Small particle at a fluid interface: Effect of contact angle hysteresis on force and work of detachment

This paper deals with the removal of a small sphere initially attached to a liquid interface. The sphere is small enough (0.3-1 mm) for the capillary force to dominate the interaction and large enough for the line tension effect to be negligible. We have measured simultaneously the force and the geometric parameters of the system as a function of the relative (sphere/interface) separation distance during the detachment process, with a high precision. This procedure allows us to quantify the effect of the contact angle hysteresis during the detachment process with respect to the force-path curve. It is shown that the previous work, which assumes a constant receding contact angle, does not describe our experimental data when the hysteresis effect dominates. By analytical integration of the capillary force experienced by the sphere during the detachment process, the first closed-form analytical expression for the detachment work was obtained. Comparison with our experimental data and with the existing numerical calculations showed good agreement. The effect of contact angle hysteresis on the detachment work is also quantified.