|Abstract:||Giant unilamellar vesicles (GUVs) composed of lipid bilayer membranes have emerged as a biomimetic substitute for investigating the non-equilibrium flow properties of biological cells. Moreover, vesicle suspensions are increasingly being used for advanced triggered release and drug delivery applications in functional materials. Prior work on vesicle dynamics in flow have largely focused on nearly-spherical membrane shapes in the low deformation regime. Departures from nearly spherical shape in cells and organelles are pervasive in biology. From the dynamic tubular shape of mitochondrial membranes to the biconcave shape of human red blood cell, membrane conformations reflect a major shift in emphasis from weakly deformed nearly-spherical shapes to strongly deformed highly anisotropic shapes. Thus, a comprehensive understanding of out-of-equilibrium dynamics of biological cells in flow, and stability of vesicle suspensions for functional material design, ultimately relies on incorporating the impact of membrane shape anisotropy on vesicle mechanics. Despite recent progress, there is a lack of fundamental understanding of how membrane shape anisotropy affects the non-equilibrium stretching dynamics of vesicles in strong flows.
In this thesis, we focus on investigating the dynamics of lipid vesicles across a wide variety of reduced volume (shape anisotropy), viscosity ratio and flow-strength using a combination of fluorescence microscopy and automated flow control under highly nonequilibrium conditions. In this way, we aim to address several fundamental questions such as (i) How do anisotropic vesicles deform and change their conformations in flow, (ii) How do vesicles deform in a time-dependent, large amplitude oscillatory extensional flow ? (iii) How do tubular and biconcave vesicles stretch and relax in strong flows ? and (iv) What is the impact of reduced volume on membrane material properties such as bending modulus and surface tension ? Importantly, answers to these question will provide a mechanistic understanding of the influence of reduced volume on vesicle shape dynamics in strong flows. In the first project, we directly observed the non-equilibrium dumbbell conformations of vesicles as a function of reduced volume, dimensionless flow strength (capillary number), and viscosity contrast using automated flow control. Moreover, we precisely characterized the flow-phase diagram of vesicle shape conformational change to dumbbell shapes in reduced volume-capillary number space.
While the vast majority of single vesicle studies on nearly spherical shapes has exclusively focused on steady linear flows, there is a clear need to implement more complicated time-dependent flow fields to reveal the membrane shape dynamics in oscillatory flow. In the second project, we studied the dynamics of nearly spherical and tubular vesicles under large amplitude oscillatory extension (LAOE) using automated flow control. By combining microfluidic experiments with boundary integral simulations, we uncovered three dynamical regimes of vesicle transient stretching dynamics in LAOE flow. In the third project, we directly observed the relaxation dynamics of highly deformed dumbell shaped vesicles back to their equilibrium morphology, and our results show that membrane relaxation is governed by a double-mode exponential decay, revealing two characteristic time scales: a short time scale corresponding to long-wavelength bending relaxation and a long-time scale governed by the membrane tension. Moving forward, we characterize the non-equilibrium stretching dynamics of vesicles, including transient and steady-state dynamics in extensional flow. Steady state stretching data is analyzed in the context of a membrane mechanical model in order to determine the bending modulus and membrane tension of vesicles as a function of reduced volume. Unexpectedly, our results show that bending modulus is a strong function of reduced volume, and is independent of the viscosity ratio.
In the fifth project, we demonstrated robust control over the two-dimensional center-of-mass position and orientation of anisotropic Brownian particles using only fluid flow. Moreover, we implement a path-following model predictive control scheme to manipulate colloidal particles over defined trajectories in position space, where the speed of movement along the path is a degree of freedom in the controller design. We further explored how the external flow field affects the orientation dynamics of anisotropic particles in steady and transient extensional flow using a combination of experiments and analytical modeling. Overall, this thesis aims to provide a fundamental understanding of the effect of reduced volume, flow strength, viscosity ratio and flow type on vesicle membrane dynamics in flow. On a broader perspective, this dissertation develops a systematic understanding of membrane conformations and vesicle dynamics under flowing conditions with a combination of experiments, simulations and modeling which will provide insights for answering key questions related to material processing with improved functionalities.