History-dependent droplet friction driven by contact-line aging

The motion of liquid droplets on solid surfaces is commonly described by contact-line friction, which is generally assumed to increase monotonically with instantaneous velocity. However, such a velocity-only description cannot explain why static droplet friction can exceed dynamic friction or why contact-line friction can become velocity-weakening. Here, we demonstrate that contact-line friction depends jointly on sliding velocity and contact history. Using self-assembled alkylsilane monolayers as molecularly tunable substrates and ionic liquids as low-volatility model liquids, we observe pronounced history-dependent strengthening of contact-line friction on solid-like monolayers, manifested as time-enhanced static friction and velocity-weakening dynamic friction. Both effects are progressively suppressed as the surface is tuned toward a more liquid-like state with a lower density of pinning defects, either by reducing the monolayer chain length or by increasing temperature. We attribute this behavior to nanoscopic contact-line aging, whereby contact-line segments thermally fluctuate and progressively occupy interfacial defects, leading to a progressive increase in contact-line pinning. An aging-based model quantitatively captures the history-dependent onset friction and the low-velocity friction dynamics in our system, while similar logarithmic strengthening across diverse liquid-substrate systems suggests broader applicability of the constitutive framework. These findings establish the coupled effects of contact history and sliding velocity on contact-line friction, extending dynamic wetting beyond velocity-only descriptions and providing a physical basis for controlling macroscopic droplet transport through nanoscale interfacial design.