Variation in Droplet Impact Dynamics with Change in Surface Roughness Amplitude

Previous investigations into droplet impact dynamics relied on smooth substrates with the results neglecting the possible effects of surface roughness. As such, the role of roughness amplitude (R_a) on the impact dynamics of a droplet at ambient temperature must be investigated and considered in future studies and experiments. To that end, singular water droplets with constant 2 ± 0.08 mm diameter are released at different heights to understand the change in impact behavior at higher Reynolds number between smooth and rough plates. Glass, PETG, and aluminum plates are used for their different starting wettability. Sets of 4-5 plates with roughness amplitude varying from R_a = 0.003 μm to R_a = 25 μm. This changes the advancing and receding contact angles, θ_ACA and θ_RCA signifying a change in surface wettability. It was found that for hydrophilic surfaces such as glass, the maximum spreading ratio, β_max, increases with an increased in surface roughness. However, aluminum can be made hydrophobic by increasing R_a and generating uniform micro-roughness elements. Lastly, an exact roughness analysis was conducted using a Keyence VK-X3000. The Wenzel roughness ratio was determined and compared to that obtained using the Wenzel formula and measured static contact angles.