Thermocapillary phenomena and bubble coalescence during electrolytic gas evolution

Oxygen bubbles evolved in potassium hydroxide solution during, electrolysis have been reported to be mutually attractive. A model based on thermocapillary flow and migration can explain the effect. A temperature gradient directed perpendicular to the electrode's surface into the liquid phase arises during electrolysis on a thin-layer electrode because of reaction overpotentials on the surface and ohmic losses within the electrode itself. This temperature gradient acts on a bubble at the electrode to produce a gradient of surface tension that drives flow of the adjacent liquid. Fluid next to the bubble flows away from the electrode, thus drawing liquid near the electrode laterally toward the bubble. A neighboring bubble is entrained in the thermocapillary flow and is convected toward the first bubble and vice versa. Furthermore, the presence of a bubble on a heated surface engenders a temperature gradient with a component parallel to the electrode's surface; neighboring bubbles undergo thermocapillary migration toward the bubble generating the gradient. Our theoretical model is compared with experimental data, and the agreement is good both qualitatively and quantitatively. The mutual approach of pairs of equal-size bubbles on the electrode can be modeled by considering only entrainment in each other's thermocapillary flow, because thermocapillary migration is unimportant; however, the motion of a smaller bubble toward a larger "collector" bubble can be described only when both entrainment and thermocapillary migration of the smaller bubble are included in the model.