Break-up in formation of small bubbles: Break-up in a confined volume

Abstract

Small bubbles play the critical role in flotation of providing the large interfacial area for collecting and transporting selected (hydrophobic) mineral particles. The generation of these bubbles is a physical process chemically assisted by addition of surfactants (frothers). The prime action of frothers is commonly considered to be coalescence inhibition. In contrast, the possible role of frothers in the initial air break-up has attracted little attention. In this paper, we investigate the effect of frothers on break-up. An experimental setup was designed to isolate break-up from coalescence. Single bubbles were produced through mechanically-induced deformation of a trapped air volume, the design aiming to mimic the break-up of the air cavity behind an impeller blade in a mechanical flotation machine. The results show that the bubble size produced at break-up is strongly affected by the presence of frother, but weakly by the input mechanical energy and the volume of trapped air. The proposed mechanism is that frothers induce surface tension gradients which increase the instabilities along the air/water interface. For a finite air volume, increasing the number of instabilities means that more and smaller bubbles will break away. Frothers of different types and concentrations were tested.