Viscoelasticity Enhances Contactless Adhesion of Soft Substrates

Understanding adhesion is essential for describing stability, friction, and interfacial dynamics. Here, we investigate the adhesion force dynamics between a rigid sphere and a soft surface without direct contact, mediated by a viscous fluid. By combining controlled experiments, a first-principles visco-elastohydrodynamic theory, and numerical simulations, we demonstrate that viscoelastic relaxation fundamentally modifies elastohydrodynamic adhesion. Rather than simply dissipating energy, viscoelasticity causes the substrate to behave transiently as a stiffer solid, enhancing the maximum adhesive force, changing the early-time force growth for $t^{2/3}$ to $t^{1/3}$, shortening the interaction time, and giving rise to new scaling laws governed by the Deborah number. The two proposed dimensionless parameters, the softness parameter and the Deborah number, define a unified phase diagram connecting three distinct adhesion regimes: classical Reynolds lubrication, elastohydrodynamic adhesion, and the newly identified visco-elastohydrodynamic regime.