On the origin of surface tension and its application to micro/nano-scale devices

Abstract

Surface tension, which an important interfacial property of inhomogeneous systems, strongly affects the phase transition in a system. At the macro-scale, the surface tension is regarded as constant. However, a curvature dependence of the surface tension at the nano-scale has been predicted in the seminal work of Gibbs (1928), Tolman and co-workers (1949). This thesis investigates the curvature dependence of the surface tension and the Tolman length with molecular dynamics simulations. It is found that the curvature starts to affect the surface tension and Tolman length when the curvature radius decreases to around 20 molecular diameters. The magnitude of the effect of curvature on the surface tension and Tolman length is similar for nanobubble and nanodroplet interfaces while they act oppositely. Governed by the scaling law, the surface tension dominates at millimeter-scale and below. To utilise the large Laplace pressure cross liquid menisci caused by surface tension, liquid ring bearings have recently been developed. In this thesis, I present a detailed experimental and theoretical performance analysis of such bearings. Building on the study of the liquid ring bearings, a thrust bearing consisting of an air cushion formed within a liquid ring has been developed. An important discovery is that the performance of this bearing is greatly enhanced by the sealed cushion of air within the ring. Factors which affect the performance of the liquid ring and the liquid ring sealed air bearing have been studied both experimentally and numerically providing results that can be used to optimise the design of such bearings. To conclude, the surface tension and Tolman length have a curvature dependence only when the length-scale decreases down to molecular scale. The surface tension can be utilized to confine liquid menisci at millimeter-scale and below to support the load and effectively lubricate bearings.