The electrification and size of jet droplets from bursting bubbles at an air-water interface

Abstract

The work described in this thesis is directed towards a study of the size, height of ejection and electrical charge of jet droplets which are produced from a bursting bubble at an air-liquid Interface. Maximum ejection heights of 12 and 17 cm were obtained for Jet droplets of delonized water and sea-water respectively. The ejection height of the top jet droplets were identical for sea-water and deionized water for bubble diameters less than 0.84 mm, In agreement with the results of previous workers. A relation between the top Jet droplet diameter, D(_d), and the bubble diameter D(_B), was found to be of the form D(_d) α D(_B) (^1.22). It was found that the natural charge, q, on the Jet droplets is related to the droplet radius, r, by the expression q α r(^2.4) for a range of droplet radius from 17 up to 65 micrometres. A Blow increase in the electric charge of the droplets q with bubble age t followed a relation of q α t(^0.28) for a droplet radius of 36 micrometres for tap-water. The top droplet charge increased with decreasing water conductivity and showed reasonable agreement with an electrical double-layer theory proposed by Iribarne and Klemes (1974). The natural charge of the Jet droplets was found to be positive over the range of conductivity from about 4 x 10(^4) µ mho's cm(^-1) (sea- water) down to 2.8µ mho’s cm(^-1) (deionized water). A Faraday cage system was used to measure the electric charge of droplets greater than about 50 micrometres in radius. A decrease in the droplet charge was obtained for droplet radius greater than about 70 micrometres. Possible explanations for the decrease in charge over the measured droplet radius range from 70 to 140 micrometres are given. Measurements of the induction charge in jet droplets over a range of induction filed from 0 to ± 175 V cm(^-1) were carried out for both deionized water and sea-water. A linear relation between the jet droplet charge and induction field for top droplet radii from 27 to 76 micrometres and the second jet droplet of radius 50 micrometers was found. The negatively induced charge due to normal fair weather electric field constitutes only on a small fraction of the natural positive charge of jet droplets.