Boundary lubrication: dynamics of squeeze-out

Area of Science:

• Fluid dynamics
• Surface science
• Computational physics

Background:

• Understanding the behavior of thin liquid films is crucial for various applications.
• Recent experiments have enabled direct measurement of the final stages of liquid expulsion.
• Confined liquid dynamics present unique challenges due to surface interactions.

Purpose of the Study:

• To analyze the dynamics of the final liquid monolayer expulsion between surfaces.
• To investigate the instabilities observed at the boundary of the expelled film.
• To elucidate the role of pressure gradients in liquid confinement.

Main Methods:

• Solving the two-dimensional Navier-Stokes equation.
• Employing kinetic Monte Carlo simulations.
• Combining fluid dynamics equations with particle-based simulations.

Main Results:

• Observed instabilities in the boundary line of the expelled liquid film.
• Demonstrated that instabilities create rough boundaries above a critical length scale.
• Showed smooth boundaries at length scales below the critical value.
• Identified pressure gradients in the contact area as the cause for liquid expulsion.

Conclusions:

• The expulsion of the last liquid monolayer is governed by instabilities.
• Boundary roughness is scale-dependent, influenced by critical length scales.
• Pressure gradients are the primary driver for the squeezing out of confined liquids.