Correlation of contact angle hysteresis and hydrodynamic lubrication

Abstract The strength of molecular adhesion at the lubricant-solid interface plays a key role in determining the load-carrying and film forming capacity of a lubricated system of meso- or microscale. If the liquid molecules gain higher energies than the potential energy barrier for adhesion, slip occurs at the interface. The contact angle of a liquid on a solid surface is related to the intermolecular attractive force. Some studies demonstrated the connection of the contact angle to the lubrication effect. However, there were also studies drawing the opposite conclusion. A theoretical model derived based on thermodynamic principles shows that the potential energy barrier of a surface is not only a function of contact angle, but also of another interfacial parameter, contact angle hysteresis (CAH). This study thus evaluates the two, contact angle and CAH, by conducting thin film hydrodynamic lubrication experiments with surfaces of hydrophilic and hydrophobic (contact angle ranging from 30° to 110°). The lubricating film thickness was measured under different speed-load conditions using optical interferometry with a fixed-incline slider bearing. The paper identifies CAH to be a better interfacial parameter for correlating with the thin film hydrodynamic lubrication effect. Fundamental relation between the potential energy barrier and CAH is discussed, and the experimental results are described using the critical shear stress slip model.