  In stationary nonequilibrium states a coupling between hydrodynamic modes
causes thermal fluctuations to become long ranged inducing nonequilibrium
Casimir forces or pressures. Here we consider nonequilibrium Casimir pressures
induced in liquids by a velocity gradient. Specifically, we have obtained
explicit expressions for the magnitude of the shear-induced pressure
enhancement in a liquid layer between two horizontal plates that complete and
correct results previously presented in the literature. In contrast to
nonequiibrium Casimir pressures induced by a temperature gradient, kinetic
theory shows that nonequilibrium contributions from short-range fluctuations
are no longer negligible. In addition, it is noted that computer simulations of
model fluids in shear observe effects from molecular correlations at nanoscales
that have a different physical origin. The idea that such computer simulations
probe shear-induced pressures resulting from coupling of long-wavelength
hydrodynamic modes is erroneous.
