In general, there are three ways of generating microbubbles. The most common class uses compression of the air stream to dissolve air into liquid, which is subsequently released through a specially designed nozzle system, to nucleate small bubbles as potentially nanobubbles, based on the cavitation principle. These bubbles subsequently grow into much larger bubbles through the rapid dissolution of the supersaturated liquid. The second class uses power ultrasound to induce cavitation locally at points of extreme rarefaction in the standing ultrasonic waves. The third class uses an air stream delivered under low offset pressure, and airs to break off the bubbles due to an additional feature, whether it be mechanical vibration, or flow focussing, or fluidic oscillation. Conventional air diffusers rely on the structure of porous material for the nozzles to generate small bubbles, but fluidic oscillation in general promises to break off the forming bubble while it is still a hemispherical cap - the smallest shape for which bubble formation from a pore is likely to occur given the strong adverse affect of surface tension at higher curvatures. The first two classes of microbubble generation are usually associated with high power densities and power consumption by either the compression or ultrasonic treatment. The third class should have the lowest power consumption, provided it achieves the application targets of bubble size distribution, air phase holdup, and bubble dispersion. In this paper, recent patents in microbubble generation are categorized into the first and the third classes above. The subject area is reviewed for its importance in several fields of application, particularly generalized flotation processes and bioreactor treatments.

This article was published in the journal, Applied Energy [© Elsevier]. The definitive version is available at: http://dx.doi.org/10.1016/j.apenergy.2011.02.013This study was conducted to test the feasibility of growing microalgae on steel plant exhaust gas, generated from the combustion of offgases from steel processing, which has a high CO content. Two field trials of batch algal biomass growth, mediated by microbubble transfer processes in an airlift loop bioreactor showed only steady growth of biomass with 100% survival rate. The gas analysis of CO uptake in the 2200L bioreactor showed a specific uptake rate of 0.1g/L/h, an average 14% of the CO available in the exhaust gas with a 23% composition of CO. This uptake led to a steady production of chlorophyll and total lipid constituency in the bioreactor, and an accelerating exponential growth rate of biomass, with a top doubling time of 1.8days. The gas analysis also showed anti-correlation of CO uptake and O production, which along with the apparent stripping of the O to the equilibrium level by the microbubbles, strongly suggests that the bioreactor is not mass transfer limited, nor O inhibited. Removing O inhibition results in high growth rates and high density of biomass. © 2011 Elsevier Ltd