Complex Fluid-Fluid Interface may Non Trivially Dictate Droplet Deformation in an Incipient Flow

TL;DR

This study theoretically investigates how interfacial viscosity influences the deformation and stability of compound drops in linear flows, revealing that interfacial effects can significantly alter emulsion rheology and stability.

Contribution

It introduces a theoretical model showing the impact of interfacial viscosity on drop deformation and emulsion stability, considering surfactant effects and flow conditions.

Findings

Interfacial viscosity reduces drop deformation and stabilizes emulsions.

Increased interfacial viscosity raises the effective emulsion viscosity.

High interfacial dilatational viscosity can destabilize the emulsion.

Abstract

The present study theoretically predicts the effect of interfacial viscosity on the deformation of a compound drop as well as on the bulk rheology. The system at hand comprises of a dilute emulsion of concentric compound drops, laden with surfactants and suspended in a linear flow. Two types of linear flows are considered in this study, namely, a uniaxial extensional flow and a simple shear flow. Presence of surfactants along the drop surface leads to the generation of an interfacial viscosity, which is different from the bulk. This interfacial viscosity generates a viscous drag that along with bulk flow-induced nonuniform surfactant distribution on the drop surface significantly alters drop dynamics. For the present study an asymptotic approach is used to solve the flow field under the limiting case of diffusion-dominated-surfactant transport. Assuming the surfactants to be bulk-insoluble and negligible inertia to be present in fluid flow, it is shown that presence of interfacial viscosity reduces the deformation of a compound drop and enhances the stability of a dilute double emulsion. At the same time the effective viscosity of the emulsion also increases with rise in interfacial viscosity. For large values of interfacial dilatational viscosity the drop deformation is seen to increase and hence the stability of the double emulsion is questionable.