Attachment of individual particles to a stationary air bubble in model systems

A particle-bubble attachment apparatus, similar to that used by Whelan and Brown [Bull. Inst. Min. Met. Trans. 65 (1956) 181] is developed to study free falling glass beads (d(p)=131 mum) with different surface treatment attaching to a stationary air bubble (d(b)=3 mm). The entire process of the particle-bubble attachment is photographically recorded using a high framing-rate camera and analyzed frame by frame. It is found that the hydrophilic particle slides on the top half of the bubble, and then the particle leaves the bubble, as anticipated. The particle sliding velocity increases with increasing angular position on the bubble surface. For the hydrophobic particle, it was found that the particle slides over the entire bubble surface without detaching from the bubble. The sliding velocity, however, reaches a maximum at an angular position of 90-100degrees, and then slows down to zero. A smaller sliding velocity is obtained for the hydrophobic particle than that of the hydrophilic particle at a given angular position. The attachment efficiency for the hydrophobic particle reduces with increasing initial angular positions, suggesting that the hydrophobicity alone does not directly guarantee the attachment to a bubble. Adding surfactant (sodium dodecyl sulfate [SDS]) to the test solution makes the hydrophobic particle become less hydrophobic or even hydrophilic, thereby decreasing the attachment efficiency. A much longer induction time is obtained in our present work than that reported in the literature. A simplified particle sliding velocity and attachment model is developed, and a good agreement between the theoretical prediction and experimental measurement is obtained. (C) 2003 Elsevier Science B.V All rights reserved.