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 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.

Influence of very small bubbles on particle/bubble heterocoagulation

“…Similarly, a potential and charge exist at the vapor-liquid interface, although the mechanism responsible for this charge remains somewhat obscure [14] and its measurement is fraught [15]. A similar situation exists for nanobubbles adhering to a particle surface.…”

Dynamic interactions between microbubbles in water

“…On the other hand, using the fully mobile or no tangential stress boundary condition, normally assumed to be valid at the air-water interface, will underestimate the magnitude of the dynamic force by over four orders of magnitude, and the predicted coalescence times will be too short because hydrodynamic effects under this boundary condition are much weaker (20). Although measurements of the terminal velocities of rising bubbles (21,22) show that the fully mobile boundary condition is applicable in ultraclean water, such degree of cleanliness is impractical to achieve in our experiments. Quantitative estimates suggest that a surface concentration of surface-active species that reduces the interfacial tension by ∼1 mN∕m would be sufficient to cause bubbles to exhibit the no-slip boundary condition (23).…”

Dynamic deformations and forces in soft matter

“…In aqueous systems, inevitable surface impurities tend to arrest interfacial mobility and the mobile interface is not commonly encountered for drops or bubbles in the µm size range 20 .…”