The combined influence of a rough surface and thin fluid film upon the splashing threshold and splash dynamics of a droplet impacting onto them

Surface roughness can have a critical effect upon the splashing threshold and dynamics of a drop impacting on either a dry or rough solid surface or one coated by a thin fluid film. As most coating applications and spray systems quickly evolve to a state where the droplets impinge upon fluid deposited by preceding droplets, the combined contributions of surface roughness and a pre-deposited thin liquid film of comparable thickness upon droplet impingement dynamics are examined. For comparison, we include results for droplets impacting on a smooth, dry surface and a smooth surface wetted by a thin fluid film. The inclusion of surface roughness considerably lowers the splashing threshold and alters the splashing dynamics such that differences in fluid surface tensions between 20.1 and 72.8 dynes/cm or viscosities between 0.4 and 3.3 cP have little effect.