Abstract:

Vesicles are membrane-bound compartments that play a central role in biology. Despite recent progress, the dynamics of single- and multi-component lipid vesicles are not fully understood, particularly far from equilibrium where complex nonspherical shapes undergo large deformations in flow. In this talk, I will present recent work from our group on the non-equilibrium dynamics of lipid vesicles in precisely defined flows using the Stokes trap – a new method that enables full 3D control of position and orientation of molecules or particles using active feedback control, without the need for external optical, magnetic, or electric fields. After characterizing equilibrium properties including bending modulus and membrane tension, we study vesicle deformation as a function of dimensionless flow strength (capillary number, Ca) and vesicle deflation (reduced volume). Our results show that vesicles are remarkably deformable, exhibiting reversible shape changes with aspect ratios exceeding 20 in repeated stretch-relax cycles in the bending-dominated regime. Single-component vesicles show a rich variety of shapes and conformations, including asymmetric and symmetric dumbbells, in addition to pearling, wrinkling, and buckling instabilities, depending on membrane properties. Based on these observations, we construct a detailed flow-phase diagram for vesicles in extensional flow, and we further analyze transient stretching and relaxation dynamics. Two distinct relaxation processes emerge for deformed vesicles, including a fast relaxation process corresponding to bending modes and a slow process governed by membrane tension relaxation. Finally, we study vesicle shape dynamics in time-dependent large-amplitude oscillatory flows, revealing three distinct dynamical regimes – pulsating, reorienting, and symmetrical deformations – arising from the competition between flow and membrane deformation timescales. Together, these results provide new insight into flow-driven shape instabilities for lipid vesicles using new methods in flow automation.