Vesicles are membrane-bound compartments that play a key role in biological processes. In recent years, society has witnessed the enormous impact of research in lipid-based RNA delivery with the deployment of the COVID-19 vaccine. Despite recent progress, we do not yet fully understand how lipid membrane properties affect the mechanics and transport properties of vesicles. In this work, we study the dynamics of lipid vesicles in precisely defined flows using the Stokes trap, which is a new method to manipulate single molecules and particles using automated flow control. The Stokes trap allows for precise control over position and orientation of single and multiple particles in 2D and 3D using active feedback control - without the need for external optical or electrical fields. Using this approach, we study non-equilibrium vesicle shapes and deformation as a function of dimensionless flow strength (capillary number, Ca) and vesicle deflation (reduced volume) using fluorescence microscopy. Vesicles are found to be remarkably deformable objects that undergo reversible deformation in the bending-dominated regime, with deformed aspect ratios >20 in repeated stretch-relax cycles. Vesicles deform through a wide range of conformations in flow, including asymmetric and symmetric dumbbells, in addition to pearling, wrinkling, and buckling instabilities depending on membrane properties. We determine a precise flow-phase diagram for lipid vesicles using the Stokes trap, and we study the transient stretching dynamics of vesicles in extensional flow. We further identify two distinct relaxation processes for deformed vesicles, revealing two characteristic time scales: a short timescale corresponding to bending modes and a long timescale dictated by the relaxation of membrane tension. Finally, we study vesicle shape dynamics in time-dependent oscillatory flows, revealing three distinct dynamical regimes – pulsating, reorienting, and symmetrical deformations – that arise due to the competition between flow and membrane deformation timescales. Overall, our results provide new insights into flow-driven shape instabilities for lipid vesicles using new experimental methods.
