Contact Line Motion over a Surface Asperity: Jumping and Energy Dissipation.

Understanding the interaction between a moving contact line and surface defects is essential to explain wetting dynamics and contact angle hysteresis on real surfaces. In this work, we investigate how an isolated topographical defect influences the motion of a contact line on a cylindrical fiber, a geometry relevant to atomic force microscopy (AFM) experiments. Using numerical simulations, we analyze the quasistatic force-displacement behavior of a meniscus advancing and receding over a single nanoscale bump, revealing distinct pinning and depinning jumps of the contact line, which lead to dissipated energy. The simulations quantify the dependence of jump lengths and associated energy dissipation on physical parameters such as surface tension, contact angle, defect geometry, and tip radius. The simulations show clearly that the depinning dissipated energy is larger than the pinning one, and most of the dissipation occurs during the depinning process. We propose approximated expressions of the dissipated energy for the pinning and depinning jumps, which shows the effects of the physical parameters. This simple model gives dissipated energies, which are in good agreement with the numerical simulations and AFM experiments. We extend the simulations to other types of defects, such as pits and chemical heterogeneities, and demonstrate how their signatures in force curves differ. Finally, we establish that the contact line jumps occur at a velocity close to the velocity of capillary waves, in agreement with recent results in the literature. This study provides insights into the fundamental mechanisms of contact angle hysteresis and energy dissipation in wetting phenomena.