A double laser Doppler electrophoresis apparatus has been used to study the effects of bubble size, pH, type and concentration of electrolytes and electric field strength, on the electrophoretic mobilities of electrolytically generated oxygen and hydrogen bubbles in aqueous electrolyte solutions. The pH dependence of the (very large) bubbles' electrophoretic mobilities implied the presence of an isoelectric point between pH 2 and 3 and their (negative) mobilities increased in magnitude with increasing bubble diameter, in the experimentally accessible range of about 60-100 pm. The apparent negative charge of the bubbles in the surfactant-free electrolyte solutions was ascribed to preferential adsorption of O Hions. The pH-dependent mean charge density was estimated as ca. -18 pC rn-, at pH 6.9 by addition of sufficient cationic surfactant (DoTAB) of known adsorption isotherm, to bring the bubble electrophoretic mobility to zero. Although very small, this was similar to the value of ca. -10 pC rn-, derived from the diameter dependence of the electrophoretic mobilities in surfactant-free NaClO, solutions assuming charge polarisation to the extent of greatly diminishing the electrophoretic drag. The very high mobilities enabled complementary experiments involving holding the bubbles stationary against gravity with an applied field, which is seldom possible for solid particles with the same density difference to the suspending liquid; the resulting mobilities were comparable to those obtained with laser Doppler anemometry. The high mobilities were ascribed to polarisation of adsorbed charge at the mobile gas/liquid interface decreasing the electrophoretic drag. Extrapolation to zero applied field gave a finite, though very small mobility, which may be ascribable to hydrodynamic charge polarisation.

A novel apparatus, incorporating two laser Doppler anemometers (LDA), has been designed and built to measure the rise rates and electrophoretic mobilities of gas bubbles in aqueous solutions, which were rigorously cleaned of adventitious surface active material by swarms of electrolytically generated bubbles. Single gas bubbles were then generated electrolytically at a Pt microelectrode in a specially designed cell, enabling bubble diameters to be varied between experiments. With an estimated precision of 5.3 x m s-', the velocity of each bubble was determined as it rose naturally through the first LDA measurement volume, then again after it rose through a hole in the lower of two horizontal plate electrodes. In some experiments, the second measurement was taken after applying a vertical electric field, which increased or decreased the bubble velocity, depending on the sign of the apparent adsorbed charge on the bubble and the sign and magnitude of the applied electric field.Rise rates of oxygen bubbles in lo-, M NaClO, solutions increased with purging time, reaching a plateau value after about 1.5 h, when the diameter dependence of the rise rates obeyed Levich's theoretical predictions for < 100 pm bubbles with surfactantfree, mobile interfaces. The feasibility of using the apparatus for determination of bubble electrophoretic mobilities was also established.