The Irregular Motion of a Liquid Contact Line is a Path Dependent Memory

A water drop on a horizontal surface has an irregular shape that does not relax to equilibrium, revealing the disorder of the solid substrate beneath. In this talk we show that the shape of the contact line at the edge of a drop encodes a memory of its motion. To move the contact line, a constant volume of water is slowly added and then removed from a water drop placed within a Hele-Shaw cell. Repeating this process will "train" the contact line to a steady state, where further driving at the same volume amplitude no longer changes its shape. Reducing the amplitude significantly changes the shape of the contact line when it returns to its initial volume, but when driven again with even one cycle at the training amplitude, the contact line transitions back to its steady state. Driving above the training volume erases the memory, making the steady state inaccessible. This behavior is reminiscent of return-point memory, a phenomenon best known in ferromagnets. Return-point memory, and the contact line's evolution to steady state, can offer a framework to study contact line hysteresis in other cyclically driven multiphase flows, give insight on how local regions in a disordered system cooperate to reach global metastability, and provide a comparative memory to other higher dimensional disordered systems far from equilibrium.