Transient effects and the role of wetting in microbubble generation

Microbubble dispersions are now commonly deployed in industrial applications ranging from bioprocesses to chemical reaction engineering, at full scale. There are five major classes of microbubble generation devices that are scalable. In recent years, some of these approaches have been explicitly studied for the influence of wetting properties on microbubble performance, for which the major proxy is the bubble size distribution (BSD). In this piece, the methodologies for inferring BSD are explored, with several recent advances as well as their potential pitfalls. Subsequently, studies where microbubble generation has been investigation for wetting effects are assessed, and in some cases, those that were not allow the deduction that wetting is a significant factor. Two particular studies are highlighted – (i) systematic variation of wetting effects within a venturi with removable walls substituted with coated walls of known contact angle with hydrodynamic cavitation induced microbubbles; (ii) variation of ionic liquids with staged fluidic oscillation before steady flow. The first shows that even in scenarios where high inertial effects would be expected to dominate, wetting influences are significant. The second shows that transient effects are strongly influenced by both imbibition into pores and surface wetting, but that viscous resistance is always a key factor. From the exploration of these recent studies, specific recommendations are made about how to assess the influence of wetting in those mechanisms/devices where it has not been explicitly studied, via deduction from those mechanisms/devices where the effects are demonstrable significant, and indeed in some cases, controlling. In study (ii), which is the first to blow micro/bubbles into ionic liquids, wetting and transient effects are reasonable for between 25-50% reduction in average bubble size, although up to 70% reduction is observable when viscous effects are dominant, relative to the control of steady flow with the same pressure drop. Indeed, staging transient operations shows both bubble size reduction and increased volumetric throughput are simultaneously possible.