Supporting Information

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Liliana Booker
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1 Supporting Information High-Efficiency Fog Collector: Water Unidirectional Transport on Heterogeneous Rough Conical Wires Ting Xu, Yucai Lin, Miaoxin Zhang, Weiwei Shi, Yongmei Zheng* Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education School of Chemistry and Environment, Beihang University, Beijing, (P. R. China) * Content: Supplementary Figure Legends: Figure S1-S6; Supplementary Table S1-S2; Supplementary Analysis I; Supplementary Movie S1-S5. 1

2 Figure S1: Figure S1. Schematic diagram of the fabrication device of PCCW. DC power coupled with a microcontroller and stepping motor for providing a periodic current,while the digital syringe pump controls the rise of reaction liquid level in the reaction tube. Due to rise of the reaction liquid level, more surface is immersed to conduct the current, and per area current on wire is declined and this fabricates a conical shaped wire. The periodic current increases from 0.05A to 0.8A in a line and ponders for 5 seconds and then declines to 0.05A. As the periodic current is continuously supplied, a continuous periodic roughness gradient was cultivated along the entire wire. 2

Figure S2: Figure S2. Extreme illustration of continuous HRR and LRR on PCCW. (a)in situ optical picture of a representative periodic roughness-gradient conical copper wire.

(b) SEM images of different sites on the PCCW surface under different magnification. These regions are marked with 1, 2, 3, 4, 5 and 6.

3 Figure S2: Figure S2. Extreme illustration of continuous HRR and LRR on PCCW. (a)in situ optical picture of a representative periodic roughness-gradient conical copper wire. Two dotted squares A and B are used to indicate two continuous periods and squares 1,2,3 are forequarters of A while 4,5,6 are corresponding regions of B. Scale bar, 1mm. (b) SEM images of different sites on the PCCW surface under different magnification. These regions are marked with 1, 2, 3, 4, 5 and 6. Thereinto, 3 and 6 are rough regions while 1,2 and 4,5 are adjacent regions. Scale bars of these three lines, 20μm, 10μm, 3μm respectively. (c) Corresponding periodic current. C 3 > C 2 > C 1, C 6 > C 5 > C 4, accordingly, Ra 3 > Ra 2 > Ra 1, Ra 6 > Ra 5 > Ra 4. (d) Contact angles on different sites whithin one period of PCCW. The contact angle on the HRR is smaller than its right and left LRRs. Scale bar, 1mm. 3

b) Water collection rate of a PCCW and the uniformly rough conical wire (URCW) at different tilt angles. Squares and circles correspond to PCCW and the conical wire, respectively.

4 Figure S3 Figure S3. a) SEM of different position on a single conical wire treated by anodic corrosion. The treatment of anodic corrosion was aimed to make the surface uniformly rough. Scale bar, 5μm. b) Water collection rate of a PCCW and the uniformly rough conical wire (URCW) at different tilt angles. Squares and circles correspond to PCCW and the conical wire, respectively. The average rate of water collection of a PCCW is higher than that of a uniformly rough conical wire. Water collection performance of the URCW is mainly affected by gravity, so the water collection rate is aggrandized with tilt angle increases from 0 to 15. The error bars were obtained from four repeated measurements. (H=90%, T=25 o C, v=1.8 m/s) 4

Figure S4 Figure S4. Velocity distribution on a PCCW at different tilt angles. Velocities of the fisrt drops (from 1.0-1.5cm) are 5.6 cm/s, 7.1 cm/s and 8.

The third drops (from 2.0-2.5cm) move faster than the former since they are bigger in volume and the coalescence energy released promotes the drops motion.

5 Figure S4 Figure S4. Velocity distribution on a PCCW at different tilt angles. Velocities of the fisrt drops (from cm) are 5.6 cm/s, 7.1 cm/s and 8.3 cm/s at tilt angle 15, 5 and 0, respectively, which are faster than the second drops (from cm) with the speed of 2.1 cm/s, 3.6 cm/s and 4.2 cm/s due to the high curvature of the tip. The third drops (from cm) move faster than the former since they are bigger in volume and the coalescence energy released promotes the drops motion. As shown in the figure, due to the interaction of major factors (coalescence energy and gravity), the tendency of drop velocity from 2.0 cm to 3.5 cm is varied at different tilt angle. When the tilt angle is 0, the velocity of drop maintained the same, but it appears to increase when the angle comes to 5 and 15. And the 5 is the medium state as we can see that the velocity of the forth drop (from cm) is close to the third but promoted at the fifth (from cm) when the gravity dominates. The error bars were obtained from four repeated measurements. 5

Figure S5 Figure S5. Optical image of water collecting array. 18 PCCWs are fixxed on Teflon frame (6.5cm 6.5cm 0.5cm). there is a tube collector on the bottom of frame.

6 Figure S5 Figure S5. Optical image of water collecting array. 18 PCCWs are fixxed on Teflon frame (6.5cm 6.5cm 0.5cm). there is a tube collector on the bottom of frame. Wherein the inner side of the Teflon frame is notched as drainage channel, and a small hole is drilled at the bottom to collect water drops which flow through the drainage channel into the tube. 6

7 Table S1: Table S1 The average velocity of drop motion on PCCW at different tilt angles (H=90%,T=15 o C, v=1.8m/s). Tilt angle( ) Average velocity(cm/s) Note: The average velocity refers to the velocity of the first drop moving from the tip to the base of PCCW. The minus sign before angles means that the wire is placed with its tip pointing down. 7

8 Table S2: Table S2. The water collection rate/area of this water collecting PCCW array in different velocities of fog flow at different impact angles (H=90%,T=15 o C). Fog velocity 0.8 m/s 1.2 m/s 1.6 m/s 2.0 m/s 2.4 m/s Impact angle Water collection rate/area (g/cm 2 /h) Note: the impact angle refers to angle between the surface of this water collecting device and fog flow. 8

9 Supplementary Analysis I: In the process of drop coalescence on PCCW, the area of the solid-liquid interface reduced is equal to the area of the solid-gas interface increased. Then the surface energy released in the coalescence process is: = + + = (1) Because the copper wire is hydrophilic, is approximate to 0. Therefore, = - ) = (2) Where r 1 and r 2 refer to the radius of two drops before coalescing, and r 3 refers to the radii of the coalesced drop. During the coalescence process, the total volume of drops remains the same. Hence, + = (3) Then, the equation (1) can be transformed as follow: = = - ( + ) 2/3 ] (4) In order to visually illustrate the impact of pre-coalescing size of r 1 and r 2,we applied the meshgrid function in MATLAB and generated a surface plot of the results. As shown in figure below (Figure S6), within a certain range (smaller than capillary size), only when r 1 and r 2 reach their maximum size, the energy released is the greatest. Namely, the coalescence energy is higher when the coalescing drops are bigger. As shown in Figure S6, the curvature is greater when r 1 and r 2 reach a certain size which means much more energy will be released at this range. In this case, the energy released can propell the drop moving faster. As the drop grows, the balance between r 1 and r 2 is broken (r 1 >>r 2 ), which leads to the decline of coalescence energy. 9

10 Figure S6. MATLAB 3D Surface plot of the coalescence energy function on PCCW. a) 45 side view; b) Side view. The bigger the radii of pre-coalescing drops r 1 and r 2 are, the greater the coalescence energy released is. 10

11 Supplementary Movies: Movie S1: Transportational process of the first three drops on the PCCW (periodic roughness gradient conical copper wire) surface. After the initial deposition stage, the drop 1 started to move when it grew to the critical size of self-propelled movement, and triggered a sequence of drop coalescence. It merged with drop 2 forming drop 1+2 and then drop 1+2 coalesced with drop 3 generating drop 1+2+3, and then the merged drop moved toward the base of the PCCW very quickly. In order to observe this process more clearly, we adjusted the speed to its half in this movie (Play speed: 0.5). Movie S2: Forming process of the first drop on the PCCW surface. Firstly, there were many fog droplets condensed on the surface. The rougher the surface is, the more droplets are condensed. Then these droplets grew up and merged into a drop due to wettability gradient.. Movie S3: Coalescence process of two drops on the PCCW surface. The first drop came close to the second drop, and there was a liquid bridge formed between these two drops. Then two drops became a single drop and it was moved away after a deformation process. After then, a new round of drop would form on the same site (Play speed: 0.2). Movie S4: Water collecting process of single PCCW at different tilt angles. A PCCW was placed in fog with the following tilt angles: 15, 10, 5, 0, -10, -15 and -35, respectively. For each case but -35, firstly, there were several water drops formed on specific sites (HRRs), and then once the drops grew up to the threshold sizes, they moved from the tip to the base of the wire coalescing with the following drops. For tilt angle -35, drops formed on HRRs but they cannot move from the tip to bottom, once they grew into the threshold size, they would just fall off. Movie S5: Water collecting process of a uniformly rough conical wire (URCW) at different tilt angles. A URCW was placed in fog with the following tilt angles: 15, 10, 5 and 0, respectively. Firstly, there was a water film formed on the surface, and when the URCW was placed with its tip pointing upward, the water film moved to the base of the wire under the effect of gravity and formed a drop, once the drop grew up to the threshold size, it fell off. When the wire was placed horizontally, there were several drops formed after continuously fog impacting. These drops could not move but grew up to the threshold size and fell off on the spot, which impeded the refurbish of fog collection recycle. 11

Trichomes. Trichomes. Spines

Trichomes. Trichomes. Spines Supplementary Figures Trichomes Trichomes Spines 50 µm Supplementary Figure S The structure of the trichomes at the ase of cactus spines. These trichomes have a width of approximately 28.6 μm, a thickness