Engulfment of a drop on solids coated by thin and thick fluid films

TL;DR

This study investigates the complex interfacial dynamics of an aqueous drop contacting an oil film, revealing how inertial and viscous effects influence spreading and engulfment behaviors through simulations and experiments.

Contribution

It combines 3-phase Lattice-Boltzmann simulations with experiments to analyze the effects of inertia and viscosity on drop engulfment on coated solids, highlighting the role of the Ohnesorge number and film thickness.

Findings

Spreading radius scales as T^{1/2} for low Oh and T^{2/5} for high Oh.

Apparent spreading is independent of film height.

Engulfment dynamics depend on the ratio H/R.

Abstract

When an aqueous drop contacts an immiscible oil film, it displays complex interfacial dynamics. Upon contact the oil spreads onto the drop's liquid-air interface, first forming a curvature that drives an apparent drop spreading motion and later fully engulfing the drop. We study this flow using both 3-phase Lattice-Boltzmann simulations based on the conservative phase field model and experiments. Inertially and viscously limited dynamics are explored using the Ohnesorge number $Oh$ as a function of $R/H$, the ratio between the initial drop radius $R$ and the film height $H$. Both regimes show that the apparent spreading radius $r$ is fairly independent of the film height, and scales with time $T$ as $r\sim T^{1/2}$ for $Oh\ll 1$ and for $Oh\gg 1$ as $r\sim T^{2/5}$. For $Oh\gg 1$ we show experimentally that this immiscible apparent spreading motion is analogous with the miscible drop-film coalescence case. Contrary to the apparent spreading, however, we find that the late time engulfment dynamics and final interface profiles are significantly affected by the ratio of $H/R$.