Drop spreading with random viscosity

TL;DR

This paper models the spreading of a viscous drop with heterogeneous, solute-dependent viscosity over a thin film, revealing how initial solute distribution affects spreading dynamics through combined numerical and asymptotic analysis.

Contribution

It introduces a theoretical framework combining lubrication theory, simulations, and asymptotics to analyze how random viscosity variations influence drop spreading.

Findings

Initial solute heterogeneity impacts spreading rate.

Accumulation of solute in the precursor film hinders spreading.

Explicit variance predictions validated by simulations.

Abstract

We examine theoretically the spreading of a viscous liquid drop over a thin film of uniform thickness, assuming the liquid's viscosity is regulated by the concentration of a solute that is carried passively by the spreading flow. The solute is assumed to be initially heterogeneous, having a spatial distribution with prescribed statistical features. To examine how this variability influences the drop's motion, we investigate spreading in a planar geometry using lubrication theory, combining numerical simulations with asymptotic analysis. We assume diffusion is sufficient to suppress solute concentration gradients across but not along the film. The solute field beneath the bulk of the drop is stretched by the spreading flow, such that the initial solute concentration immediately behind the drop's effective contact lines has a long-lived influence on the spreading rate. Over long periods, solute swept up from the precursor film accumulates in a short region behind the contact line, allowing patches of elevated viscosity within the precursor film to hinder spreading. A low-order model provides explicit predictions of the variances in spreading rate and drop location, which are validated against simulations.