Electrolytic Bubble Coalescence on Hydrophobic Cavity Arrays Determines Departure Radius and Lowers Electrolyte Supersaturation

Bubble evolution on electrodes is a complex process that begins with the stochastic nucleation of bubbles on the electrode surface, followed by bubble growth due to diffusion and coalescence, and bubble departure. The stochasticity of bubble evolution on conventional electrodes is a significant challenge in efforts to study electrolytic bubbles. In this investigation, the growth of electrolytic hydrogen bubbles is studied on microfabricated silicon electrodes with arrays of hydrophobic cavities. These hydrophobic pits act as preferential nucleation sites for bubbles-thus lowering the degree of spatial-randomness in bubble nucleation and enabling the study of bubbles growing in the presence of coalescence with greater control. Substrates with different spacings between the hydrophobic pits were fabricated. It is shown that coalescence with neighboring bubbles strongly determines the departure radius of bubbles. Further analysis of the bubble growth rate and electrode coverage indicates that closer pits decrease the electrolyte supersaturation while increasing electrode coverage.