Deformation and relaxation of viscous thin films under bouncing drops

TL;DR

This study investigates how viscous thin films deform and relax after impact by water drops, using high-resolution microscopy and scaling laws, revealing detailed dynamics and underlying physical mechanisms.

Contribution

It provides a detailed experimental and theoretical analysis of oil film deformation and relaxation during water drop impact, including new scaling laws and high-resolution surface measurements.

Findings

Deformation depends on film thickness, viscosity, and impact speed.

Scaling laws accurately describe the broadening and decay of perturbations.

Lubrication analysis matches experimental results.

Abstract

Thin, viscous liquid films subjected to impact events can deform. Here we investigate free surface oil film deformations that arise due to the air pressure buildup under the impacting and rebouncing water drops. Using Digital Holographic Microscopy, we measure the 3D surface topography of the deformed film immediately after the drop rebound, with a resolution down to 20 nm. We first discuss how the film is initially deformed during impact, as a function of film thickness, film viscosity, and drop impact speed. Subsequently, we describe the slow relaxation process of the deformed film after the rebound. Scaling laws for the broadening of the width and the decay of the amplitude of the perturbations are obtained experimentally and found to be in excellent agreement with the results from a lubrication analysis. We finally arrive at a detailed spatio-temporal description of the oil film deformations that arise during the impact and rebouncing of water drops.