Force of droplet impact on thin liquid films

Inertia-dominated droplet impact transfers momentum to a dry flat target within a short span of time t characterized by (droplet diameter D)/(impact speed U). We investigate experimentally how impact force dynamics change when a droplet hits a thin liquid film of thickness H, less than or approximately equal to the droplet diameter, atop the flat target. Impact force and morphology are recorded simultaneously by piezoelectric force sensing and high-speed video imaging. Compared with a dry surface, the force of droplet impact on a thin liquid film is found to follow the same initial tU/D scaling and reach a slightly higher peak value, but at a significantly later time. Modeling the impact process as a perfect inelastic collision between the droplet and a liquid column of height equal to the film thickness yields the proper timescale (H+D)/U to characterize temporal evolution of the impact force near the inertial peak and through its subsequent exponential decay. The impact crater penetration depth developing within the thin film over the same time span is also found to collapse to a self-similar form based on this characteristic timescale, which attests to the validity of the inelastic collision model in capturing the underlying impact flow physics.