Squeeze-out and wear: Fundamental principles and applications

The dynamics of squeeze-out of thin liquid films between two solids is perhaps the most central topic in tribology. It is directly relevant for wear and indirectly involved in many other important processes, e.g., adhesion and friction. In this review we present a broad overview of the basic principles of squeeze-out, and present a number of applications to adhesion, friction and wear. We first describe the squeezing of 'thick' liquid films (thickness larger than {\sim } 100 A), which can be described using the Navier?Stokes equations of hydrodynamics, and present experimental illustrations for soft solids (rubber) and hard solids (mica). Next we consider molecularly thin liquid films. Here the squeeze-out occurs in a quantized manner involving a monolayer at each step. We discuss the nature of the nucleation of n\rightarrow n-1 monolayer squeeze-out, where n is the number of trapped monolayers. We consider in detail the nature of the spreading which follows the nucleation and show that the boundary line may exhibit instabilities. Sometimes the squeeze-out is incomplete, resulting in trapped islands. These islands may be pinned, or else they drift slowly to the periphery of the contact area where they get squeezed out through narrow liquid channels. We consider also dewetting at soft interfaces and present an application to the adhesion of soft objects on wet substrates. Finally, we present molecular dynamics and kinetic Monte Carlo simulation results on various aspects of squeeze-out for liquid-like and solid-like lubrication films, and discuss the implications for wear.