Entropy-dominated squeezout force and incipient glass formation in confined onic liquids

The squeezout force for an ionic liquid (IL) confined in a nanoscale channel between two approaching plates generally shows just short-range layering steps. Close to freezing temperature and at large squeezout speeds, recent data in a simple IL showed instead that short-range layering is replaced by a smooth intermediate range repulsive force tail. Along with the expected viscosity rise as in experiment, our squeezout simulations in a simple IL model additionally show that the confinement- induced strong and smooth repulsive squeezout force is of entropic origin, whereas the short-range layering steps are only controlled by internal energy. Our interpretation is guided by bulk glass-forming liquids, where the viscosity rise upon cooling is indeed associated with an entropy drop through the Adam-Gibbs mechanism, involving the formation of cooperatively rearranging regions (CRRs). We find that the confined IL the entropy drop and viscosity rise which accompany squeezout are also connected with remnants of a confinement-induced glassy CRRs forming at constant temperature, a mechanism that should be more general than this specific case.