Droplet Impact on a Superhydrophobic Surface at Low Weber Numbers

This study investigates the dynamic behavior of water droplets impacting a superhydrophobic surface (SHS) at low Weber numbers (We < 17). SHS is fabricated by a chemical coating process on magnesium AZ31 alloy. The surfaces exhibit a Cassie-Baxter wetting state, showing a contact angle of approximately 155°on the surfaces. Droplet impact on SHS has been extensively studied at low to moderate We, yet the low We regime remains relatively underexplored. Existing theoretical models for the maximum spreading factor (β_max) primarily rely on empirical correlations or scaling laws. We propose an improved theoretical model for β_max incorporating a geometric approximation of the droplet at maximum spread. The model is found to be in good agreement with the present experimental results and compared with other models available in the literature. A comprehensive analysis of droplet morphology reveals the emergence of transient toroidal structures and dual minima in height evolution, providing new insights into the inertial-capillary interplay. We introduce the holding time (t_h), defined as the duration over which a droplet maintains its maximum spread, found to exhibit a decreasing trend with We (t_h = 5.2-2.84 We^0.2). The findings offer more accurate insights into droplet dynamics in the low We regime, which help in designing SHS for applications involving low-energy liquid interactions, such as self-cleaning surfaces and microfluidic devices.