Aging Effect of Contact Line Dynamics on Monolayer Surfaces*

Contact line dynamics are crucial for various applications, including microfluidics and self-cleaning technologies. This study utilized a micro-cantilever method to investigate contact line friction as ionic liquid droplets slide over self-assembled alkylsilane monolayers. A distinct peak force was observed at the onset of droplet movement, exhibiting near log-linear growth with holding time until saturation. Steady-state friction initially decreased with increasing sliding velocity until reached a critical velocity. A pronounced transition at elevated temperatures was observed with accelerated saturation, reduced aging strength, and altered the friction-velocity relationship. Molecular analysis correlated these changes with a transition from ordered to amorphous monolayer molecular structures. We propose a general model that combines the aging effect of the contact line pinning-depinning processes and viscous dissipation effect at high velocities to explain the observed phenomena. This work advances the understanding of contact line dynamics and offers insights for the design of slippery surfaces.