Avalanche statistics of a moving contact line

Many disordered systems show crackling noise when driven by an external force or filed, such as Barkhausen noise in magnetization of ferromagnetic materials, acoustic emission in plastic deformation and seismic activity in earthquakes. An important feature of these noises is the power law distribution of avalanche sizes, implying a universal mechanism underlying the avalanche dynamics in these systems. Here we report avalanche statistics of a moving contact line (CL) formed on a vertical hanging fiber, which is driven through a liquid-air interface by an atomic-force-microscope (AFM) cantilever. The measured capillary force exerted on the circular CL intersecting the liquid-air interface exhibits zig-zag-like fluctuations, indicating a stick-slip motion of the CL. In the stick state, the capillary force increases linearly with the CL displacement. Once the capillary force reaches its local maximal value F_c, the CL slips in the form of avalanches, accompanied by a loss of capillary force δf. The measured δf is found to follow a power-law distribution, as predicted by the ABBM model. The experimental results provide new insights into the avalanche dynamics of CLs, which may also help to understand other avalanche phenomena.