Two-Point Microrheology of Phase Separated Domains in Lipid Bilayers

Abstract

Though membrane fluidity is of paramount importance for cellular function, methods for measuring lipid bilayer viscosity are often difficult to implement. While approaches based on quantifying the Brownian dynamics of individual tracers do yield insight in to membrane properties, in general methods based on single-particle trajectories provide a limited view of hydrodynamic response. Two-point microrheology, in which correlations between pairs of tracers measure the properties of the intervening medium, grants a broader picture of membrane hydrodynamics by characterizing viscosity at length-scales that are larger than that of individual tracers. We present the first two-point microrheological study of lipid bilayers, examining the correlated motion of domains in phase-separated lipid vesicles. We measure two-point correlation functions in excellent agreement with the forms predicted by two-dimensional hydrodynamic models, and find that these measurements are sensitive to the viscosity of both bilayer phases, indicating that we can ascertain global fluid properties rather than just the viscosity of the local neighborhood of the tracer.