Bubble coalescence dynamics and supersaturation in electrolytic gas evolution

Abstract

1.Bubble coalescence on gas-evolving electrodes disturbs the fluid immediately adjacent to the surface, thereby significantly impacting mass transport to and from the electrode. Previous efforts to quanti@ coalescence phenomena have not been successfbl despite the application of high speed cinematography (up to 1 O4 framesls) because of the extremely short duration (< 1 ms) of such events. To overcome this difficulty, a new technique was developed to study the interfacial velocities and shape changes associated with the coalescence of two electrolytically generated bubbles, 50 to 600 micron in diameter. The optical technique devised and used here employs a linear photodiode array to detect bubble movement with a resolution of 10" s. The apparatus and procedures developed in this research permit the observation and measurement of electrolytic bubble 3. The accumulation of dissolved gas in the electrolyte adjacent to an electrode creates a concentration overpotential that lowers the energy efficiency of the electrochemical system. The concentrations of dissolved gas in aqueous electrolytes can exceed saturation even at low production rates because the difiksivity and solubility of the dissolved gas are low (about 10"cm2/s and 10"moVcm3, respectively). In this research, the dependence of gas supersaturation on current density is determined under conditions approaching zero convection and with a low density of bubble nucleation sites. These conditions minimize hydrodynamic and bubble nucleation variables and provide high degrees of supersaturation.Supersaturations ranged fiom relative activities of 26 to 178, proportional to the square root of the applied current densities in the range of 0.4 to 72 mA/cm2. The supersaturation values were consistently higher than those at a coarse electrode (rough surface) or at a vertically oriented non-recessed electrode. These results indicate that bubble nucleation and mass transfer in the bulk electrolyte are restricted by the electrode texture, orientation, and geometry. Even a highly polished surface provides sites for bubble nucleation that lower gas supersaturation. A simulation obtained by a model developed in this research showed that bubble departure does not reduce the dissolved gas concentration at an electrode surface to the extent assumed in the surface renewal model of Ibl. A proposed partial renewal model, which reflects the replacement of departing bubbles with supersaturated electrolyte, fits the experimental results.

Bubble coalescence dynamics

“…The poor agreement between experimental and calculated damping rates at the larger values of u/R2 is, however, a cause for concern. Good agreement between the simulations and experiments was obtained in the 1-M H,SO, for 600-pm bubbles using the measured surface tension of 73 mN/m, but good agreement was obtained for the smaller bubbles only by reducing the surface tension in the simulations to 60 mN/m for 375-pm bubbles and to 49 mN/m for 150-pm bubbles (Stover, 1996). Since the electrolytic cell was open to the atmosphere, it is possible that the surface tension changed with time as surface-active species accumulated on the bubbles (Levich, 1962;Kelsall et al, 1996).…”

“…The experimental values of u, are of the order of meters per second (cf. Figure 3b), requiring initial film thicknesses of the order of tens of microns, which is clearly unrealistic, and the Egan and Tobias analysis does not agree well with the data; comparisons in Stover (1996) show that the experimental deceleration is much faster than predicted by the analysis.…”

“…Simulations were carried out for bubble sizes of 50, 150, 375 and 600 p m for the kinematic viscosities and surface tensions used in the experiments. Detailed results are in Stover (1996). The experimental and computed damping rates are shown in Figure 12 as a function of v/R2.…”

Mechanism of damped oscillation in microbubble coalescence

“…If we consider that the surface tension acts as a restoring force, the surrounding fluid acts as a mass, and viscosity damps the motion, the 1-D motion of the saddle point can be modelled as a damped harmonic oscillator [36] A Then, we look at the green side in Fig. 3.…”

MEMS electrolytic inchworms for movable neural probe applications

“…The topic of electrogenerated bubbles has been investigated by many researchers, for example [88][89][90][91][92][93][94]. The behavior of bubbles on gas-evolving electrodes is of great interest because it significantly affects the mass transport.…”