Research ArticleMATERIALS SCIENCE

Compact nanoscale textures reduce contact time of bouncing droplets

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Science Advances  17 Jul 2020:
Vol. 6, no. 29, eabb2307
DOI: 10.1126/sciadv.abb2307
  • Fig. 1 Examples of water-repellent insects equipped with high solid fraction nanoscale surface textures.

    (A) Optical and scanning electron microscopy (SEM) images of mosquito eyes, a springtail, and cicada wings showing the presence of high solid fraction nanoscale surface textures (Photo credit: L.W., Pennsylvania State University). (B) A plot summarizing the solid fraction Φs and the corresponding texture size D for various water-repellent insects (4, 7, 8, 37, 38). Note that the solid fraction of different insect surfaces is in the range of ~0.25 to ~0.64, which is substantially higher than that of the plant surfaces (e.g., Φs ~ 0.01). Error bars indicate SDs for five independent measurements.

  • Fig. 2 Comparison of contact time of bouncing water droplets on textured surfaces.

    (A) Time-lapse images of bouncing water droplets (diameter d0 ~ 2.3 mm, Weber number We ~ 31.6) on surfaces with solid fraction Φs = 0.44. The droplet detached faster from ~100 nm textures than the one from ~300 nm textures. D denotes the texture cap size of each reentrant pillar, and tc denotes contact time. (B) Identical drop impact experiments on surfaces with solid fraction Φs = 0.25. Droplets detached simultaneously from both surfaces. Insets showing the SEM images of fabricated nanoscale reentrant textures. Scale bars in all SEM images, 200 nm; scale bar in the optical image, 1 mm. See also movies S1 and S2.

  • Fig. 3 Experimental measurements and theoretical predictions of contact time of bouncing water droplets on textured surfaces.

    (A) Comparison between the measured contact time tc (scatter dots) of bouncing droplets and those predicted by Eq. 2 (dash lines) on textured surfaces. The purple circles and the orange triangles represent the measured contact time obtained from surfaces with solid fractions Φs = 0.25 and Φs = 0.44, respectively. Error bars represent the SDs of three independent measurements. (B) A phase map showing the ratio (E*) of line energy to surface energy as a function of texture size and solid fraction. The line energy begins to dominate over the surface energy in dictating the macroscopic surface wettability when the surface texture size approaches ~100 nm at Φs > 0.25.

  • Fig. 4 Droplet bouncing and wetting kinematics on textured surfaces.

    (A) Spreading and receding kinematics of bouncing water droplets on various surfaces. Black silicon with Φs ~ 0.01 showed a superhydrophobic bouncing behavior with a constant receding velocity. Surfaces with 150 and 400 nm textures at Φs = 0.25 showed a characteristic bouncing behavior in which the receding velocity reached a plateau at ~10 to 12 ms. The surface with 300 nm textures at Φs = 0.44 showed a similar bouncing behavior in which the receding velocity reached a plateau at ~11 to 14 ms, while the surface with 100 nm textures at Φs = 0.44 showed a superhydrophobic bouncing behavior with a constant receding velocity similar to that on the black silicon. (B) Comparison of the receding angle θr between surfaces with texture sizes of ~100 and ~300 nm at Φs = 0.44 during the receding process. Scale bar, 2 mm. (C) Measured contact angle hysteresis on surfaces with texture size ranging from ~100 nm to 30 μm and Φs = 0.25 or 0.44. Error bars indicate SDs for three independent measurements. Our measured contact angle hysteresis data are in good agreement with those reported in the literature (11, 31).

  • Fig. 5 Pressure stability of reentrant textured surfaces against impacting raindrops.

    (A) A phase map showing the pressure stability of reentrant textured surfaces against impacting raindrops as a function of texture size and solid fraction. To repel impacting raindrops, it requires a sufficient capillary pressure PC on textured surfaces to withstand the raindrop hammer pressure PH. P* is defined as the ratio between PC and PH, i.e., P* = PC/PH. Note that the textured surfaces are pressure stable when texture size D is small at high solid fraction Φs. It is shown that all the geometrical parameters of the surface textures on water-repellent insects fall within or near the pressure stable regime. (B) Experimental results showing droplets impacting on reentrant textured surfaces with different geometrical parameters (movies S3 to S6). Water droplets with terminal velocity ~4.0 m/s impacted the reentrant pillars, resulting in a water hammer pressure PH ~ 1.2 MPa and We ~ 505.5. The surface with texture size of 200 nm and solid fraction of 0.44 was able to maintain the droplet at the Cassie-Baxter state (solid star symbol), while the droplets on other surfaces were in partial Wenzel state (empty star symbols). Scale bar, 2 mm.

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