Observing the impact force and low-speed droplet behavior of wet surfaces with very-thin liquid films

The impact of a low-speed droplet on a very-thin liquid film on a solid surface was analyzed experimentally by observing the impact force and droplet behavior. A high impact force and delayed peak impact time were observed with very-thin liquid films (with h/d < 0.05; h is the liquid-film thickness and d is the droplet diameter). On increasing the film thickness, the impact force gradually increased. The increased impact force could be attributed to splash formation on the wet surface arising from lateral-jet formation at the instant of droplet impact, and interactions with the liquid film. Furthermore, this result was in agreement with numerical simulation results of the surface impact-pressure variation which indicated impact-pressure damping immediately after droplet impact, with a subsequent spread of the wet-surface impact-pressure area, thereby causing an increase in the impact force at a later stage. Thus, impact-pressure damping and force increments occurred at different time ranges after droplet impact.