ABSTRACT

The depinning of contact lines is an old fluid mechanics problem that has received considerable new interest in recent years. The commonly observed stick-slip motion is believed to be a generic feature found in a broad range of seemingly unrelated physical systems. Some well known examples include the motion of magnetic domain walls in response to changing magnetic fields [1], and the dynamics of flux lines in high-Tc superconductors with increasing electric current [2]. In all cases, pinning results from the inherent disorder in the system, and depinning occurs when the applied force exceeds a certain threshold. It is commonly postulated that the pinned state and the moving state are separated by a dynamical phase transition that exhibits universality in its critical behavior, i.e., independent of the details of the systems [3, 4]. The advantage of studying contact line dynamics is that one can make direct visual observations of the critical behavior. The challenge, however, is that one has to apply a mechanical driving force in such a way that it does not externally introduce stick-slip motion or other fluctuations. We have studied the depinning transition by simple capillary rise and fall of a water column in glass

tubes [5, 6]. Geometric roughness and chemical inhomogeneities on the tube wall provide the random pinning forces. The motion of the contact line is driven by its own interaction with the environment and no external device is needed to push it. So we may describe the system as self-organized. This paper is a summary report of our work.