Shear-Modulated Adhesion of Liquid Bridges on Hydrophobic Surfaces: A Dominant Role of Contact Angle Hysteresis

The evolution of capillary adhesion mediated by sheared liquid bridges between hydrophobic surfaces plays a pivotal role in microfluidics, biomimetic adhesives, and surface engineering applications, yet the underlying mechanisms remain poorly understood. A custom-built surface force apparatus (SFA) was used to measure both lateral and normal adhesion forces generated by constant-volume liquid bridges confined between two identical poly(tetrafluoroethylene) (PTFE) surfaces during low-velocity shear. The dynamic meniscus geometry was captured in real time via synchronized dual-CCD imaging, enabling a correlated analysis of force evolution with contact angle, contact line dynamics, and curvature variations. The results showed that (1) contact angle hysteresis dominated the shear-dependent evolution of lateral and normal adhesion forces at the solid-liquid interface, (2) the normal adhesion became more sensitive to shear as the bridge separation distance decreased, and (3) the normal force reversed from repulsive to attractive during meniscus deformation, with a 155% amplitude change, which has never been observed in any reported studies of hydrophilic surfaces. Moreover, the random appearance of moving contact lines on both solid surfaces was addressed by replacing the conventional upper plate with a finite-sized circular pad. We proposed a novel characterization method for solid-liquid interactions based on sheared liquid bridges capable of simultaneously quantifying the contact angle hysteresis and measuring bidirectional (lateral/normal) adhesion forces.