Avalanches and extreme value statistics of a moving contact line*

Avalanche dynamics is widely observed in various out-of-equilibrium disordered systems, ranging from the seismic activities of tectonic plates to the plastic deformation of crystalline materials and the proceeding of a domain wall in the ferromagnets. Among the systems that exhibit avalanche behaviors, the moving contact line (CL) between a liquid interface and a solid substrate, which is often pinned by the physical roughness and/or chemical inhomogeneity on the solid surface, is an ideal system for the study of the avalanche dynamics. In this talk, we present our recent experimental efforts in developing a mesoscale “long needle” AFM (atomic force microscope) to study the stick-slip dynamics of a moving three-phase contact line [1]. From a large volume of individual avalanche events collected over a wide range of sampling rates and long durations, we established, for the first time, three statistical laws of the avalanche dynamics at the critical state in the forms of the probability density functions (PDFs) of the slip length Dz_slip, the maximal force F_c needed to trigger the avalanches and the local force gradient k' of the pinning force field. This work bridges the gap between the microscopic behavior of individual depinning events and the macroscopic laws of contact angle hysteresis. It thus represents a breakthrough in our understanding of the avalanche dynamics of a moving CL.