Lipid bilayer hydrodynamic drag

The hydrodynamic drag at a lipid bilayer surface determines in part the flow properties of suspensions of cells and liposomes. Given the fluidity of lipid bilayers, it is not obvious a priori whether boundary conditions like those of solid objects in water, inviscid fluids in water, or something intermediate should apply at the water-bilayer interface. Though boundary conditions equivalent to those of no-slip solid-liquid interfaces have been widely assumed for decades, a direct measurement of this fundamental aspect of membrane rheology for free bilayers is lacking. We apply light sheet fluorescence microscopy to image freely diffusing phospholipid vesicles and determined the hydrodynamic drag coefficient C pi eta R, where eta is the external fluid viscosity, R is the vesicle radius, and the dimensionless C characterizes the surface boundary condition. We find that C = 5.92 +/- 0.13 (stat.) +/- 0.16 (syst.), matching the theoretical value of C = 6 for a rigid sphere and far from the C = 4 value for a boundary with zero shear stress.