Tribonucleation of bubbles

We report on the nucleation of bubbles on solids that are gently rubbed against each other while immersed in a gas-supersaturated liquid. For given supersaturation and surface material, bubble nucleation is only observed beyond a certain threshold for the rubbing force and velocity. Above this threshold, a regularly spaced row of growing bubbles is left behind on the surface. Direct observation through the bottom of a transparent solid shows that each bubble in the row results from the early coalescence of several microscopic bubbles. From a detailed study of the wear tracks it seems that these gas nuclei originate from a local fracturing of the surface asperities in the contact area.