THERMOCAPILLARY PHENOMENA AND BUBBLE COALESCENCE DURING ELECTROLYTIC GAS EVOLUTION

Oxygen bubbles evolved in potassium hydroxide solution during, electro lysis have been reported to be mutually attractive. A model based on t hermocapillary flow and migration can explain the effect. A temperatur e gradient directed perpendicular to the electrode's surface into the liquid phase arises during electrolysis on a thin-layer electrode beca use of reaction overpotentials on the surface and ohmic losses within the electrode itself. This temperature gradient acts on a bubble at th e electrode to produce a gradient of surface tension that drives flow of the adjacent liquid. Fluid next to the bubble flows away from the e lectrode, 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 g radient with a component parallel to the electrode's surface; neighbor ing bubbles undergo thermocapillary migration toward the bubble genera ting 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 thermoc apillary flow, because thermocapillary migration is unimportant; howev er, the motion of a smaller bubble toward a larger ''collector'' bubbl e can be described only when both entrainment and thermocapillary migr ation of the smaller bubble are included in the model.