The effects of surface hydration on capillary adhesion under nanoscale confinement

Nanoscale phenomena such as surface hydration and the molecular layering of liquids under strong nanoscale confinement play a critical role in liquid-mediated surface adhesion that is not accounted for by available models, which assume a uniform liquid density with or without considering surface forces and associated disjoining pressure effects. This work introduces an alternative theoretical description that via the potential of mean force (PMF) considers the strong spatial variation of the liquid number density under nanoscale confinement. This alternative description based on the PMF predicts a dual effect of surface hydration by producing: (i) strong spatial oscillations of the local liquid density and pressure and, more importantly, (ii) a configuration-dependent liquid-solid surface energy under nanoscale confinement. Theoretical analysis and molecular dynamics simulations for the case of an axisymmetric water bridge with nanoscale heights show that the latter hydration effect is critical for the accurate prediction of the surface energy and adhesion forces when a small volume of liquid is nanoscopically confined by two surfaces approaching contact.