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Multistep wettability gradient in bioinspired triangular patterns for water condensation and transport
Journal Pre-proofs Multistep Wettability Gradient in Bioinspired Triangular Patterns for Water Condensation and Transport Wei Feng, Bharat Bhushan PII: DOI: Reference:
S0021-9797(19)31300-1 https://doi.org/10.1016/j.jcis.2019.10.113 YJCIS 25607
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
23 September 2019 29 October 2019 30 October 2019
Please cite this article as: W. Feng, B. Bhushan, Multistep Wettability Gradient in Bioinspired Triangular Patterns for Water Condensation and Transport, Journal of Colloid and Interface Science (2019), doi: https:// doi.org/10.1016/j.jcis.2019.10.113
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier Inc.
Oct. 29, 2019 Multistep Wettability Gradient in Bioinspired Triangular Patterns for Water Condensation and Transport Wei Feng1, 2 and Bharat Bhushan1,*
* Corresponding
author: [email protected] 1
1
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLBB) The Ohio State University 201 W. 19th Avenue, Columbus, OH 43210-1142, USA 2
College of Mechanical Engineering Chongqing University
No.174 Shazhengjie, Shapingba, Chongqing, 40044, P.R. China Abstract Water scarcity is becoming increasingly worse in the world. To address water shortages, the water supply needs to be supplemented. In arid regions, flora and fauna use fog or the moisture in air as a source of water. An important consideration in efficient water collection is transport of water droplets collected as rapidly as possible to storage/use before they are evaporated. Wettability gradient on a surface is expected to increase the rate of transportation. In this study, a bioinspired triangular surface with multistep wettability gradient was fabricated. Water condensation and transport studies were carried out on triangular surfaces with wettability gradient in ambient air. Wettability gradient is found to accelerate the water transport. The droplets can climb even if a surface is mounted on an inclined plane.
Keywords: Wettability gradient, Triangular, Laplace pressure gradient, Condensation, Transport 1. Introduction Water scarcity is becoming increasingly worse in the world (Anonymous, 2019). To address water the shortage, water supply needs to be supplemented. In arid regions, nature uses fog or the moisture in air as a source of water for flora and fauna (Cloudsley-Thompson and Chadwick, 1964; Seely, 1979; Mares, 1999; Agam and Berliner, 2006; Bhushan, 2019). After water is collected, it needs to be transported to a location where it is either consumed or stored 2
before it is evaporated. Flora and fauna rely on their surface morphology and wettability to collect water to sustain life (Brown and Bhushan, 2016; Bhushan, 2019). In arid deserts, two examples of such species are desert beetles and cacti. An important consideration in efficient water collection is to transport as rapidly as possible. As an example, desert beetles rely on an array of hydrophilic bumps surrounded by a hydrophobic background to create a heterogeneous wettability that transports water into their mouths. Inspired by this, wettability has been modified by attempting to construct hydrophilic spots in the hydrophobic region, which produces a heterogeneous wettability that effectively collects water from the fog (Gurera and Bhushan, 2019). Inspired by cactus, conical and triangular geometries have been used to drive droplets by Laplace pressure gradient (Gurera and Bhushan, 2019; Song and Bhushan, 2019). Droplets can be driven on surfaces with wettability gradient (Brochard, 1989; Chaudhury and Whitesides, 1992). When a droplet sits on a surface with heterogeneous wettability, wettability gradient provides an unbalanced Young force on both sides of the droplet as a driving force for directional transport of the droplet. When the droplet is placed on an inclined plane, the driving force is equal to the unbalanced Young’s force experienced by a cross section of a droplet force minus the gravitational force component along the surface, given as (Israelachvili, 2011; Bhushan, 2018), Fhetero = γ(cos𝜃𝐵 ― cos𝜃A) 𝑤 ― mg sin ∝
(1)
where γ is the surface tension of water, 𝜃𝐴 and 𝜃𝐵 are the contact angles of the adjacent steps on a surface with heterogeneous wettability, and w is the width and m is the mass of the droplet. If the right step is more hydrophilic than the left step, the droplet will be driven toward the right side of the surface. Droplet on an inclined plane will move as long as the Young force is larger than gravitational component. In this article, multistep wettability gradient was introduced on a bioinspired surface with triangular geometry for rapid droplet transport. The droplets were captured by condensation from ambient air, and the effects of wettability gradient and triangular geometry on water condensation and transport were investigated. The effect of inclined plane on droplet transport was also studied. 2. Experimental details 3
2.1 Experimental set up The schematic of the apparatus used in the study is shown in Fig. 2(a). The water collection chamber was constructed with acrylic sheets. The sample was placed on top of an aluminum block platform which was cooled to 5±1ºC by a Peltier cooler. The air temperature in the chamber was 22±1ºC, and the relative humidity (RH) was controlled by injecting moist water vapor. The water vapor was produced by heating the water in the distillation flask. By varying the flow rate of the air stream as well as the temperature of the hot water, the relative humidity in the chamber was stabilized to about 95±2%. Since the sample temperature of about 5ºC was below the dew point, the water vapor continuously condensed on the sample surface. The water condensation and the transport process of droplet transport process were captured by a digital camera (Koolertron, 5MP 20-300X). 2.2 Fabrication method of multistep wettability gradient surfaces Samples with the multistep wettability gradient were fabricated on polydimethylsiloxane (PDMS) coatings on a glass slide. For deposition of PDMS (Sylgard® 184, Dow Corning) coating, a silicone elastomer curing agent was mixed with the base at the weight ratio of 1:10. The uncured PDMS was degassed for 15 min at room temperature to remove entrained air bubbles. The mixture was subsequently poured onto a glass slide and allowed to flow for 1 h to form approximately 3 mm thick flat samples. The mixture was cured by heating at 80°C for 2 h. Finally, the desired rectangular and triangular samples, were produced by cutting the cured PDMS. PDMS surfaces are hydrophobic with an angle of about 110º. Ultraviolet-ozone (UVO) treatment is commonly used to activate surfaces to change their degree of wettability (Bhushan, 2018). The wettability gradient with various degrees of wettability can be achieved by treating various segments of a pattern for different lengths of treatment times. The UVO exposure was generated from a U-shaped, ozone-producing and ultraviolent lamp (18.4 W, Model G18T5VHU, Atlantic Ultraviolet Co.) and samples were placed 10 cm underneath the light source. By using a moving mask in selected steps, samples were treated for various times, to realize surfaces with wettability gradient (Fig. 2(b)). The mask was moved every 10 minutes with the first step of 2.5 mm or 5 mm and the other five steps of 1.5 mm or 3 mm.
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Figure 2(c) shows the details of two samples with wettability gradients. The size of the rectangular surface was 10 mm × 2.5 mm. The triangular surface was 20 mm long and 8 mm wide at the base. The base was 5 mm long and the triangular region was 15 mm long with tip angle of 9º. Both rectangular and triangular samples had 6 wettability steps, with storage steps of 2.5 mm and 5 mm long, and the other five steps of 1.5 mm and 3 mm long, respectively. From left to right, the CA of the surfaces was decreased in six steps (81º, 68º, 53º, 47º, 34º and 20º, respectively). The wettability properties of various steps on the surface with the wettability gradient are shown in Fig. 3(a). The angles were measured by using a standard automated goniometer (Model 290, Ramé-Hart Inc.) and 5 μL distilled water droplets which were deposited onto the surfaces. The contact angle data was reproducible within ±1°. Figure 3(b) shows the change in the static contact angle with UVO treatment over time. 3. Results and discussion For fundamental understanding of the mechanism of droplet transport on the surfaces with the wettability gradient, droplet transport experiments were carried out on two homogeneously hydrophilic surfaces and one surface with wettability gradient. The relationship between droplet transport and volume at sample temperatures of 22±1ºC was explored. Next, the water condensation and transport studies of the droplets on the rectangular surfaces with the different wettability properties were carried out. Finally, the water condensation and transport studies on the triangular surfaces were carried out, in order to study the combined effect of the wettability gradient and the triangular geometry. To study if the droplets can be transported on an inclined plane, studies were carried out on samples mounted on an inclined plane. 3.1 Droplet transport on the wettability gradient patterns The droplet transport experiment was carried out by depositing droplets using a microsyringe at the left end of sample areas in a volume increment of 5 μL. Experiments were carried out on two hydrophilic surfaces with CA of 81º and 20º, and another with multi step wettability gradient (CA = 81º- 20º). Selected optical images are shown in Fig. 4. The rectangular array below the photo in the bottom row shows the steps.
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The hydrophilic surface with CA = 81º did not have a wettability gradient, so there was no droplet transport. But due to the increased volume of the liquid and the limitation of the left edge, the droplet could expand to the right. When the volume of the droplet reached above 60 μL, the width of the surface could not hold the droplet. For another hydrophilic surface with CA = 20°, the droplet spread more on the surface due to the smaller contact angle, and the entire surface was covered with a thin water film when the droplet was at 10 μL. As the droplet volume increased, the water film became thicker. For the surface with the wettability gradient, the droplets rapidly spread over the surface as the droplets were deposited. As the contact angle of the next steps became smaller, the spread steps of depositing droplets will gradually increase. As the droplet volume increased higher than 20 μL, the droplets traversed more steps and after coalescing, the droplets were transported to the storage step (CA = 20º). 3.2 Water condensation and transport on rectangular samples To compare the effects of samples with multistep wettability gradient (CA = 81º-20º) and two hydrophilic samples (CA = 81º and CA = 20º), the transport of condensed droplets on different samples were studied. The selected optical images are shown in Fig. 5(a). In the hydrophilic surface with CA = 81º, at the beginning stage, the droplets coalesced and grew larger on the surface. As the condensation continued, more droplets condensed and coalesced to form larger ones (recorded as 1-7). Eventually, when the droplets were large enough to reach adjacent droplets, the coalescence of droplets formed larger ones (1+2+3, 4+5 and 6+7) at a coalescence time of 98 minutes. After the formation of the larger droplets, the center of the entire droplets did not change, which meant that the droplets were not transported after coalescence. On another hydrophilic surface with CA = 20º, since the surface was more hydrophilic, the droplet spread more quickly. Droplets coalesced to form larger droplets more quickly. This took place in 13 minutes. Then in a short period of time, a water film was formed, and the entire process took only 32 minutes. No directional transport of any droplets was observed due to the high adhesion of the surface. The initial process of condensation of tiny droplets on the surface with multistep wettability gradient (CA = 81º-20º) was the same as on the two hydrophilic surfaces (CA = 81º and 20º). However, at 42 minutes after the droplets condensed, they were not uniform in size 6
(recorded as 1-3), and the right step was more hydrophilic than the left step, thus causing the droplets in the right step to spread more. Between 51 and 63 minutes, as the droplets condensed and became larger, the wettability gradient provided an unbalanced force at the two sides of a droplet which acted as a driving force for directional movement of the droplet to the more hydrophilic step. Droplets 2 and 3 coalesced into large droplets (2+ 3) at 51 minutes. After that, the size of the large droplets was insufficient to coalesce the droplets next to them due to the randomness of the droplets. Therefore, with the help of continuous condensation to create new droplets, they finally coalesced with large droplets (2+3) at 63 minutes and transported to the storage step, at which point the droplet transport was completed over the entire surface. The exposed area on the left steps could continue to have condensation and have new transport. To better understand the transport mechanism of droplets on hydrophilic surfaces and the surfaces with wettability gradient, the growth, coalescence and transport of droplets are schematically shown in Fig. 5(b). For three different surfaces, first, droplets were condensed, which coalesced with other droplets to form larger droplets. For hydrophilic surfaces (CA = 81º and CA = 20º), the forces of the droplets were equal when coalescing. In the case of a more hydrophilic surface, and the droplets spread over larger area under the same volume. Therefore, coalescence was more likely to occur. In addition, the more hydrophilic surface takes the shorter time to form the water film. However, since the surface was more hydrophilic, the adhesion was higher. The droplets formed a very thin film of water that did not fall off when the surface was tilted. The surface with a wettability gradient provided directional transport of droplets. The CA on the left and right sides were different, which led to droplet transport. Next, the sample with multistep wettability gradient was placed on an inclined plane with an inclination angle of 5º. The droplets condensed, coalesced and transported upward. The selected optical images of droplets on samples sitting on flat and inclined planes are shown in Fig. 6. As expected, it took longer for droplets on an inclined plane to reach the storage step. However, the driving force due to heterogeneous wettability was high enough to overcome gravitational forces. 3.3 Water condensation and transport on triangular samples To investigate the combined effects of Laplace pressure gradient and multistep wettability gradient, three samples with triangular geometries were studied. They included two 7
hydrophilic triangular surfaces (CA = 81º and CA = 20º) and one triangular surface with wettability gradient. Selected optical images are shown in Fig. 7. Due to the Laplace pressure generated by the shape of the triangular sample, the shape of the droplet is affected. When the droplets nucleated and coalesced, once they touched the sides, the droplets began to move because of the Laplace pressure gradient (Song and Bhushan, 2019). The entire droplet transport time was also affected by surface wettability. In the hydrophilic surfaces with CA = 81º, the condensed water droplets were relatively small at the onset of condensation. As condensation continues, the growing droplets began to coalesce into larger droplets. Ultimately, they were large enough to reach the boundary, triggering transport driven by Laplace pressure gradient. For example, after 36 minutes of condensation on the triangular sample, 5 large droplets (numbers 1-5) were formed to touch the boundary. At 48 minutes, droplets 1, 2 and 3 coalesced into one large droplet (1 + 2 + 3). In the next period of time of 3 minutes, large droplets (1 + 2 + 3) coalesced droplets 4 and 5, the droplet volume increased, and the Laplace pressure gradient could drive the droplets to the storage area. On another more hydrophilic surface with CA = 20º, the initial stage of condensation was the same, but the whole process was very fast. At 14 minutes, all of the droplets on the entire surface coalesced to form a water film. Similar to the rectangular hydrophilic surface (CA = 20º), a thin water film was formed. The Laplace pressure gradient along the coalesced droplets could not overcome the high adhesion resulting in the transport of droplets. The process of droplet condensation, coalescence and transport on two hydrophilic surfaces was similar. However, the triangular sample combined with the multistep wettability gradient and the Laplace pressure gradient had an increase in the coalescence rate of the droplets. By comparing the rectangular surface with wettability gradient, the entire transport time was shortened by approximately 50%. It was found that the key to rapid transport of droplets was the rapid coalescence of the droplets and the sufficient directional driving force so that the overall condensation and transport time was greatly reduced. Next, the three samples with triangular geometries placed on a plane inclined at 15º were studied. Selected optical images of droplets are presented in Fig. 8. As expected, it took longer for droplets on all surfaces on inclined planes to reach the storage steps. However, the driving forces in all samples were high enough to overcome gravitational forces. 8
4. Conclusions and outlook In this study, water condensation and transport were systematically studied for triangular surfaces with multistep wettability gradient. For comparison, experiments were also carried out on rectangular surfaces with various homogeneous wettability. On the rectangle surfaces with wettability gradient, when the water in the ambient air was cooled to about 5º, it condensed, grew, and coalesced on the rectangle surface. When the coalesced droplets were driven by the wettability gradient, the droplets began to move and were transported to the storage step. The triangular surface combined with the wettability gradient and the Laplace pressure gradient had an increase in the coalescence rate of the droplets and the transport time was shorter than on the rectangular samples. Even on surfaces placed on inclined planes, the driving forces were high enough to transport droplets to the storage steps. It is demonstrated that methods combining triangular geometry and wettability gradient can be used for more efficient water collection systems.
Acknowledgments Wei Feng gratefully acknowledges the financial support from the China Scholarship Council. Authors would like to thank Dr. Dong Song for his suggestions on planning, execution and writing of the paper. The authors would also like to thank Dev Gurera for the insightful discussions.
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References Agam, N. and Berliner, P. R. (2006), “Dew Formation and Water Vapor Adsorption in SemiArid Environments– A Review,” J. Arid Environ. 65, 572-590. Anonymous (2019), The United Nations World Water Development Report 2019: Leaving No One Behind, UNESCO World Water Assessment Programme, Paris, France. Bhushan, B. (2018) Biomimetics: Bioinspired Hierarchical-Structured Surfaces for Green Science and Technology, third ed., Springer International, Cham, Switzerland. Bhushan, B. (2019), "Bioinspired Water Collection Methods to Supplement Water Supply," Phil. Trans. R. Soc. A 377, 20190119. Brochard, F. (1989), "Motions of Droplets on Solid Surfaces Induced by Chemical or Thermal Gradients," Langmuir 5, 432-438. Brown, P. S. and Bhushan, B. (2016), "Bioinspired Materials for Water Supply and Management: Water Collection, Water Purification and Separation of Water from Oil," Phil. Trans. R. Soc. A 374, 20160135. Chaudhury, M. K. and Whitesides, G. M. (1992), “How to Make Water Run Uphill,” Science 256, 1539–1541. Cloudsley-Thompson, J. L. and Chadwick, M. J. (1964), Life in Deserts, Dufour Editions, Philadelphia, Pennsylvania. Gurera, D. and Bhushan, B. (2019), "Designing Bioinspired Surfaces for Water Collection from Fog," Phil. Trans. R. Soc. A 377, 20180269. Israelachvili, J. N. (2011), "Intermolecular and Surface Forces," third ed., Academic Press, Cambridge, Mass. Mares, M. A. (Ed.) (1999), Encyclopedia of Deserts, University of Oklahoma Press, Norman, Oklahoma. Song, D. and Bhushan, B. (2019), "Optimization of Bioinspired Triangular Patterns for Water Condensation and Transport," Phil. Trans. R. Soc. A 377, 20190127. Seely, M. K. (1979), “Irregular Fog as a Water Source for Desert Dune Beetles,” Oecologia 42, 213–227.
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Figure Captions Fig. 1
Schematic of the droplet movement on an inclined surface with heterogeneous wettability.
Fig. 2
Schematics (a) of the experimental setup for water condensation system, (b) of fabrication steps of the PDMS surfaces with multistep gradient by UVO treatment, and (c) samples with rectangular and triangular patterns with wettability gradient.
Fig. 3
(a) Wettability characterization of the surfaces along the multistep wettability gradient direction. Reproducibility was ±2°. (b) The relationship between the contact angle and the UVO treatment time.
Fig. 4
Optical observation of droplet deposition and transport on the rectangular surfaces with two wettability (CA = 20° or 81°) and with multistep wettability gradient (CA=81°20°) at different droplet volumes.
Fig. 5
(a) Water condensation and transport on rectangular surfaces won a horizontal plane with two wettability (CA = 20° or 81°) and with multistep wettability gradient (CA=81°- 20°). Selected photographs (top view) of the droplet collection at different times are shown. Arrows shown above some droplets represent droplet movement observed in videos. (b) Schematic of droplet growth, coalescence and transport on rectangular surfaces with two wettability and with wettability gradient.
Fig. 6
Water condensation and transport on rectangular surfaces with multi step wettability gradient (CA=81°- 20°) placed on a horizontal plane and another inclined at 5°.
Fig. 7
Water condensation and transport on triangular surfaces placed on horizontal planes with two wettability (CA = 20° or 81°) and with multi step wettability gradient (CA=81°- 20°), respectively. Selected photographs (top view) of the droplet collection at different times are shown. Arrows shown above some droplets represent droplet movement observed in videos.
Fig. 8
Water condensation and transport on triangular surfaces placed on a plane inclined at 15° with two wettability (CA = 20° or 81°) and with multistep wettability gradient (CA=81°- 20°), respectively. Selected photographs (top view) of the droplet collection at different times are shown. Arrows shown above some droplets represent droplet movement observed in videos.
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Graphical abstract
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S0021-9797(19)31300-1 https://doi.org/10.1016/j.jcis.2019.10.113 YJCIS 25607
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
23 September 2019 29 October 2019 30 October 2019
Please cite this article as: W. Feng, B. Bhushan, Multistep Wettability Gradient in Bioinspired Triangular Patterns for Water Condensation and Transport, Journal of Colloid and Interface Science (2019), doi: https:// doi.org/10.1016/j.jcis.2019.10.113
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier Inc.
Oct. 29, 2019 Multistep Wettability Gradient in Bioinspired Triangular Patterns for Water Condensation and Transport Wei Feng1, 2 and Bharat Bhushan1,*
* Corresponding
author: [email protected] 1
1
Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLBB) The Ohio State University 201 W. 19th Avenue, Columbus, OH 43210-1142, USA 2
College of Mechanical Engineering Chongqing University
No.174 Shazhengjie, Shapingba, Chongqing, 40044, P.R. China Abstract Water scarcity is becoming increasingly worse in the world. To address water shortages, the water supply needs to be supplemented. In arid regions, flora and fauna use fog or the moisture in air as a source of water. An important consideration in efficient water collection is transport of water droplets collected as rapidly as possible to storage/use before they are evaporated. Wettability gradient on a surface is expected to increase the rate of transportation. In this study, a bioinspired triangular surface with multistep wettability gradient was fabricated. Water condensation and transport studies were carried out on triangular surfaces with wettability gradient in ambient air. Wettability gradient is found to accelerate the water transport. The droplets can climb even if a surface is mounted on an inclined plane.
Keywords: Wettability gradient, Triangular, Laplace pressure gradient, Condensation, Transport 1. Introduction Water scarcity is becoming increasingly worse in the world (Anonymous, 2019). To address water the shortage, water supply needs to be supplemented. In arid regions, nature uses fog or the moisture in air as a source of water for flora and fauna (Cloudsley-Thompson and Chadwick, 1964; Seely, 1979; Mares, 1999; Agam and Berliner, 2006; Bhushan, 2019). After water is collected, it needs to be transported to a location where it is either consumed or stored 2
before it is evaporated. Flora and fauna rely on their surface morphology and wettability to collect water to sustain life (Brown and Bhushan, 2016; Bhushan, 2019). In arid deserts, two examples of such species are desert beetles and cacti. An important consideration in efficient water collection is to transport as rapidly as possible. As an example, desert beetles rely on an array of hydrophilic bumps surrounded by a hydrophobic background to create a heterogeneous wettability that transports water into their mouths. Inspired by this, wettability has been modified by attempting to construct hydrophilic spots in the hydrophobic region, which produces a heterogeneous wettability that effectively collects water from the fog (Gurera and Bhushan, 2019). Inspired by cactus, conical and triangular geometries have been used to drive droplets by Laplace pressure gradient (Gurera and Bhushan, 2019; Song and Bhushan, 2019). Droplets can be driven on surfaces with wettability gradient (Brochard, 1989; Chaudhury and Whitesides, 1992). When a droplet sits on a surface with heterogeneous wettability, wettability gradient provides an unbalanced Young force on both sides of the droplet as a driving force for directional transport of the droplet. When the droplet is placed on an inclined plane, the driving force is equal to the unbalanced Young’s force experienced by a cross section of a droplet force minus the gravitational force component along the surface, given as (Israelachvili, 2011; Bhushan, 2018), Fhetero = γ(cos𝜃𝐵 ― cos𝜃A) 𝑤 ― mg sin ∝
(1)
where γ is the surface tension of water, 𝜃𝐴 and 𝜃𝐵 are the contact angles of the adjacent steps on a surface with heterogeneous wettability, and w is the width and m is the mass of the droplet. If the right step is more hydrophilic than the left step, the droplet will be driven toward the right side of the surface. Droplet on an inclined plane will move as long as the Young force is larger than gravitational component. In this article, multistep wettability gradient was introduced on a bioinspired surface with triangular geometry for rapid droplet transport. The droplets were captured by condensation from ambient air, and the effects of wettability gradient and triangular geometry on water condensation and transport were investigated. The effect of inclined plane on droplet transport was also studied. 2. Experimental details 3
2.1 Experimental set up The schematic of the apparatus used in the study is shown in Fig. 2(a). The water collection chamber was constructed with acrylic sheets. The sample was placed on top of an aluminum block platform which was cooled to 5±1ºC by a Peltier cooler. The air temperature in the chamber was 22±1ºC, and the relative humidity (RH) was controlled by injecting moist water vapor. The water vapor was produced by heating the water in the distillation flask. By varying the flow rate of the air stream as well as the temperature of the hot water, the relative humidity in the chamber was stabilized to about 95±2%. Since the sample temperature of about 5ºC was below the dew point, the water vapor continuously condensed on the sample surface. The water condensation and the transport process of droplet transport process were captured by a digital camera (Koolertron, 5MP 20-300X). 2.2 Fabrication method of multistep wettability gradient surfaces Samples with the multistep wettability gradient were fabricated on polydimethylsiloxane (PDMS) coatings on a glass slide. For deposition of PDMS (Sylgard® 184, Dow Corning) coating, a silicone elastomer curing agent was mixed with the base at the weight ratio of 1:10. The uncured PDMS was degassed for 15 min at room temperature to remove entrained air bubbles. The mixture was subsequently poured onto a glass slide and allowed to flow for 1 h to form approximately 3 mm thick flat samples. The mixture was cured by heating at 80°C for 2 h. Finally, the desired rectangular and triangular samples, were produced by cutting the cured PDMS. PDMS surfaces are hydrophobic with an angle of about 110º. Ultraviolet-ozone (UVO) treatment is commonly used to activate surfaces to change their degree of wettability (Bhushan, 2018). The wettability gradient with various degrees of wettability can be achieved by treating various segments of a pattern for different lengths of treatment times. The UVO exposure was generated from a U-shaped, ozone-producing and ultraviolent lamp (18.4 W, Model G18T5VHU, Atlantic Ultraviolet Co.) and samples were placed 10 cm underneath the light source. By using a moving mask in selected steps, samples were treated for various times, to realize surfaces with wettability gradient (Fig. 2(b)). The mask was moved every 10 minutes with the first step of 2.5 mm or 5 mm and the other five steps of 1.5 mm or 3 mm.
4
Figure 2(c) shows the details of two samples with wettability gradients. The size of the rectangular surface was 10 mm × 2.5 mm. The triangular surface was 20 mm long and 8 mm wide at the base. The base was 5 mm long and the triangular region was 15 mm long with tip angle of 9º. Both rectangular and triangular samples had 6 wettability steps, with storage steps of 2.5 mm and 5 mm long, and the other five steps of 1.5 mm and 3 mm long, respectively. From left to right, the CA of the surfaces was decreased in six steps (81º, 68º, 53º, 47º, 34º and 20º, respectively). The wettability properties of various steps on the surface with the wettability gradient are shown in Fig. 3(a). The angles were measured by using a standard automated goniometer (Model 290, Ramé-Hart Inc.) and 5 μL distilled water droplets which were deposited onto the surfaces. The contact angle data was reproducible within ±1°. Figure 3(b) shows the change in the static contact angle with UVO treatment over time. 3. Results and discussion For fundamental understanding of the mechanism of droplet transport on the surfaces with the wettability gradient, droplet transport experiments were carried out on two homogeneously hydrophilic surfaces and one surface with wettability gradient. The relationship between droplet transport and volume at sample temperatures of 22±1ºC was explored. Next, the water condensation and transport studies of the droplets on the rectangular surfaces with the different wettability properties were carried out. Finally, the water condensation and transport studies on the triangular surfaces were carried out, in order to study the combined effect of the wettability gradient and the triangular geometry. To study if the droplets can be transported on an inclined plane, studies were carried out on samples mounted on an inclined plane. 3.1 Droplet transport on the wettability gradient patterns The droplet transport experiment was carried out by depositing droplets using a microsyringe at the left end of sample areas in a volume increment of 5 μL. Experiments were carried out on two hydrophilic surfaces with CA of 81º and 20º, and another with multi step wettability gradient (CA = 81º- 20º). Selected optical images are shown in Fig. 4. The rectangular array below the photo in the bottom row shows the steps.
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The hydrophilic surface with CA = 81º did not have a wettability gradient, so there was no droplet transport. But due to the increased volume of the liquid and the limitation of the left edge, the droplet could expand to the right. When the volume of the droplet reached above 60 μL, the width of the surface could not hold the droplet. For another hydrophilic surface with CA = 20°, the droplet spread more on the surface due to the smaller contact angle, and the entire surface was covered with a thin water film when the droplet was at 10 μL. As the droplet volume increased, the water film became thicker. For the surface with the wettability gradient, the droplets rapidly spread over the surface as the droplets were deposited. As the contact angle of the next steps became smaller, the spread steps of depositing droplets will gradually increase. As the droplet volume increased higher than 20 μL, the droplets traversed more steps and after coalescing, the droplets were transported to the storage step (CA = 20º). 3.2 Water condensation and transport on rectangular samples To compare the effects of samples with multistep wettability gradient (CA = 81º-20º) and two hydrophilic samples (CA = 81º and CA = 20º), the transport of condensed droplets on different samples were studied. The selected optical images are shown in Fig. 5(a). In the hydrophilic surface with CA = 81º, at the beginning stage, the droplets coalesced and grew larger on the surface. As the condensation continued, more droplets condensed and coalesced to form larger ones (recorded as 1-7). Eventually, when the droplets were large enough to reach adjacent droplets, the coalescence of droplets formed larger ones (1+2+3, 4+5 and 6+7) at a coalescence time of 98 minutes. After the formation of the larger droplets, the center of the entire droplets did not change, which meant that the droplets were not transported after coalescence. On another hydrophilic surface with CA = 20º, since the surface was more hydrophilic, the droplet spread more quickly. Droplets coalesced to form larger droplets more quickly. This took place in 13 minutes. Then in a short period of time, a water film was formed, and the entire process took only 32 minutes. No directional transport of any droplets was observed due to the high adhesion of the surface. The initial process of condensation of tiny droplets on the surface with multistep wettability gradient (CA = 81º-20º) was the same as on the two hydrophilic surfaces (CA = 81º and 20º). However, at 42 minutes after the droplets condensed, they were not uniform in size 6
(recorded as 1-3), and the right step was more hydrophilic than the left step, thus causing the droplets in the right step to spread more. Between 51 and 63 minutes, as the droplets condensed and became larger, the wettability gradient provided an unbalanced force at the two sides of a droplet which acted as a driving force for directional movement of the droplet to the more hydrophilic step. Droplets 2 and 3 coalesced into large droplets (2+ 3) at 51 minutes. After that, the size of the large droplets was insufficient to coalesce the droplets next to them due to the randomness of the droplets. Therefore, with the help of continuous condensation to create new droplets, they finally coalesced with large droplets (2+3) at 63 minutes and transported to the storage step, at which point the droplet transport was completed over the entire surface. The exposed area on the left steps could continue to have condensation and have new transport. To better understand the transport mechanism of droplets on hydrophilic surfaces and the surfaces with wettability gradient, the growth, coalescence and transport of droplets are schematically shown in Fig. 5(b). For three different surfaces, first, droplets were condensed, which coalesced with other droplets to form larger droplets. For hydrophilic surfaces (CA = 81º and CA = 20º), the forces of the droplets were equal when coalescing. In the case of a more hydrophilic surface, and the droplets spread over larger area under the same volume. Therefore, coalescence was more likely to occur. In addition, the more hydrophilic surface takes the shorter time to form the water film. However, since the surface was more hydrophilic, the adhesion was higher. The droplets formed a very thin film of water that did not fall off when the surface was tilted. The surface with a wettability gradient provided directional transport of droplets. The CA on the left and right sides were different, which led to droplet transport. Next, the sample with multistep wettability gradient was placed on an inclined plane with an inclination angle of 5º. The droplets condensed, coalesced and transported upward. The selected optical images of droplets on samples sitting on flat and inclined planes are shown in Fig. 6. As expected, it took longer for droplets on an inclined plane to reach the storage step. However, the driving force due to heterogeneous wettability was high enough to overcome gravitational forces. 3.3 Water condensation and transport on triangular samples To investigate the combined effects of Laplace pressure gradient and multistep wettability gradient, three samples with triangular geometries were studied. They included two 7
hydrophilic triangular surfaces (CA = 81º and CA = 20º) and one triangular surface with wettability gradient. Selected optical images are shown in Fig. 7. Due to the Laplace pressure generated by the shape of the triangular sample, the shape of the droplet is affected. When the droplets nucleated and coalesced, once they touched the sides, the droplets began to move because of the Laplace pressure gradient (Song and Bhushan, 2019). The entire droplet transport time was also affected by surface wettability. In the hydrophilic surfaces with CA = 81º, the condensed water droplets were relatively small at the onset of condensation. As condensation continues, the growing droplets began to coalesce into larger droplets. Ultimately, they were large enough to reach the boundary, triggering transport driven by Laplace pressure gradient. For example, after 36 minutes of condensation on the triangular sample, 5 large droplets (numbers 1-5) were formed to touch the boundary. At 48 minutes, droplets 1, 2 and 3 coalesced into one large droplet (1 + 2 + 3). In the next period of time of 3 minutes, large droplets (1 + 2 + 3) coalesced droplets 4 and 5, the droplet volume increased, and the Laplace pressure gradient could drive the droplets to the storage area. On another more hydrophilic surface with CA = 20º, the initial stage of condensation was the same, but the whole process was very fast. At 14 minutes, all of the droplets on the entire surface coalesced to form a water film. Similar to the rectangular hydrophilic surface (CA = 20º), a thin water film was formed. The Laplace pressure gradient along the coalesced droplets could not overcome the high adhesion resulting in the transport of droplets. The process of droplet condensation, coalescence and transport on two hydrophilic surfaces was similar. However, the triangular sample combined with the multistep wettability gradient and the Laplace pressure gradient had an increase in the coalescence rate of the droplets. By comparing the rectangular surface with wettability gradient, the entire transport time was shortened by approximately 50%. It was found that the key to rapid transport of droplets was the rapid coalescence of the droplets and the sufficient directional driving force so that the overall condensation and transport time was greatly reduced. Next, the three samples with triangular geometries placed on a plane inclined at 15º were studied. Selected optical images of droplets are presented in Fig. 8. As expected, it took longer for droplets on all surfaces on inclined planes to reach the storage steps. However, the driving forces in all samples were high enough to overcome gravitational forces. 8
4. Conclusions and outlook In this study, water condensation and transport were systematically studied for triangular surfaces with multistep wettability gradient. For comparison, experiments were also carried out on rectangular surfaces with various homogeneous wettability. On the rectangle surfaces with wettability gradient, when the water in the ambient air was cooled to about 5º, it condensed, grew, and coalesced on the rectangle surface. When the coalesced droplets were driven by the wettability gradient, the droplets began to move and were transported to the storage step. The triangular surface combined with the wettability gradient and the Laplace pressure gradient had an increase in the coalescence rate of the droplets and the transport time was shorter than on the rectangular samples. Even on surfaces placed on inclined planes, the driving forces were high enough to transport droplets to the storage steps. It is demonstrated that methods combining triangular geometry and wettability gradient can be used for more efficient water collection systems.
Acknowledgments Wei Feng gratefully acknowledges the financial support from the China Scholarship Council. Authors would like to thank Dr. Dong Song for his suggestions on planning, execution and writing of the paper. The authors would also like to thank Dev Gurera for the insightful discussions.
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References Agam, N. and Berliner, P. R. (2006), “Dew Formation and Water Vapor Adsorption in SemiArid Environments– A Review,” J. Arid Environ. 65, 572-590. Anonymous (2019), The United Nations World Water Development Report 2019: Leaving No One Behind, UNESCO World Water Assessment Programme, Paris, France. Bhushan, B. (2018) Biomimetics: Bioinspired Hierarchical-Structured Surfaces for Green Science and Technology, third ed., Springer International, Cham, Switzerland. Bhushan, B. (2019), "Bioinspired Water Collection Methods to Supplement Water Supply," Phil. Trans. R. Soc. A 377, 20190119. Brochard, F. (1989), "Motions of Droplets on Solid Surfaces Induced by Chemical or Thermal Gradients," Langmuir 5, 432-438. Brown, P. S. and Bhushan, B. (2016), "Bioinspired Materials for Water Supply and Management: Water Collection, Water Purification and Separation of Water from Oil," Phil. Trans. R. Soc. A 374, 20160135. Chaudhury, M. K. and Whitesides, G. M. (1992), “How to Make Water Run Uphill,” Science 256, 1539–1541. Cloudsley-Thompson, J. L. and Chadwick, M. J. (1964), Life in Deserts, Dufour Editions, Philadelphia, Pennsylvania. Gurera, D. and Bhushan, B. (2019), "Designing Bioinspired Surfaces for Water Collection from Fog," Phil. Trans. R. Soc. A 377, 20180269. Israelachvili, J. N. (2011), "Intermolecular and Surface Forces," third ed., Academic Press, Cambridge, Mass. Mares, M. A. (Ed.) (1999), Encyclopedia of Deserts, University of Oklahoma Press, Norman, Oklahoma. Song, D. and Bhushan, B. (2019), "Optimization of Bioinspired Triangular Patterns for Water Condensation and Transport," Phil. Trans. R. Soc. A 377, 20190127. Seely, M. K. (1979), “Irregular Fog as a Water Source for Desert Dune Beetles,” Oecologia 42, 213–227.
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Figure Captions Fig. 1
Schematic of the droplet movement on an inclined surface with heterogeneous wettability.
Fig. 2
Schematics (a) of the experimental setup for water condensation system, (b) of fabrication steps of the PDMS surfaces with multistep gradient by UVO treatment, and (c) samples with rectangular and triangular patterns with wettability gradient.
Fig. 3
(a) Wettability characterization of the surfaces along the multistep wettability gradient direction. Reproducibility was ±2°. (b) The relationship between the contact angle and the UVO treatment time.
Fig. 4
Optical observation of droplet deposition and transport on the rectangular surfaces with two wettability (CA = 20° or 81°) and with multistep wettability gradient (CA=81°20°) at different droplet volumes.
Fig. 5
(a) Water condensation and transport on rectangular surfaces won a horizontal plane with two wettability (CA = 20° or 81°) and with multistep wettability gradient (CA=81°- 20°). Selected photographs (top view) of the droplet collection at different times are shown. Arrows shown above some droplets represent droplet movement observed in videos. (b) Schematic of droplet growth, coalescence and transport on rectangular surfaces with two wettability and with wettability gradient.
Fig. 6
Water condensation and transport on rectangular surfaces with multi step wettability gradient (CA=81°- 20°) placed on a horizontal plane and another inclined at 5°.
Fig. 7
Water condensation and transport on triangular surfaces placed on horizontal planes with two wettability (CA = 20° or 81°) and with multi step wettability gradient (CA=81°- 20°), respectively. Selected photographs (top view) of the droplet collection at different times are shown. Arrows shown above some droplets represent droplet movement observed in videos.
Fig. 8
Water condensation and transport on triangular surfaces placed on a plane inclined at 15° with two wettability (CA = 20° or 81°) and with multistep wettability gradient (CA=81°- 20°), respectively. Selected photographs (top view) of the droplet collection at different times are shown. Arrows shown above some droplets represent droplet movement observed in videos.
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Graphical abstract
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