Surface design of superhydrophobic parallel grooves for controllable petal bouncing and contact time reduction

Abstract

This study numerically investigates the bouncing characteristics of impacting droplets on superhydrophobic sub-millimeter parallel grooves by the level-set method. Once the Weber number ( We) is increased to a critical value ( Wec), a unique petal-like droplet bouncing off the parallel grooves without horizontal retraction is found, dramatically reducing the contact time ( tc) by up to ∼75%. Such a bouncing mode is attributed to the rectification of capillary energy stored in the penetrated liquids into upward motion. To achieve controllable petal bouncing, the coupling effects of impact velocity and surface geometric characteristics on tc and Wec are elucidated from the perspective of timescale, momentum, and energy. The numerical results indicate that narrowing the center-to-center spacing contributes to shortening tc and slowing down the growth of tc with We. In contrast, the effect of ridge height is negligible. By establishing the model of emptying time, the relationships of tc with impact velocity and geometric parameters are quantitatively identified. Furthermore, along with the strengthened anisotropic property, a large center-to-center spacing promotes the conversion of horizontal momentum into vertical momentum and suppresses the increment of surface energy, thus inducing the reduction in Wec. Distinct from known anisotropic surfaces in the previous work, the anisotropic property of parallel-grooved surface plays an opposite role in shortening tc. Finally, incorporating the energy balance approach, a semi-empirical model is developed to predict Wec, exhibiting good agreement with present simulation. This work provides physical insights into petal bouncing and inspires the design of textured surfaces to reduce contact time.