Biased Motions of a Droplet on the Inclined Micro-conical Superhydrophobic Surface

Abstract

Anisotropic superhydrophobic surfaces that have many superior properties, such as directional self-cleaning, droplet transport, heat transfer, and so on, are widely used in various fields. Different from symmetric surfaces, a water droplet often shows directional spreading, moving, and bouncing on asymmetric surfaces. To investigate the mechanisms and achieve controllability of droplet motions on asymmetric surfaces, a series of surfaces with inclined micro-conical arrays are fabricated by integrating the methods of soft lithography, hot-pressing, and crystal growth. We found that the droplet would spread along the reverse direction of micro-cone's orientation but bounce and detach off the surface and move toward the direction of micro-cone's orientation. To understand these interesting performances, a mathematical model is established from the perspective of force balance, and a series of numerical simulations are performed. Additionally, the relationship between the droplet motions and the micro-structural parameters, including the inclined angle, line space, and height, are studied. This work may provide useful insights into droplet controlling, anisotropic surface designing, and its applications.

Droplet impact on asymmetric hydrophobic microstructures

“…The directional displacement is observed only for V 0 > 0.5 m/s and on P = 30 µm. This is similar to the observation made by Li et al 45 They also observed a larger horizontal displacement on arrays of inclined cones with a smaller spacing.…”

“…9(a), smaller displacement and velocity are observed. Li et al 45 reported that on inclined cone structures with H ∼ 300 µm and P ∼ 300 − 400 µm , the displacement is 0.7 mm for V 0 = 0.71 m/s and 2.0 mm for V 0 = 1.73 m/s. The horizontal rebound distance is similar to this study.…”

“…On such hydrophobic surfaces, upon impact, droplets bounce off towards the direction in which the surface structures are oriented. 37,[42][43][44][45] However, the detailed mechanisms of bouncing are not fully understood. In particular, the influence of the surface geometry, i.e., the pitch and height of structural features, and the impact velocity remain to be fully elucidated.…”

Droplet Impact on Asymmetric Hydrophobic Microstructures

“…For a smaller structure where the rebound mechanisms are described in Figure 9 a, smaller displacement and velocity are observed. Li et al 49 reported that on inclined cone structures with H ∼ 300 μm and P ∼ 300–400 μm the displacement is 0.7 mm for V 0 = 0.71 m/s and 2.0 mm for V 0 = 1.73 m/s. The horizontal rebound distance is similar to this study.…”

Efficient fabrication of tilt micro/nanopillars on polypropylene surface with robust superhydrophobicity for directional water droplet rebound

“…Similarly, Li et al. ( Li et al. (2021) also utilized a combination of soft lithography and crystal growth techniques to prepare microcone-structured array surfaces with different tilt angles.…”