Measurement of rise and electrophoretic velocities of gas bubbles

Abstract

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 × 10^–5 m s^–1, 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 10^–4M NaClO₄ 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 µm bubbles with surfactant-free, mobile interfaces. The feasibility of using the apparatus for determination of bubble electrophoretic mobilities was also established.