  Recent measurements suggest the possibility to exploit ionic liquids (ILs) as
smart lubricants for nano-contacts, tuning their tribological and rheological
properties by charging the sliding interfaces. Following our earlier
theoretical study of charging effects on nanoscale confinement and squeezout of
a model IL, we present here molecular dynamics simulations of the frictional
and lubrication properties of that model under charging conditions.First we
describe the case when two equally charged plates slide while being held
together to a confinement distance of a few molecular layers.The shear sliding
stress is found to rise as the number of IL layers decreases stepwise. However
the shear stress shows, within each given number of layers, only a weak
dependence upon the precise value of the normal load, a result in agreement
with data extracted from recent experiments.We subsequently describe the case
of opposite charging of the sliding plates, and follow the shear stress when
the charging is slowly and adiabatically reversed in the course of time, under
fixed load. Despite the fixed load, the number and structure of the confined IL
layers changes with changing charge, and that in turn drives strong friction
variations. The latter involve first of all charging-induced freezing of the IL
film, followed by a discharging-induced melting, both made possible by the
nanoscale confinement. Another mechanism for charging-induced frictional
changes is a shift of the plane of maximum shear from mid-film to the
plate-film interface, and viceversa. While these occurrences and results
invariably depend upon the parameters of the model IL and upon its specific
interaction with the plates, the present study helps identifying a variety of
possible behavior, obtained under very simple assumptions, while connecting it
to an underlying equilibrium thermodynamics picture.
