Conformational dynamics and phase behavior of lipid vesicles in a precisely controlled extensional flow

TL;DR

This study uses a Stokes trap to precisely observe lipid vesicle shape transitions in extensional flow, revealing three distinct conformational changes and confirming simulation predictions, advancing understanding of vesicle non-equilibrium dynamics.

Contribution

We demonstrate the use of a Stokes trap to control vesicle position and directly observe shape transitions in extensional flow, providing experimental phase diagrams that match theoretical predictions.

Findings

Identified three shape transitions: tubular to dumbbell, spheroid to asymmetric dumbbell, and quasi-spherical to ellipsoid.

Phase boundaries are independent of viscosity contrast.

Experimental results agree with recent simulation predictions.

Abstract

Lipid vesicles play a key role in fundamental biological processes. Despite recent progress, we lack a complete understanding of the non-equilibrium dynamics of vesicles due to challenges associated with long-time observation of shape fluctuations in strong flows. In this work, we present a flow-phase diagram for vesicle shape and conformational transitions in planar extensional flow using a Stokes trap, which enables control over the center-of-mass position of single or multiple vesicles in precisely defined flows [Shenoy, Rao, Schroeder, \textit{PNAS}, 113(15):3976-3981, 2016]. In this way, we directly observe the non-equilibrium conformations of lipid vesicles as a function of reduced volume $\nu$, capillary number $Ca$, and viscosity contrast $\lambda$. Our results show that vesicle dynamics in extensional flow are characterized by the emergence of three distinct shape transitions, including a tubular to symmetric dumbbell transition, a spheroid to asymmetric dumbbell transition, and quasi-spherical to ellipsoid transition. The experimental phase diagram is in good agreement with recent predictions from simulations [Narsimhan, Spann, Shaqfeh, \textit{J. Fluid Mech.}, 2014, \textbf{750}, 144]. We further show that the phase boundary of vesicle shape transitions is independent of the viscosity contrast. Taken together, our results demonstrate the utility of the Stokes trap for the precise quantification of vesicle stretching dynamics in precisely defined flows.