  Lipid bilayer membranes undergo rapid bending undulations with wavelengths
from tens of nanometers to tens of microns due to thermal fluctuations. Here,
we probe such undulations and the membranes' mechanics by measuring the
time-varying orientation of single gold nanorods (GNRs) adhered to the
membrane, using high-speed dark field microscopy. In a lipid vesicle, such
measurements allow the determination of the membrane's viscosity, bending
rigidity and tension as well as the friction coefficient for sliding of the
monolayers over one another. The in-plane rotation of the GNR is hindered by
undulations in a membrane tension dependent manner, consistent with
simulations. The motion of single GNRs adhered to the plasma membrane of living
cultured cells similarly reveals that membrane's complex physics and coupling
to the cell's actomyosin cortex.
