摘要

The paper reports an experimental study to understand the movement of a single liquid water plug of length L, placed inside a long, dry, horizontal, circular mini-channel (I.D. 1.5 mm). The plug (of specified L/D ratio) is pushed from rest by controlled injection of air from one side, till a quasi-steady terminal plug velocity is achieved. The plug, in presence of dominating surface forces, forms distinct front and rear menisci. The focus of the study is to understand the contribution of the contact angle of these menisci, and its hysteresis, in determining the pressure required to initiate plug motion. The air pressure required to move the plug from rest and the deformation of advancing and receding contact angles are observed for different plug lengths. It is observed that due to dominating surface effects, the minimum pressure (which corresponds to 'static' friction) required to commence plug motion from rest remains identical, irrespective of the plug length. In addition, the deformation in advancing contact angle is observed to be greater than deformation in receding contact angle. Once the plug starts to move, wall shear opposes the motion, eventually leading to a terminal velocity. At this stage, both the dynamic contact angles remain unchanged. The net force required to sustain plug motion at terminal velocity is substantially larger than the corresponding estimate by Poiseuille flow approximation. Future work involves modeling the relation between critical pressure and the contact angles, with a given hysteresis.