(490j) Giant Unilamellar Vesicle Dynamics in Oscillatory Extension

Vesicles are widely used as systems for studying cell-like dynamics, and their dynamics in simple shear and extensional flows have been thoroughly studied. However the flow types present in microfluidic devices or biological systems are not always described by those flows alone. Firstly, we present our work on the nonlinear dynamics of vesicles in large amplitude oscillatory extensional (LAOE) flows using both experiments and boundary integral (BI) simulations. Our results characterize the transient membrane deformations, dynamical regimes, and stress response of vesicles in LAOE in terms of reduced volume (vesicle asphericity), capillary number (Ca, dimensionless flow strength), and Deborah number (De, dimensionless flow frequency). Results from single vesicle experiments are found to be in good agreement with BI simulations across a wide range of parameters. Our results reveal three distinct dynamical regimes based on vesicle deformation: pulsating, reorienting, and symmetrical regimes. We construct phase diagrams characterizing the transition of vesicle shapes between pulsating, reorienting, and symmetrical regimes within the two- dimensional Pipkin space defined by De and Ca. Contrary to observations on clean Newtonian droplets, vesicles do not reach a maximum length twice per strain rate cycle in the reorienting and pulsating regimes. The distinct dynamics observed in each regime result from a competition between the flow frequency, flow time scale, and membrane deformation timescale. By calculating the particle stresslet, we quantify the nonlinear relationship between average vesicle stress and strain rate. Additionally, we present results on tubular vesicles that undergo shape transformation over several strain cycles. Broadly, this work provides new information regarding the transient dynamics of vesicles in time-dependent flows that directly informs bulk suspension rheology. We will also present some preliminary work on the dynamics of multicomponent vesicles.