(334bw) Dynamics of Vesicles in Strong Flows Using a Stokes Trap

Research Interests - My Ph.D. thesis research focuses on: (i) developing new tools for trapping and manipulating micro and nanoparticles in free solution using only fluid flow, and (ii) understanding the physics and emergent properties of vesicle suspensions using these control-based techniques. To this end, I have developed a millisecond time nonlinear model predictive control algorithm with real-time image processing (written in C++, LabVIEW and MATLAB) to control the center-of-mass and orientation of single and multiple anisotropic particle using only fluid flow. In tandem, I study the dynamics of lipid vesicles using fluidic trapping tools and advanced statistical tools. My research has direct implications to the processing of food active ingredient encapsulants, and personal care products such as shampoos and detergents. Overall, my Ph.D. research entails a distinct combination of numerical, experimental and theoretical/modeling skills involving fluid dynamics, process control techniques, and numerical methods that has provided me with the confidence to address a wide spectrum of challenges faced by modern society.

Abstract Text-

In this work, we study the shape 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 vesicle’s equilibrium shape anisotropy. 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 that of flow time scale. We further probe the stability boundary at two different viscosity ratios to understand how the onset of dumbbell shape 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. 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 as a function of vesicle reduced volume 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.