Marangoni forces on electrolytic bubbles on microelectrodes

In this work we perform simulations of combined thermo- and solutocapillary convection in which the solute is the electrolyte. These simulations are inspired by measurements by Park (2023) of the detachment radius of hydrogen bubble for various acidic electrolytes. The results indicated that the detachment radius depends on Marangoni flow caused by the electrolyte concentration gradient (Park, 2023). They were able to attribute this to the solutal Marangoni force. We simulate some of these measurements and quantify the Marangoni flow and the Marangoni force on a hydrogen bubble for various acidic electrolytes. To distinguish between the thermocapillary and solutocapillary effect we also perform simulations for the solutocapillary effect only. In addition, we perform simulations of oxygen bubbles growing on an anode in acidic electrolytes. For a given electrolyte, the direction of the solutal Marangoni force on an anodic bubble relative to the cathode is opposite to the direction of the solutal Marangoni force on a cathodic bubble relative to the cathode. If the force delays detachment at the cathode it accelerates at the anode, or vice versa. Furthermore, a similar behavior is simulated for bubbles in alkaline electrolysis. We summarize our findings into a general phase diagram that indicates the importance of the Marangoni force in the force balance. This also gives insight into the detachment radius of bubbles. For example, it provides an explanation why in alkaline water electrolysis oxygen bubbles tend to be much larger than hydrogen bubbles.