Direct Observation of State-dependent Contact Line Forces at Solid–Liquid Interfaces

The friction between two solid surfaces is normally state-dependent and increases with contact time due to the plastic creeping of contact asperities. However, it remains unclear whether similar state-dependence exists in contact line forces (CLF) at solid–liquid interfaces. The challenge primarily arises from the lack of an experimental framework to precisely characterize a single moving contact line under a well-controlled contact state. In this work, using an atomic force microscope (AFM) equipped with a hanging-fiber testing probe, we systematically measured the dynamic CLF at a variety of liquid-solid interfaces with different state conditions. In our experiments, we directly observed the state-dependent CLF in both advancing and receding processes, which are determined by the interplay between physical roughness and chemical heterogeneity on the surfaces of solid substrates. By considering the relaxation dynamics of metastable liquid and air traps on solid surfaces, we proposed a phenomenological model that connects the state-dependent CLF to the microscopic aging effects occurring at liquid-solid interfaces.