Electrophoresis of electrogenerated bubbles

Abstract

The fundamental understanding of the interfacial charge on gas bubbles and the consequences of such charge are essential in understanding the behaviour of physicochemical systems involving liquid/gas and solid/liquid/gas interfaces. Such interfaces are involved in many industrial processes such as electrolytic gas evolution, particle flotation and bubble coalescence. The knowledge of such interfaces will aid mass transfer calculations. This thesis describes the application of a laser Doppler anemometer (LDA) system to the measurement of bubble electrophoretic mobilities, giving a measure of adsorbed charge. Single bubbles were electrogenerated in surfactant-free electrolytes, characterised by bubble rise rates, and their behaviour investigated in an electric field applied parallel to the direction of rise, so that, depending on the field direction, an increase or a decrease in the rise velocities was obtained. This field orientation served to decouple the hydrodynamic and field-induced charge polarisation. The velocity measurements using LDA showed a large degree of scatter despite numerous modifications to the optics and the signal processing. This culminated in the belief that a double LDA system was necessary to optimise the reliability and accuracy of the technique. Measurements using a Kodak high speed camera and recording system showed that the bubbles were negatively charged over the pH range studied (3-11), as indicated by their migration towards the anode under the influence of an applied electric field, with mobilities showing a radius and field dependence, implying that the adsorbed charge at the gas/electrolyte interface was mobile and polarisable. Large mobilities (10-60 x 10"® m2 s"^ V"^) were observed in comparison with results from previous bubble electrophoresis experiments with lateral fields. This was explained in terms of the enhanced charge polarisation occurring in the parallel electric field to the rise vector. A qualitative explanation for the decoupling of the hydrodynamic and field-induced charge polarisation has also been provided. In a separate series of experiments, under sufficient field conditions to overcome buoyancy forces, rising bubbles were stopped and held stationary. It was shown by extrapolation that bubbles possessed an iso electric point between pH 2 and 3, being positively charged below pH 2 and negatively charged above pH 3, supporting the hypothesis that the preferential adsorption of OH /H+ ions gives rise to the net charge. A laser reflection technique was investigated to measure the thickness of a liquid film formed between a bubble and the planar gas/electrolyte interface when they are in close proximity of each other. Preliminary investigations on macroscopic soap films showed the technique to be suitable for studying film thinning rates, though further refinement is necessary to study microscopic transient films. Electrophoresis measurements using a high speed camera have shown that bubbles preferentially adsorbed OH-/H+ ions from the solution in the absence of surfactants. This charge resided on a highly mobile interface and could be polarised by the actions of the hydrodynamics and the electric field. The laser Doppler anemometer system requires further development to achieve more accurate bubble velocity profiles in order to detect the small changes that occur.Open acces