Tribonucleation of bubbles

TL;DR

This paper investigates the phenomenon of bubble nucleation during gentle rubbing of solids in liquids, revealing how material properties, surface roughness, and mechanical forces influence bubble formation, with implications for controlling bubble patterns.

Contribution

It uncovers the microscopic mechanisms of bubble nucleation on solids, linking surface fracturing and gas pocket formation to the tribonucleation process.

Findings

Bubble trails form above certain force and velocity thresholds.

Microscopic gas pockets originate from surface asperity fracturing.

Surface chemistry may enhance gas pocket formation.

Abstract

We report on the nucleation of bubbles on solids that are gently rubbed against each other in a liquid. The phenomenon is found to depend strongly on the material and roughness of the solid surfaces. For a given surface, temperature, and gas content, a trail of growing bubbles is observed if the rubbing force and velocity exceed a certain threshold. Direct observation through a transparent solid shows that each bubble in the trail results from the early coalescence of several microscopic bubbles, themselves detaching from microscopic gas pockets forming between the solids. From a detailed study of the wear tracks, with atomic force and scanning electron microscopy imaging, we conclude that these microscopic gas pockets originate from a local fracturing of the surface asperities, possibly enhanced by chemical reactions at the freshly created surfaces. Our findings will be useful either for preventing undesired bubble formation or, on the contrary, for "writing with bubbles," i.e., creating controlled patterns of microscopic bubbles.