Reversible Motion of a Contact Line

We study the evolution of the liquid-solid-vapor contact line of water held in a narrow gap between two acrylic plates. A syringe pump injects and withdraws a constant, small volume of the water, driving the contact line back and forth repeatedly and changing its shape. We take photos of the contact line after each cycle. Comparing these images to each other, we find that below a critical value of infused volume, after several cycles the contact line reaches a steady state in which it always returns to the same shape, despite depinning and executing many small jumps during its motion. Above that value the shape fluctuates in the steady state. This suggests a transition reminiscent of those seen in other systems including cyclically deformed particle suspensions and solids. We discuss possible explanations in terms of the microscopic dynamics of the contact line.