  Theoretically and experimentally, we study electroviscous phenomena resulting
from charge-flow coupling in a nanoscale capillary. Our theoretical approach
relies on Poisson-Boltzmann mean-field theory and on coupled linear relations
for charge and hydrodynamic flows, including electro-osmosis and charge
advection. With respect to the unperturbed Poiseuille flow, we define an
electroviscous coupling parameter $\xi$, which turns out to be maximum where
the film thickness $h_0$ is comparable to the screening length $\lambda$. We
also present dynamic AFM data for the visco-elastic response of a confined
water film in sphere-plane geometry; our theory provides a quantitative
description for the electroviscous drag coefficient and the electrostatic
repulsion as a function of the film thickness, with the surface charge density
as the only free parameter. Charge regulation sets in at even smaller
distances.
