  Viscosity is a key mechanical property of cell membranes that controls
time-dependent processes such as membrane deformation and diffusion of embedded
inclusions. Despite its importance, membrane viscosity remains poorly
characterized because existing methods rely on complex experimental designs
and/or analyses. Here, we describe a facile method to determine the viscosity
of bilayer membranes from the transient deformation of giant unilamellar
vesicles induced by a uniform electric field. The method is non-invasive, easy
to implement, probe-independent, high-throughput, and sensitive enough to
discern membrane viscosity of different lipid types, lipid phases, and polymers
in a wide range, from 10$^{-8}$ to 10$^{-4}$ Pa.s.m. It enables fast and
consistent collection of data that will advance understanding of biomembrane
dynamics.
