(381e) Transient Stretching and Relaxation of Highly Deformed Vesicles in Strong Flows Using a Stokes Trap

Vesicles are membrane-bound soft containers that play an integral role in key biological processes. In this work, we study the non-equilibrium stretching and relaxation dynamics of giant unilamellar vesicles in precisely defined steady and time-dependent extensional flow. In particular, we use a Stokes trap to control the position and time-dependent strain and strain rate schedules applied to single vesicles in flow. In this way, we directly observe non-equilibrium vesicle shapes as a function of reduced volume, viscosity contrast, and Capillary number using fluorescence microscopy. Vesicles are found to deform through a wide range of interesting shapes in flow, including asymmetric and symmetric dumbbells, in addition to pearling, wrinkling, and buckling instabilities depending on membrane properties. Using this approach, we study the non-equilibrium stretching dynamics of vesicles, including transient and steady state stretching dynamics in extensional flow. Our results show that vesicle stretching dynamics are a strong function of reduced volume and viscosity ratio, such that the steady-state deformation of vesicles exhibits power-law behavior as a function of reduced Capillary number. We identify two distinct relaxation processes for vesicles stretched to high deformation, revealing two characteristic time scales: a short time scale corresponding to bending relaxation and a long-time scale dictated by the membrane tension. We further discuss a model to estimate the bending modulus of lipid membranes from the steady-state stretching data. Overall, our results provide new insights into the flow-driven shape-dynamics for vesicles using new experimental methods based on the Stokes trap.