Flowrate effects on the lateral coalescence of two growing bubbles

Abstract

The coalescence of two growing bubbles presents unique characteristics compared to static bubble coalescence. The gas injection flowrate significantly affects the different stages of bubble evolution, which is poorly understood. In this study, we investigate the flowrate effects on the lateral coalescence of two growing bubbles experimentally. The synchronous bubbling from adjacent needles is achieved using water to push air. During the bubble growth process, we find that the initial nonlinear evolution of bubble volume is because the bubble emerges as a small spherical cap with a large curvature radius and apparent contact angle. As the neck expands after bubble coalescence, the injection flowrate accelerates the neck evolution compared to the case without air injection. We find the neck expansion time decreases linearly with increasing flowrate, while the expansion speed increases with flowrate, but only in the early stage. Moreover, we propose a new theoretical expression that predicts the neck radius well at all the flowrates. At the post‐coalescence oscillation stage, the average projection area of the coalesced bubble increases linearly with time, except for periodic oscillations. Besides, we find that the injected air primarily influences the coalesced bubble's height, which in turn affects the projection area.

Coalescence of Two Equal-Sized Bubbles with Air Injection

“…As a result, the unbalanced surface tension at the neck propels, while the inertia hinders the neck expansion process. Moreover, continuous air injection exerts a significant impact on neck evolution …”

“…As for the theoretical expression for the neck radius, we proposed eq which includes air injection effects previously . The air-injection-induced pressure difference at the neck is modeled as a multiplier (α) of the Laplace pressure at the neck.…”

“…Moreover, continuous air injection exerts a significant impact on neck evolution. 17 Figure 8a depicts the temporal evolution of the gas−liquid interface during neck expansion (D = 1.05 mm, S = 2.10 mm, and Q = 20 mL•min −1 ). The overall bubble size and shape remain consistent as the neck expands outwardly.…”

“…14−16 Experimental images illustrate that r n increases over time following the bubble merging until it reaches the value equal to the bubble size at contact. 12,17 As for the theoretical description of the neck radius, the power-law scaling is introduced for the self-similar universal solutions. 3,11,18 Paulsen et al 19 found that neck expansion is always dominated by the gas viscosity during asymptotically early times.…”

“…In the presence of gas injection during bubble coalescence, our previous study has demonstrated that continuous air injection significantly promotes neck expansion. 17 Nevertheless, the neck evolution with air injection under other needle configurations and air injection flow rates has not been thoroughly investigated. Moreover, a universal description of neck evolution is still lacking.…”