Viscous resuspension of droplets

TL;DR

This study investigates the behavior of buoyant droplets under shear in an emulsion, revealing that normal viscosity driving droplet migration is largely unaffected by droplet deformation and varies with volume fraction.

Contribution

It provides direct measurements of normal stress in droplet suspensions and links these to the suspension balance model, highlighting the role of volume fraction over droplet deformation.

Findings

Normal viscosity is independent of capillary number.

Viscous resuspension behavior resembles rigid particles at low volume fractions.

Normal viscosity is significantly smaller at high volume fractions.

Abstract

Using absorbance measurements through a Couette cell containing an emulsion of buoyant droplets, volume fraction profiles are measured at various shear rates. These viscous resuspension experiments allow a direct determination of the normal stress in the vorticity direction in connection with the suspension balance model that has been developed for suspensions of solid particles. The results unambigously show that the normal viscosity responsible for the shear-induced migration of the droplets is independant on the capillary number, implying that particle deformation does not play a great role. It is similar to that of rigid particles at volume fractions below 40\% but much smaller at higher ones.