High-speed impact of water droplets on microtextured surfaces: Effect of roughness and wettability on corona splashing

Abstract

A study of the high-speed impact of water droplets on smooth and microtextured fluoropolymer-coated titanium surfaces is presented. The experimental samples had an average roughness R_a from 0.04 μm to 15.4 μm and a static contact angle θ from 74° to 164°. The 0.5–1.3-mm droplets were impacted on the surfaces at velocities U₀ = 5–20 m/s (the Weber number We = 450–2,800). Using a high-speed video camera with a sample rate of 60,000 frames per second, the values of the opening angle α, the maximum diameter D_cor, and the lifetime of the corona were measured and analyzed. In addition, the mean splashing velocities of both large and small secondary fragments were captured. A dimensionless ratio, α/θ, which characterizes the predominance of inertial or adhesive forces, was proposed for the development of an empirical model for predicting D_cor. This model was validated using data from other authors, which proved its applicability in the ranges of We = 450–2,800, R_a = 1.05–38 µm, θ = 69–164° (water) and θ $\approx$ 0° (ethanol). The research elucidated that superhydrophobic microtextured surfaces provide greater symmetry in corona splash and a larger opening angle. However, these surfaces also delayed liquid removal during splashing, which has the potential to impact the effectiveness of their water-repellent properties.