(354b) 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 vesicles in precisely-defined steady or time-dependent flow fields using a Stokes trap. Vesicles are membrane-bound soft containers that are often used for triggered release or reagent delivery and play an integral role in key biological processes such as molecular transport in cells. Giant unilamellar vesicles (GUVs) have been used as model systems to study the equilibrium and non-equilibrium dynamics of simplified cells that do not contain a cytoskeleton or polymerized membrane commonly found in cells. A grand challenge in the field of membrane transport lies in understanding how interfacial mechanics and fluid dynamics on the inside or outside of a soft vesicle contributes to the overall shape instabilities. Here, we study the dynamics of single floppy vesicles under large strain rates (~20 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, 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. In some cases, experimental results are compared to the recent boundary-integral simulations [Narsimhan et al. Journal of Fluid Mechanics 750, 144-190]. We further probe the stability boundary at two different viscosity ratios to understand how the onset of asymmetric 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 spheroidal shapes after the cessation of flow. We study vesicles with shapes ranging from symmetric to asymmetric dumbbells with a long thin tether (extremely large fractional extensions with flattened thermal fluctuations), and we report on the influence of initial conditions in determining dynamic behavior. We further discuss the possibility of estimating the bending modulus of lipid membranes from the transient shape of vesicles during the relaxation process. Overall, our results provide new insights into the flow-driven shape-instabilities for vesicles which has been achieved using new experimental methods involving the Stokes trap and related precise control over the center-of-mass position of vesicles, resulting in observation times on order of the time required for instabilities to form.