A high throughput method to measure shear viscosity of bilayer membranes reveals shear-thinning and charge-dependence*

Lipid bilayers are the main structural component of cell membranes. The nanometrically thin bilayer  behave as a two-dimensional fluid and its shear viscosity controls the transport of embedded biomolecules and membrane deformation. In this study, we devise a high throughput and non-invasive method to measure shear surface viscosity of bilayer membranes based on transient deformation of giant vesicles (closed membrane sacs) in an uniform electric field. Our method is probe-free and sensitive enough to allow for robust measurement of membrane viscosity of single-and multi-component lipid and polymer membranes. The amassed data reveals that homogeneous membranes of liquid disordered and ordered phase are Newtonian fluids while phase separated bilayer membranes and polymersomes exhibit shear thinning behavior. Surprisingly, we find that shear surface viscosity depends on the imbalance of the induced surface charges on the opposite membrane surfaces. This has important implications for biomolecular transport in living cell membranes that are intrinsically charged due to a transmembrane potential.