(268j) Dynamics, Rheology, and Breakup of Droplets with Complex Interfaces – Role of Interfacial Viscosity and Bending Resistance

There is a lot of interest in characterizing the mechanics of droplet-like systems with interfaces that cannot be solely described by surface tension. Some examples include red blood cells, vesicles, pickering emulsions, and asphaltenes. In this talk, we discuss some of our recent work on such systems. The first part of the talk discusses droplets with a thin layer of viscous, insoluble surfactant whose mechanics are described by interfacial viscosity, i.e., a Boussinesq-Scriven constitutive law. We develop analytical theories to quantify droplet shape under flow in the limit of weak deformation, to a sufficient level of approximation where one can extract information about non-linear rheology and droplet breakup. In shear flow and extensional flows, we calculate how interfacial viscosity alters the extra stress of a dilute suspension of droplets, in particular the shear stress, extensional stress, and normal stress differences. We also investigate how shear and dilatational viscosities influence droplet breakup and droplet migration in wall-bounded shear flow. These theories highlight the extent to which surface viscosity alters droplet dynamics, and we conclude by discussing how one can extend our theories to include effects such as surface tension gradients and viscoelastic surfaces. In the last part of the talk, we present some experimental and simulation results of our work on another droplet-like system with a complex interface -- giant unilamellar vesicles, i.e., droplets of ~20 microns with a phospholipid membrane that exhibits dilatational and bending resistance. We show that the mechanical stability of these systems depend intimately on flow type and flow history, and that the presence of a phospholipid membrane gives rise to dynamical shapes that are very different than that of a clean droplet.