Electrostatic attraction between a hydrophilic solid and a bubble

The contact between fine hydrophilic α-Al2O3 particles and nitrogen bubbles was studied as a function of solution composition in single bubble capture experiments, where the bubble collection efficiency was measured. The surface charges of both bubble and particle were controlled by varying the electrolyte concentration and pH of the solution. In all experiments the bubbles were negatively charged while the α-Al2O3 particles were either negatively (above pH of the isoelectric point, pHIEP) or positively (below pHIEP) charged. The collection efficiency was found to be strongly influenced by the surface charge of the particles. The maximum collection efficiency occurred when the bubble and particle were oppositely charged (at low pH values) and at low salt concentration, i.e. when a long range attractive electrostatic interaction is present. In the case where both bubble and particle were of the same charge, the collection efficiency was near to zero within experimental error and was not influenced by either salt concentration or pH. This is the first experimental proof of the concept of ‘contactless flotation’, first proposed by Derjaguin and Dukhin in 1960, with far reaching implications from minerals processing to biology.