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

Abstract

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 behaviour are even more complex when forces of non-hydrodynamic origin come into play, two major classes of such forces being 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 fluid-mediated oscillatory motion of a rigid sphere over an ultra-thin soft coating, delineating 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 upto four orders of magnitude and upto 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 substratedeformation characteristics exhibits a tendency towards amelioration of the increment due to solvation force. arXiv:1908.03923v1 [cond-mat.soft]

Fluid-Elastic Interactions Near Contact at Low Reynolds Number

“…Assuming small deformations, Urzay (2010) analyzed the role of DLVO interactions on the motion of a sphere near a soft wall, predicting both reversible and irreversible adhesion behaviors. More recently, Karan et al (2020Karan et al ( , 2021 studied the effect of these interactions on time-dependent normal motion in a similar configuration. In addition to deformation due to flow, van der Waals pressure produces elastic deformations of magnitude AR 1/2 /(Gd 5/2 ) [note that d is clearance between the surfaces (see Section 2)], which defines another dimensionless compliance AR 1/2 /(Gd 7/2 ).…”

Soft hydraulics: from Newtonian to complex fluid flows through compliant conduits

“…Overall, it appears that prior studies have not considered the combined effect of non-Newtonian rheology, deformation of the compliant flow conduit, and electrokinetics all together, leaving this problem open. Furthermore, beyond electrokinetics, non-hydrodynamic effects, such as van der Waals and solvation forces [235,236] as well as finite-size effects and steric interactions [237], arise at nanometer level confinements. These effects can also be incorporated into the physical theories and mechanistic models reviewed herein, assuming the continuum hypothesis still holds.…”

Soft hydraulics: from Newtonian to complex fluid flows through compliant conduits

“…Overall, it appears that prior studies have not considered the combined effect of non-Newtonian rheology, deformation of the compliant flow conduit, and electrokinetics all together, leaving this problem open. Furthermore, beyond electrokinetics, non-hydrodynamic effects, such as van der Waals and solvation forces [229] as well as finitesize effects and steric interactions [230], arise at nanometer level confinements. These effects can also be incorporated into the physical theories and mechanistic models reviewed herein, assuming the continuum hypothesis still holds.…”