Non-equilibrium deformation and relaxation of giant floppy vesicles in a precisely controlled extensional flow

In this work, we study the non-equilibrium dynamics of single floppy vesicles under large strain rates (~15 s^-1) using a Stokes trap, which is a new technique developed in our lab for controlling the center-of-mass position of multiple particles or single molecules in a free solution. In this way, we directly observe the vesicle shape and conformations as a function of reduced volume,which is a measure of a vesicle’s equilibrium shape departure from sphericity. We observe the formation of asymmetric dumbbell shapes, symmetric dumbbell, pearling, and wrinkling and buckling instabilities for vesicles depending upon the nature of flow and amount of membrane floppiness. We report the precise stability boundary of the flow-based phase diagram for vesicles in Capillary number (Ca)-reduced volume space, where Ca is the ratio of the bending time scale to the of flow time-scale. We further probe the stability boundary at two different viscosity ratios to understand how the onset of instability in vesicles depends on viscosity ratio. We also present results on the long-time relaxation dynamics of vesicles from high deformation back to their equilibrium shapes after the cessation of flow.