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Title: Bubble charge and coalescence
Author: Tang, Shanyu
ISNI:       0000 0001 3498 4130
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 1995
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This thesis describes an experimental investigation of the interfacial electrical properties of bubbles in aqueous solutions and of their coalescence behaviour, which were studied using laser Doppler anemometry (LDA) and laser light reflection techniques. A double laser Doppler anemometer system was developed to measure the electrophoretic mobilities for single bubbles, generated electrolytically at a Pt microelectrode in aqueous solutions, across which electric fields were subsequently applied using two horizontal electrodes. The effects of solution composition on the interfacial electrical properties of bubbles were then investigated by measurement of the natural rise rates (v) of bubbles and those (v^) in applied electric fields (£). In principle, the derived electrophoretic mobilities, defined by (vg - v)IE, may be related to the effective surface potentials and apparent adsorbed electric charge densities on the bubbles. However, classical theory cannot be used to effect such transformations, nor could it be assumed that the mobilities would correlate with coalescence behaviour of bubbles, as would be the case for solid particles with fixed charge sites. For electrolytes carefully purified of adventitious surfactants by purging with swarms of bubbles over a few hours, bubble electrophoretic mobilities showed a clear pH dependence, with an apparent iso-electric point at about pH 2, indicating that OIT ions were preferentially adsorbed at gas/liquid interfaces. Also, their (negative) mobilities increased with increasing bubble diameter, in the experimentally accessible range of 60 - 100 |im. The behaviour of bubbles in such clean electrolytes did not obey predictions of classical theory, as their gas/liquid interfaces were mobile, unlike that of solid/liquid interfaces, for which the interfacial velocity is zero. However, adsorption of surfactants rigidifies bubble interfaces, so that with a complete monolayer adsorbed, they behave as solid particles. Bubble rise rates and their electrophoretic velocities were sensitive to the adsorption of adventitious surfactants from the laboratory atmosphere. Electrophoretic mobilities of oxygen bubbles in surfactant-free sodium perchlorate aqueous solutions increased with the applied electric field. Experimental results and calculations based on the theory of Brandon et al. indicated that, in surfactant-free electrolyte solutions, the electric field was primarily responsible for polarisation of the adsorbed charge, rather than the hydrodynamic effects resulting from the bubble rise rate. A simple model was presented to describe the polarisation of charge distribution on bubble surfaces in electrolyte solution with the electric field parallel to the bubble rise vector. The initial surface charge density on oxygen bubbles generated in ICT* M surfactant-free NaCl04 solution was estimated to be - 17.4 |xC m'^, which compares favourably with the value of - 9.8 |iC m"^ derived from the diameter dependence of bubble electrophoretic mobilities. The LDA technique was extended to determine the coalescence times of individual bubbles as they approached a planar air/water interface. These results showed no dependence of coalescence times on the pH of surfactant-free electrolytes, as the electrostatic repulsion between the two interfaces as they approached, presumably caused the very small pH-dependent charge to move out of the zone of maximum interaction. However, addition of ionic surfactants caused coalescence times to increase, due to electrostatic repulsion between the partially rigidified surfaces. A laser reflection technique was developed to measure the time dependence of the thickness of the liquid film formed as a bubble approaches a planar gas / aqueous solution interface. Preliminary results indicated that the technique is suitable for studying film thinning, in that the apparent coalescence times were comparable to those derived from the LDA technique, although further refinement of the experiments is needed.
Supervisor: Kelsall, G. H. ; Smith, A. L. Sponsor: K.C. Wong Education Foundation, Hong Kong ; Committee of Principals and Vice-Chancellors (CVCP)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physical chemistry