Wetting-induced attraction of thin plates due to capillary flow

Abstract

Wetting-induced attraction are widely observed in microstructures where liquid flows along solid surfaces. Unexpected bending or collapse occurs if wetting-induced forces are neglected in the structural design, such as high aspect ratio pillars in the process of wet-etching. In this study, a simple experiment is designed to capture the evolving deformation of a cantilever beam due to capillary flow. A pair of polymer plates fixed at one end with a small gap is submerged into liquid, so that capillary rise between the plates and their attraction can be simultaneously observed. The plate dimension is sub-millimeter scale, which is rather large in observation of capillarity, in order to clearly capture deformation process of the plates until their contacts. Different types of liquids are prepared to investigate the influence of wettability, surface tension, and viscosity. Velocity of capillary flow is also considered by changing submergence rate of the plate. The experimental results of plate deflection are compared to analytical estimation obtained from an equation of motion for capillary rise and an equilibrium between capillary attraction and elastic force of plate. This estimation corresponded well with experimental results regardless of liquid types. In addition, the relationship between plate deflection and material constants is derived in a non-dimensional form. Therefore, plate deformation due to wetting-induced attraction, considering velocity of capillary flow, became predictable only from dimension of plates and material constants.