Tribology of thin wetting films between a bubble and a moving solid surface

TL;DR

This study investigates the tribology of a bubble sliding on a solid surface using interferometry to measure film thickness and develop a model for friction behavior, revealing different friction regimes and the influence of speed.

Contribution

It introduces a novel experimental setup for detailed 3D measurement of wetting films and develops a theoretical model linking film thickness to friction coefficient and slip behavior.

Findings

Friction regime resembles dry friction below 170 μm/s.

Friction force peaks at higher speeds.

Friction coefficient decreases with increasing speed.

Abstract

The tribology of a bubble rubbing on a solid surface is studied via interferometry. A unique experimental setup is designed for monitoring the thickness profiles of a wetting film, intercalated between the bubble and hydrophilic glass moving with speed up to 412 um/s. The determination of the 3D film thickness profiles allows us to calculate 3D maps over the wetted surface of the local capillary, disjoining and lift pressures, viscous stress and friction force. In this way the average friction force and the corresponding friction coefficient are obtained. A theoretical model for the dependence of the friction coefficient on the film thickness is developed. The relevant slip coefficient, being a measure for the slip between liquid and solid, is determined as a function of the speed of the solid surface. It is found out that below 170 um/s a friction regime exists which formally resembles dry friction, while at larger speed the friction force between the bubble and solid passes through a maximum. Furthermore, the friction coefficient has a large value at low speed of the solid and reduces substantially with the speed increase.