Spreading Characteristics of a Single Water Droplet Impinging on Hydrophobic Micro-Textured Metal Surfaces

We reports on extensive experimental investigation on the impact dynamics of a single water droplet impinging on micro-textured aluminum (Al 6061) surfaces fabricated by micro computer numerical control (μ-CNC) milling machine. The texture surfaces are composed of micro-holes (diameter: 125 , depth: 125 ) with a texture area fraction ((πd0 2/4)/(s+d)2, Φs) ranging from 0 to ~0.568. We measured apparent contact angles to examine wetting characteristics of the textured metal surfaces with different texture area fractions by using a de-ionized (DI) water droplet of 4.3 μl which impinges on textured surfaces with varying impact velocity from 0.0 to ~1.45 m/s (which are limited to low Weber number region). After impact, we measured spreading diameter and dynamic contact angle with time, and estimated a maximum spreading diameter by using a high-speed camera and the image analysis. It was found that for designed textured surfaces, equilibrium contact angle(CA) increased up to 125.8°, compared to bare surface (CA ~ 80.6°), and the spreading rate decreased with the increase in the texture area fraction. In addition, we found that at lower Weber number, the texturing effects would be more prominent, and increase dynamic contact angle during impact process.