Trichomes. Trichomes. Spines

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Aileen Leonard
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1 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 of approximately.4 μm, and a length of approximately μm.

2 a Width (μm) S S2 S S4 S5 S6 S7 S8 S9 S0 Sample Decreasing width of microgrooves 200 μm 50 μm c Distance (%) Roughness gradient Sample Sample μm 50 μm 200 μm 50 μm Smaller roughness Larger roughness 80% 70% 60% 50% 40% 0% Supplementary Figure S2 Statistical measurements of the gradient microgrooves along the cactus spines. a, Ten cactus spines were selected randomly; for each spine, the average widths of the microgrooves at 80%, 70%, 60%, 50%, 40% and 0% of the entire spine length away from the tip were measured. From the region near the ase to the location near the tip (80% to 0%), the width of these microgrooves decreased., Three representative cactus spine SEM images (Sample, Sample 2 and Sample ) illustrate the width gradient of the microgrooves along the cactus spines. For each spine, the lower six images correspond with the six regions (80%, 70%, 60%, 50%, 40% and 0% of the entire spine length away from the tip) on the upper single spine. The microgrooves near the ase are wider than those near the tip. c, The gradient of the microgrooves in the middle of the spine gives rise to a gradient of roughness along the spine; the roughness near the ase is smaller than at the top. 2

a Multi level grooves () Wider microgrooves (c)

Narrower microgrooves 20 μm e Su microgrooves 2

of the multi-level grooves along the cactus

The tip was covered with oriented ars, while

and was divided into two regions () and (c)

-c, Magnified images of regions () and (c).

spine near the ase was larger than the width

d-e, Magnified images of the areas depicted

3 a Multi level grooves () Wider microgrooves (c) c 20 μm d Su microgrooves 2 μm Bars 00 μm Narrower microgrooves 20 μm e Su microgrooves 2 μm Supplementary Figure S Detailed oservations of the multi-level grooves along the cactus spines. a, SEM image of a spine. The tip was covered with oriented ars, while the middle was covered with multi-level grooves and was divided into two regions () and (c) with different grooved structures. -c, Magnified images of regions () and (c). The width of the microgrooves on the cactus spine near the ase was larger than the width near the tip. d-e, Magnified images of the areas depicted with the lue and yellow oxes in and c, respectively. The su-micron grooves near the ase and tip demonstrated no significant differences.

a c d e 2 7.4 s 49.95 s 64.24 s 64.2 s f g h i j 2 64.6 s 2 2 k 64.57 s 64.72 s 64.96 s 69.95 s 500 µm 70.

4 a c d e s s s 64.2 s f g h i j s 2 2 k s s s s 500 µm s Supplementary Figure S4 In situ optical microscopic oservation and an illustration of the asorption process of the water drops on two adjacent spines with clustered trichomes at the ase. a, Two water drops denoted as and 2 were collected on the two adjacent spines and moved toward the ase of the spines with increasing size (-c). d-g, As water drop contacted the trichomes, an immediate asorption occurred within a half second (from 64.2 s to s). A new collection cycle egan with the appearance of drop. h-j, Water drop 2 also moved along the spine to e asored y the same trichomes. k, The scheme illustrates the ehaviour of the water drops on adjacent spines. These water drops were quickly asored upon contact with the trichomes. 4

a c d e f 2 2.9 s 56.77 s 06.6 s 06.29 s 06.4 s g h 2 4 4 4 4 4 i j 07.4 s 07.64 s 07.

77 s k Coalescence Supplementary Figure S5 In situ optical microscopic oservation and an

adjacent spines with clustered trichomes at the ase.

transported toward the ase of the spines with increasing size ().

e-j, The coalesced water drop was quickly asored upon contact with the trichomes.

5 a c d e f s s 06.6 s s 06.4 s g h i j 07.4 s s s s 500 µm s k Coalescence Supplementary Figure S5 In situ optical microscopic oservation and an illustration of the coalescence and the asorption process of the water drops on two adjacent spines with clustered trichomes at the ase. a, Two water drops (denoted as and 2) were collected on two adjacent spines and transported toward the ase of the spines with increasing size (). c-d, Water drops and 2 were located close enough to coalesce into larger drop. e-j, The coalesced water drop was quickly asored upon contact with the trichomes. A new collection cycle egan with the appearance of drop 4. k, The scheme illustrates the movement ehaviour of the water drops on adjacent spines. These water drops coalesced to form larger drops that were susequently asored upon contact with the trichomes. 5

6 a CA =. CA = 26.5 Supplementary Figure S6 Typical optical images of two sessile water drops on the regions near the ase and the tip of a cactus spine. The water contact angle near the ase (a) and the tip () is approximately. and 26.5, respectively. 6

a 2R 2 2R 2R2 2R c Top View d r 2 r r 2 r Supplementary Figure S7 Comparison of the gradient of the Laplace pressure along the smooth and the grooved

The circular-arc-shaped red lines denote the corresponding three-phase contact lines (TCLs). 2R and 2R 2 are the local diameters of the spines.

The local curvature radii r (r 2 ) and r (r 2 ) of the TCLs on the smooth surface and the gradient grooved surface with r < r, r 2 < r 2.

7 a 2R 2 2R 2R2 2R c Top View d r 2 r r 2 r Supplementary Figure S7 Comparison of the gradient of the Laplace pressure along the smooth and the grooved conical spines. Water drop sites on the conical spine with a smooth surface (a) and gradient grooves (). The circular-arc-shaped red lines denote the corresponding three-phase contact lines (TCLs). 2R and 2R 2 are the local diameters of the spines. c-d, Top-view images of the liquid-solid contact area. The lue dashed lines in (d) correspond with the red lines in (c). The local curvature radii r (r 2 ) and r (r 2 ) of the TCLs on the smooth surface and the gradient grooved surface with r < r, r 2 < r 2. Assuming the very small liquid-solid contact area to e a flat surface, as shown y the green colour in the Top View images, the Laplace pressure difference of this small area on a gradient grooved surface γ ( ) r ' r ' is 2 larger than that on a smooth surface γ ( ) r r. 2 7

a c Smooth Parallel to grooves d Rugged Perpendicular to grooves Supplementary Figure S8 Anisotropic hysteresis of the water drops moving on the grooved cactus spines.

8 a c Smooth Parallel to grooves d Rugged Perpendicular to grooves Supplementary Figure S8 Anisotropic hysteresis of the water drops moving on the grooved cactus spines. a, A water drop moving on the grooved cactus spine., Magnified region near TCL indicated in (a) y a red rectangle. The red and lue arrows denote the motion parallel and perpendicular to the grooves, respectively. c-d, the motion parallel to the grooves had a relatively small hysteresis that was identical to that on a smooth surface (c). The motion perpendicular to the grooves had an increased hysteresis that was identical to that oserved for a rugged surface (d). 8

9 a c d Water Supplementary Figure S9 Schematic diagrams of the application of a io-mimicking artificial continuous fog collection system with different outer contours and surface structures. a-, Two types of artificial fog collection systems. Clustered conical spines dispersed evenly on the ellipsoid with the hemispherical structures covered (a) and on the sphere with the ridges covered (). c, Magnified single cluster of spines with conical ars on the tip side, gradient grooves on the middle side, and elt-structured trichomes on the ase side. d, A sapling living in the arid environment can otain water supply from the artificial fog collection system suspended from the sapling. 9

10 Supplementary Note Background and design principles of the artificial fog collector. Fog has een widely recognised as an important alternative water source in many regions worldwide. Bio-inspired artificial fog collectors have een extensively studied, and many products, such as the two-dimensional planar collector 6, the standard fog collector 7 and the three-dimensional omnidirectional fog collector 8, have een developed. Water drops collected on these collectors rely primarily on the force of gravity to convey the water to the containers attached elow. The openings of the water containers designed for catching the water drops may also increase the risk of evaporation. Inspired y the integral fog collection system of the cactus O. microdasys, we designed a three-dimensional, omnidirectional fog collector y introducing the main characteristics of the cactus without considering the oundary layer prolems. The fog collector ased on this asic idea could collect water drops from fog, directing the water to a container without the use of gravitational force or wind velocity. In addition, the water container emedded in the collector did not require a large opening, decreasing evaporation. A consideration of the oundary layer 40-4 on the surface of the artificial fog collector indicated that a smaller size of the construction would have een a etter choice. 0

11 Supplementary References 40 Martorell, C. & Ezcurra, E. The narrow-leaf syndrome: a functional and evolutionary approach to the form of fog-harvesting rosette plants. Oeologia 5, (2007). 4 Ritter, A. & Regalado, C. M. Fog water collection in a sutropical Elfin Laurel Forest of the Garajonay National Park (Canary Islands): a comined approach using artificial fog catchers and a physically ased impaction model. J. Hydrometeorol. 9, (2008).

Supporting Information

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