Bubble formation by water release in nozzle---II. Influence of various parameters on bubble size

Our aim was to study the parameters influencing the size of bubbles formed by the release of water under pressure through a nozzle in the dissolved air flotation process. The nozzles were simulated by glass tubes with a constriction (Reynolds tubes). A micro-cinematographic technique was used to visualize of the bubbles formed. We were particularly interested in the influence of a physical parameter (the tube geometry) and some physical-chemical parameters (connected with the surface energy of the tubes and the composition of the water). Tube geometry plays a key role: The greater the conical divergence angle, the greater the degree of turbulence and the smaller the bubbles obtained. On the other hand, the coalescence of bubbles on the walls of hydrophobic tubes leads to the formation of macrobubbles. Surfactants and ethanol encourage the largest bubbles to break up into microbubbles. Electrolytes, by reducing electrostatic repulsion, favour bubble coalescence.