the critical area to form one water nucleus on a biphilic surface is given by,

[

𝐽𝐻𝑃𝐿(𝑇)

𝑇

𝐴 ∗ = ― ln (1 ― 𝑃𝐵𝑃) ⋅ 𝜑∫𝑇𝑠𝑎𝑡 𝑠

𝐶(𝑇)

𝐽𝐻𝑃𝐵(𝑇)

𝑇

𝑑𝑇 + (1 ― 𝜑)∫𝑇𝑠𝑎𝑡 𝑠

𝐶(𝑇)

]

𝑑𝑇

(2)

Considering the random Poisson distribution of water nuclei during phase change, the characteristic droplet nucleation spacing can be quantified as LN =0.5 A * ,35 in which Ns = A*-1 is the initial nucleation density of embryo droplets on a condensing surface. Figure 2f shows the characteristic droplet nucleation spacing as a function of the hydrophilic area fraction on a biphilic surface. As depicted by the experiment data obtained in atmospheric environment (T ∞ = 23 ± 0.6 ℃ and RH = 50 ± 2 %), under a fixed supersaturation, the distribution of hydrophilic nanobumps can effectively tune the nucleation spacing LN of condensed droplets. For the “nucleation” stage shown in Figure 2 (t

~ 10s), LN on the three biphilic surfaces was controlled at 22.0 µm,

12.7 µm, 6.3 µm, respectively, with φ increasing from 0.18% to 1.56%. The water nucleation rate and droplet spacing is also subject to the supersaturation S between the vapor phase and condensing surface. When S rose from 1.14 to 1.86, LN on surface B-2 (φ ~ 0.58%) decreased from 12.7 µm to 2.5 µm. This dependency indicates that water nucleation on the biphilic surface results from the


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coupling effect of the surface features and environmental conditions. For efficient water harvesting, the hydrophilic area fraction has to be tailored according to the supersaturation in the phase transition. It is important to note that, for nucleation at the atmospheric conditions, the non-condensable gases (e.g., N2 and O2) dramatically hinder the vapor diffusion during phase change. The accumulation of the non-condensable gases at the vapor-droplet interface reduces the interfacial vapor pressure significantly;51,52 thus, the supersaturation adopted for theoretical estimation (Figure 2f) was selected as a fitting parameter, which was less than the actual values in the ambient environments. Substituting appropriate values for the supersaturation, the theoretical model (solid lines) show a fairly good prediction for the experimental results (symbols) in Figure 2f.

Efficient water harvesting by balancing nucleation and transport For coalescence-induced droplet jumping condensation, the characteristic nucleation spacing LN determines the total amount of water vapor nucleated at the solid interface, while also governing the water transport efficiency during the droplet interaction. We quantified the proportion of droplet departing events involved in the occurrence of droplet coalescence with different nucleation spacing LN (see Figure 3a). When the droplet spacing ranged between 10 ~ 20 µm, about 78% of coalesced droplets spontaneously jumped away from the condensing surface upon coalescence. However, the departing probability sharply dropped to less than 8% for droplets of LN less than 2.5 µm. This is because, when droplets coalesce at small LN, there is insufficient energy released to trigger droplet jumping on the condensing surface.53, 54 In our experiments, we found that keeping the droplet nucleation spacing at around 10 ~ 20 m rendered superior probability of droplet departure.


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The time evolution of the droplet average diameter (Figure 3b) further emphasizes the correlation between droplet nucleation spacing LN and associated water transport rate. We found that, at the initial growth stage without coalescing (t < 30 s), the average diameter of condensed droplets strictly followed the classical power law function, ~ ρtα. Here, the power law exponent was fitted as α = 0.42 and 0.45 for the biphilic and superhydrophobic surfaces, respectively. The difference in droplet nucleation spacing had little impact on the individual droplet growth at the beginning, but it resulted in a large disparity of average droplet size at steady state. As the droplet spacing LN = 12.7 µm on the surface B-2 favors droplet self-jumping, the rapid water refreshing and re-nucleation rate allowed the growing diameter to reach a plateau at t ~ 30s, which was one quarter of the time required for those on the superhydrophobic surface. The appropriate LN on the surface B-2 resulted in balanced droplet nucleation and transport; thus the droplet average diameter at the steady-state condition stayed at 10.7 ± 2.4 µm, which was ~ 50% smaller than that on the superhydrophobic surface ( = 21.8 ± 3.6 µm). It also implies higher heat and mass transfer rates on the biphilic surface during the water harvesting. To directly determine the steady-state water harvesting rate, the biphilic surface (B-2), superhydrophobic, and hydrophilic surfaces were tested in an industry-standard testing chamber over 6 hours (see Supplementary Section 5). Throughout the experiments, the dry and wet bulb temperatures in the testing chamber were consistently controlled with an accuracy of ± 0.3 ℃. The dry bulb temperature of atmospheric environment was kept at Tdry = 26.7 ± 0.3 ℃ based on the ANSI standard test condition, and the wet bulb temperature was adjusted to achieve different relative humidity (30 ~ 80%). Figure 4a shows the photograph of a specifically designed experimental setup for measuring the water collection on different surface topographies. By installing an untreated hydrophilic surface opposite to the biphilic/superhydrophobic surface, the


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self-propelled micro-droplets during condensation can be collected by the hydrophilic surface without evaporation (see Figure 4a inset). The condensing surface temperature was stabilized by the cooling water flow-loop attached to the surfaces. The condensed water was collected by a groove underneath the testing sample. The water collection flux was calculated according to the collected water weight and condensing surface area over a 6-hour test. The experiment on two hydrophilic surfaces facing each other was taken as a control to obtain the water collection flux of a hydrophilic interface. Then, one surface was replaced with a superhydrophobic, lubricantinfused, or biphilic surface allowing the associated water collection flux to be determined by comparing the experimental data with the control case of the hydrophilic interface. Figure 4b shows the measured water collection flux with the relative humidity on four surface topographies. Because of the balanced nucleation and transport enhancement, the steady-state water condensation rate of the biphilic surface (B-2) substantially outperformed those counterparts in the arid climates (RH = 30 ~ 40%). In particular, in the environment with RH = 40%, surface B-2 yielded more than twofold higher water harvesting rate compared to the hydrophilic, superhydrophobic and lubricant-infused surfaces, owing to the cyclical water capture on nanoscale biphilic structures. When the relative humidity increased to 60 ~ 80%, we note that the water condensation enhancement on the biphilic surface gradually dropped to below 20%. The larger supersaturation in humid air greatly reduced the energy barrier for water nucleation, thus changing the droplet nucleation spacing LN and the associated water transport dynamics on the testing surfaces. The overdense droplet distribution broke the balance between the droplet departure and nucleation rate on surface B-2. Thus, in a moist environment, the water collection rate on the biphilic surface became comparable to that of the superhydrophobic surface. Nevertheless, given the arid climates in desert regions, the nanoscale biphilic surface topography is still a very


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promising strategy for improving the water harvesting efficiency of existing water collection systems. Another thermal characterization in pure vapor environments also validated the enhanced condensation heat transfer on the biphilic surface in lower supersaturations. The overall thermal performance on aluminum surfaces with biphilic, superhydrophobic, and hydrophilic surfaces, were tested in an environmentally-controlled chamber (see Supplementary Section 6). Figure 5 shows the measured condensation heat transfer coefficient (h) as a function of the supersaturations between the saturated vapor and condensing surface. Throughout the experiments, the supersaturation was carefully controlled below S =1.06. The flow rate of cooling water was stabilized at ~0.6 L/min to achieve a larger temperature difference between the inlet to outlet and to improve the accuracy of the measurements. The reliability of thermal characterization had been validated using the measurement of a hydrophilic surface, which showed a good agreement with the theoretical estimation by the Nusselt model (Figure 5). The experimental measurements indicate that the biphilic surface performed much better than other surfaces in low supersaturations. At S = 1.01, the condensation heat transfer coefficient on the biphilic surface (h = 166.6 kW/m2K) was ~1.8 times higher than the superhydrophobic surface (90.2 kW/m2 K) because the optimal nucleation spacing LN balances the water nucleation and transport in moisturelacking air. With the increase of supersaturation, the narrowing droplet nucleation spacing LN on the biphilic surface gradually deviated from the optimal distance, thereby degrading the droplet departure and re-nucleation rate. As a result, the condensation heat transfer coefficient on the biphilic surface dropped to ~50 kW/m2 K when S reached ~1.06. The results of pure vapor condensation heat transfer were consistent with the water harvesting performance shown in Figure 4b, which suggests that the droplet nucleation spacing LN needs to be carefully tuned to enhance


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the condensation heat and mass transfer. With the balanced water nucleation and transport rate, the nanoscale biphilic topography enables an ultra-efficient water collection from thin moisture, which has great potential in the areas of vacuum desalination, sludge drying, and dew harvesting. CONCLUSIONS In summary, we report a strategy that successfully balance the tradeoff between water nucleation and transport during the phase change process. A scalable approach was developed to fabricate biphilic topography with nanoscale bumpy geometry that exploits the merits of biological surfaces in desert and humid regions. Through experiments and modeling, we showed that the droplet position, density, interaction frequency, and transport rate can be manipulated by the sprayed hydrophilic nano-bumps. As a result of the continuous water capture on nanoscale hydrophilic patterns, more than twofold enhancement in water collection rate was realized in arid climates (RH = 40%) compared to homogeneous superhydrophobic surfaces. The experimental results expand the horizon of designing surfaces for tuning condensation dynamics according to the environmental conditions. We anticipate that our discovery of fundamentals in tunable water nucleation and scalable manufacturing technology will aid the development of numerous applications based on condensation, transport, and phase-change heat transfer, particularly for water harvesting in drought-affected regions. METHODS Fabrication of a Superhydrophobic Nanostructured Substrate: Commercially available aluminum foils (99.99% purity) and tubes (99.95% purity) were used as test samples. The aluminum foils with thickness of 0.2 mm were electrochemically polished in a 1:4 mixture of 65% HClO4 and 99.5% ethanol (5 ◦C) under 15 V for 4 min, which was vigorously stirred at 250 rpm. The polished sample was then immersed in 0.05 M NaOH solution at 80 °C for 5 min to form Al(OH)3 layer on


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the interface. Then boehmite nanostructures were created by immersing the sample into hot water (90 °C) for an hour. The fluorinated hydrophobic coating was subsequently formed on the asprepared nanostructured surface via chemical vapor deposition. Preparation of Nylon-CNT Composite Solution: Methanol-soluble nylon (ES-6, Shanghai Zhenwei) was selected to form hydrophilic patterns on the nanostructured substrate. Solid-state Nylon (350 mg) was dissolved into 99.8% methanol (10 ml). To enhance the thermal conductivity, 55,56

short multi-walled carbon nanotubes (diameter: 20 - 30 nm, length < 500 nm) were added into

the Nylon-methanol solution with a concentration of 0.7 mg ml-1. The short carbon nanotubes were prepared as reported by Zhang et al.57 Electrospraying of Nylon-CNT Composite Solution: The as-prepared nylon-CNT composite solution was atomized into ultrafine liquid droplets using an electrospray technique. The composite solution was propelled out of a 1 ml syringe through a spraying emitter with inner diameter of 0.21 mm. An electric field of 0.55 kV cm-1 was applied between the spraying emitter and the aluminum substrate (Figure S1). Driven by the applied electric field, the composite solution sprayed outwards as charged droplets and deposited as spherical nanoparticles on the nanostructured aluminum surface. The surface was then heated on a hot plate at 60 °C for 30 min, and then cooled at room temperature. After that, the electrosprayed nanoparticles transitioned to bumpy morphology and adhered on the underlying nanostructures. In Situ Observations of the Condensation Dynamics: Condensation experiments were performed under identical conditions by attaching the sample on a customized cooling stage for microscopic examination (Figure S3). The backside temperature of the testing sample was measured using two cement-on surface thermocouples (CO1-K, Omega). For each experimental trial, the sample temperature was cooled to 3 ± 0.5 ℃ at 0.5 ℃/s via a Peltier module coupled with cooling water


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flow. The ambient temperature and relative humidity of condensation environment, monitored by a humidity/temperature transmitter (HX 94C, Omega), were stabilized at 23 ± 0.6 ℃ and 50 ± 2 %, respectively. Water Collection Characterization: Characterization of the water collection rate on various surfaces was conducted using a specifically designed condensation setup in an industry standard environment-controlled laboratory. By installing a hydrophilic surface opposite to the biphilic/superhydrophobic surface, the self-jumping micro-droplets were captured by the hydrophilic surface, and collected by the lower groove (Figure S5). To ensure efficient droplet collection, an external electric field (voltage difference, V = 80 V) was applied between the condensing and collecting surfaces. The condensing and collecting surfaces were cooled by a water flow-loop set at 1 ± 0.01℃ (AD07R-20, PolyScience). The water collection flux was calculated according to the collected water weight and condensing surface area in a 6-hr experiment. The experiment of two untreated surfaces was taken as the case-control to obtain the water collection flux of a hydrophilic interface. Then one surface was replaced with a superhydrophobic or biphilic surface; the associated water collection flux was obtained by comparing the experimental data with the control case of the hydrophilic interface. Thermal Characterization in Pure-Vapor: All the thermal characterizations of condensation were carried out in a custom environmental chamber which was composed of a stainless-steel vessel (Kurt J. Lesker) connected to a vapor generator and a cooling loop (Figure S5). The condensing surface, 80 mm in diameter, was mounted vertically in a specifically designed cooling stage, which was installed in the environmental chamber. The water-vapor pressure inside the chamber was stabilized at 2000 ± 64 Pa, measured by a capacitance diaphragm gauge (CDG-500, Agilent). The flow rate of cooling water was set at 0.6 L/min, measured by a flow meter (S-114, McMillan)


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integrated in the inflow line. The fluid inlet and outlet temperatures of the cooling stage were measured by T-type thermocouple probes (Omega) which were calibrated in advance using a highprecision thermostatically controlled circulating bath (PolyScience). For extrapolating the temperature of the condensing surface, three thermocouple probes (T-type, Omega) were inserted into the drilled holes, distributed in a triangle, at the backside of testing sample for the temperature measurement (see details in Supplementary Information). All the experimental data were collected by the data acquisition devices (NI-6353 and 9214, National Instruments). The condensation dynamics was recorded using a high speed camera (Phantom v7.3, Vision Research) for further image processing and analysis.


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Figure 1 Topology of the biphilic surface. a Schematics showing the fabrication of biphilic topography by electrospraying a nylon solution to a functionalized nanostructured aluminum surface. b Scanning electron micrograph showing the morphology of a nanostructured aluminum substrate and randomly scattered nylon particles. Inset shows the zoom-in view of a single nanobump. Optical micrograph c and ESEM d showing tuned droplet nucleation and cyclical renucleation by the hydrophilic nanobumps.


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Figure 2 Condensation and nucleation characterization. a-d Selected microscope snapshots showing the initial nucleation, early coalescence, and steady state condensation on superhydrophobic, biphilic B-1, B-2 and B-3 surfaces under the ambient temperature T∞ = 23 °C and relative humidity RH = 50%. e The time evolution of droplet number density during 10minute condensation on different surface topographies. f Theoretical (lines) and experimental (symbols) nucleation spacing on the three biphilic surfaces (B-1, B-2 and B-3) under supersaturation from 1.14 to 1.86.


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Figure 3 Balancing droplet nucleation and water transport. a Histogram showing the jumping probability of coalesced droplets versus the nucleation spacing. b The variation of average droplet diameter with condensation time t for various surface topographies. The gray dash lines denote the average droplet diameter at the steady state, while the dash circles highlight the timescale for activating droplet coalescence on B-2 and superhydrophobic surfaces, respectively. The error bar is one standard deviation.


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Figure 4 Water harvesting tests. a Photograph showing the water harvesting device consisting of a condensing surface (e.g., a biphilic surface, a superhydrophobic surface, a lubricant-infused surface and a hydrophilic surface) and a water collecting surface (a hydrophilic surface), connected with a cooling system for maintaining a stable surface temperature.

Inset highlights

the jumping droplets collected by the hydrophilic collecting surface. b The steady-state water collection flux during a 6-hour test under different environmental humidity (RH = 30% ~ 80%). The dry bulb temperature in all tests was stabilized at 26.7 °C.


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Figure 5 Thermal characterization at pure-vapor conditions. The condensation heat transfer coefficient variation with supersaturation. The error bars were determined from the propagation of errors associated with measurements of temperature, and flow rate.


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Table 1. Nanoscale bumpy structural topographies via various electrospraying parameters Case

Voltage [kV]

Flow rate [l min-1]

Spray time [s]

Bump diameter [m]

Density [×109m-2]

Area fraction [%]

B-1

6

2

5

0.98±0.28

1.68

0.18

B-2

6

2

12

0.97±0.22

4.72

0.58

B-3

6

2

35

0.95±0.32

8.60

1.56


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ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Electrospraying of Nylon-CNT composite solution, contact angle measurements, heterogeneous nucleation theory for biphilic surface, condensation setup and supplementary control experiments, water collection characterization and thermal characterization in pure-vapor condition, data reduction. (PDF) Video S1: Continuous spatial-control of droplet nucleation on biphilic surface. (WMV) Video S2: Cyclical water capture on the biphilic surface. (WMV) Video S3: Condensation dynamics on biphilic surface B-2 and superhydrophobic surface. (WMV) Video S4: Condensation dynamics on biphilic surface B-1 and B-3. (WMV) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions Y. H. and Y. S. conceived the research idea and designed the experiments. Y. S. and M.Y. fabricated the samples. Y. H., Y. S. and M. Y conducted the experiments and analyzed the data with the assistance and contribution from C. F.

S. Y. and H. Y. wrote the manuscript, and all

authors participated in manuscript proofreading.


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ACKNOWLEDGMENT We kindly acknowledge funding from the Research Grants Council of Hong Kong under Collaborative Research Fund (C6022-16G and C1018-17G), the Science and Technology Planning Project of Guangdong Province, China (No. 2017A050506010) and Guangzhou Science and Technology Program (No.201704030077).

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Graphic for manuscript


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Tunable Water Harvesting Surfaces Consisting of Biphilic Nanoscale

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