Abstract
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 solid-like no-slip, liquid-like no-stress, or intermediate boundary conditions apply at the water-bilayer interface. Though no-slip conditions have been widely assumed for many decades, this fundamental aspect of membrane rheology has, to our knowledge, never been directly measured for free bilayers. We applied 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 flow boundary condition. We find that $C = 5.92 \pm 0.13$ (stat.) $\pm 0.16$ (syst.), matching the theoretical value of $C=6$ for a no-slip boundary and far from the $C=4$ value for a zero shear stress boundary
Highlights
Interfaces between lipid bilayers and aqueous solutions are present in countless environments both natural, such as at cell and organelle membranes, and artificial, such as in suspensions of liposome-encapsulated drugs
An ideal spherical shell composed of an incompressible fluid would behave to a rigid solid sphere, as the fluid cannot move to the interior space
We provide the complete set of positions, radii, diffusion coefficients, and boundary coefficient values for every microsphere, water droplet, and lipid vesicle examined in [34]
Summary
Interfaces between lipid bilayers and aqueous solutions are present in countless environments both natural, such as at cell and organelle membranes, and artificial, such as in suspensions of liposome-encapsulated drugs. The rheology of red blood cells in vivo [9] and suspensions of cells and liposomes in vitro [10,11] depends directly on the nature of the flow boundary condition of the bilayer-water interface. It has been widely assumed throughout work spanning many decades that this interface is well described by the boundary condition characteristic of a rigid solid [3,12,13,14,15,16]. The extent to which a real lipid vesicle behaves, in this sense, as an ideal spherical shell is experimentally
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