Influence of non-hydrodynamic forces on the elastic response of an ultra-thin soft coating under fluid-mediated dynamic loading

The force between two approaching solids in a liquid medium becomes increasingly large with decreasing separation—a phenomenon that prevents contact between the two solids. This growth in force occurs because of the intervening liquid, and studies of such physical systems constitute the classical discipline of lubrication. Furthermore, when the solid(s) are soft, there are quantitative as well as qualitative alterations in the force interaction due to the solids’ deformation. The underlying physics as well as resultant system behavior is even more complex when forces of non-hydrodynamic origin come into play. Two major classes of such forces are the DLVO (Derjaguin–Landau–Verwey–Overbeek) forces and the non-DLVO molecular forces. Studies assessing the coupling of these physical phenomenon are avenues of contemporary research. With this view, we perform an analytical study of oscillatory motion of a rigid sphere over an ultra-thin soft coating with an electrolytic solution filling the gap between them. We delineate the distinctive effects of solvation force as well as substrate compliance. Our key finding is the major augmentation in the force and substrate-deformation characteristics of the system due to solvation force when the confinement reduces to a few nanometers. Consideration of solvation force leads to up to four orders of magnitude and up to three orders of magnitude increment in force and substrate-deformation, respectively. While higher softness leads to higher deformation (as expected), its effect on force and substrate-deformation characteristics exhibits a tendency toward amelioration of the increment due to solvation force.The force between two approaching solids in a liquid medium becomes increasingly large with decreasing separation—a phenomenon that prevents contact between the two solids. This growth in force occurs because of the intervening liquid, and studies of such physical systems constitute the classical discipline of lubrication. Furthermore, when the solid(s) are soft, there are quantitative as well as qualitative alterations in the force interaction due to the solids’ deformation. The underlying physics as well as resultant system behavior is even more complex when forces of non-hydrodynamic origin come into play. Two major classes of such forces are the DLVO (Derjaguin–Landau–Verwey–Overbeek) forces and the non-DLVO molecular forces. Studies assessing the coupling of these physical phenomenon are avenues of contemporary research. With this view, we perform an analytical study of oscillatory motion of a rigid sphere over an ultra-thin soft coating with an electrolytic solution filling the gap between them. We ...