Fundamentals And Kinetics Of The Colliding Bubble Attachment To Hydrophobic Surfaces

Bubble collisions with ore grains and a selective formation of the bubble-grain aggregates are necessary steps for flotation separation of useful and gangue components of an ore. For the attachment of valuable mineral grain to the colliding bubble a liquid film, separating the bubble and the grain needs to rupture during short time of their collision. High hydrophobicity of the grain surface is favorable because stability of the separating (wetting) film is much lower at such surfaces, that is, the film ruptures faster and at greater thickness. Simultaneously, it needs to be kept in mind and taken into consideration that high affinity to air is a typical feature of hydrophobic surfaces. Thus, when the hydrophobic surface is immersed into aqueous phase the air can stay attached to such surfaces and affect kinetics of the bubble attachment. The paper presents results and analysis of dynamic phenomena occurring upon the collision of the rising bubble followed by its attachment to hydrophobic (Teflon) plates of different roughness. The influence of the Teflon surface roughness, solution concentration, bubble size and impact velocity on bouncing of the colliding bubble, time of the three phase contact (TPC) formation and drainage time of the liquid film, formed between the solid and bubble surfaces, is described. High hydrophobicity, as showed earlier (Malysa et al, 2005), does not assure that each collision leads to the TPC formation and attachment of the colliding bubble. Roughness is the parameter of crucial importance for kinetics of the bubble attachment to hydrophobic surfaces (Krasowska and Malysa, 2007; Krasowska et al, 2009; Kosior et al, 2011). Enlarged roughness of the Teflon plate, from ca. 1 to 80µm, results in shortening the time of the TPC formation (bubble attachment) by over an order of magnitude - from 105 to a few milliseconds only. Higher surface roughness means that larger amount of air can be entrapped in bigger scratches and cavities of the Teflon plate during its immersion into aqueous phase. Results documenting importance of the air presence are showed and mechanism of the air facilitated TPC formation and bubble attachment to hydrophobic surfaces is described. Keywords: bubble collisions, hydrophobic surfaces, teflon plate roughness, TPC formation