Detachment forces during parallel-plate gap separation mediated by a simple yield-stress fluid

In this work we have monitored the multiple stages of the normal traction force response of a yield-stress fluid confined between two circular parallel plates that are separated at constant velocity. At narrow initial gaps, the air–fluid interface suffers from the Saffman–Taylor instability, confirmed by visual inspection of fingering patterns imprinted on the fluid. At larger initial gaps, the fluid preserves the initially imposed circular symmetry of the confining plates, indicating the absence of instability. Due to the system characteristics and experimental environment, the multiple traction force contributions occurred in cascade, permitting us to isolate the adhesion responses associated with viscosity, capillarity, and yield stress. Employing a standard Herschel–Bulkley model, we assessed the scaling of the traction force in multiple regimes—specifically, evaluating the dependencies of the fingering to yield-stress transitions.