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.