Capillary waves with surface viscosity

TL;DR

This paper provides a theoretical analysis of how surface viscosity influences the damping and stability of capillary waves on surfactant-laden interfaces, revealing scale-dependent effects and differences from Marangoni influences.

Contribution

It introduces an analytical model incorporating surface viscosity effects on capillary waves, extending understanding of interfacial damping mechanisms.

Findings

Surface viscosity causes damping of capillary waves.

Critical damping wavelength increases with surface concentration.

Surface viscosity and Marangoni effects influence damping differently.

Abstract

Experiments over the last 50 years have suggested a tentative correlation between the surface (shear) viscosity and the stability of a foam or emulsion. We examine this link theoretically using small-amplitude capillary waves in the presence of a surfactant solution of dilute concentrations where the associated Marangoni and surface viscosity effects are modelled via the Boussinesq-Scriven formulation. The resulting integro-differential initial value problem is solved analytically and surface viscosity is found to contribute an overall damping effect on the amplitude of the capillary wave with varying degrees depending on the lengthscale of the system. Numerically, we find the critical damping wavelength to increase for increasing surface concentration but the rate of increase remains different for both the surface viscosity and the Marangoni effect.