Reducing Bubble Size Detached from Thin-wall Needles Submerged in Liquids with Pulsating Gas Flows

The present work explores the feasibility of generating axisymmetric sub-millimeter bubbles with pulsating gas flows from thin-wall needles submerged in liquids through numerical simulations using an OpenFOAM® volume-of-fluid solver. The results for needles of inner diameters D from 0.05 to 1 mm suggest that bubbles of diameters d less than 2 D can be produced with sufficient pulse amplitude, i.e., bubbles of d < 0.1 mm may be obtained with needles of D ~ 0.05 mm, especially for liquids of relatively low viscosity. An investigation of the numerically simulated effects of liquid viscosity and surface tension reveals that the normalized bubble size d / D (regarded here as the minimum feasible value under given conditions) depends primarily on Reynolds number (Re) and Weber number (We). For example, to obtain d / D < 2 appears to require Re > 1000 while We is above a threshold value, with the feasibility window shrinking when D is reduced. Hence, using smaller needles for generating smaller bubbles could encounter more stringent requirements. As expected, liquids with a smaller contact angle (e.g., < 45o) on the needle wall are more desirable for generating smaller bubbles. Moreover, using the pulsating gas flow is likely to enable well-controlled operations with reasonable gas throughput while repeatably generating microbubbles. It can also facilitate various types of bubble-on-demand applications.