Generation of a Monodisperse Microbubble Jet in Microgravity

Abstract

A new method to create a jet of a virtually monodisperse microbubble suspension of prescribed bubble size into a quiescent cavity is proposed. The method is insensitive to gravity and is based on the creation of a slug flow at a T junction in a capillary tube before injection. We develop a theoretical analysis that establishes the validity and efficiency of the method, as controlled by the crossflow Weber number, and yields a simple explicit prediction for the bubble size in terms of the injection parameters. The method operates efficiently for small Weber numbers, yet it generates small bubbles of very uniform size. The reduced size dispersion is also explained within the theoretical model. The method of bubble formation, injection, and spreading by the resulting turbulent jet is validated experimentally in 4.7 s of free fall in the drop tower at the University of Bremen. Experiments demonstrate the physical principle behind the method of bubble formation and allow us to explore the dynamics of the resulting bubble jet after injection of the slug flow into a quiescent cavity in microgravity conditions as an efficient method of bubble spreading and transport. Detailed measurements of average local velocities show that bubbles are essentially passive with respect to the carrier mean flow, and the inherent turbulence of the flow is crucial for optimal spreading of the bubble distribution and reduction of bubble coalescence. The shape of the bubble jet is studied as a function of the Reynolds number. Finally, the degree of coalescence is also characterized and found to be remarkably small.

Characterization of the performance of a minibubble generator in conditions relevant to microgravity

“…The linear behaviour is a consequence of the bubble size being independent from the gas flow rate for small fluxes [6]. The physical idea is that in this regime the detachment of the forming bubble at the T-junction occurs when the drag with the liquid is large enough to overcome capillary forces [6].…”

“…The linear behaviour is a consequence of the bubble size being independent from the gas flow rate for small fluxes [6]. The physical idea is that in this regime the detachment of the forming bubble at the T-junction occurs when the drag with the liquid is large enough to overcome capillary forces [6]. On the contrary, for large gas flow rates there is a limiting scale for the bubble generation process, which is the time needed by the liquid flow to cross a distance of the order of the capillary diameter [7].…”

“…Then U SG0 is the superficial gas velocity at the crossover point between linear and saturation regimes, defined as the point where a line starting at the origin with a slope a intersects the horizontal line of the saturation region. Of these three parameters two of them carry a clear physical meaning and can be related to theoretical considerations, namely the initial slope (related to the bubble size, which in this regime is expected to depend only on the liquid flow [6]) and the saturation frequency (also related with the liquid flow [7]). The length of the crossover region is of no clear interest as long as it is small enough for allowing both regimes to be observable and does have a negligible effect on the computed results for the other two, physically more relevant, parameters.…”

“…Typically the liquid flow is used to aid bubble detachment, both in co-flow configuration [4], in which liquid flows parallel to the gas injection direction, and cross-flow configuration [5], in which liquid flows perpendicularly to the gas injection. We proposed a configuration [6] which is a particular case of the cross-flow one. A gas and a liquid flow are injected separately into a T-junction with circular section capillaries of the same diameter (1 mm i.d.).…”

“…A gas and a liquid flow are injected separately into a T-junction with circular section capillaries of the same diameter (1 mm i.d.). At the T-junction, gas bubbles are formed by the action of the liquid cross-flow [6,7]. In the nominal regime of the bubble generator, capillary forces dominate over buoyancy (small Bond number 1 , B0 =0.138), and over inertial forces (small Weber number 2 , We from 6·10 -3 to 22).…”