Effect of bubble spacing on bubble breakup dynamics in a T-junction microchannel

Simulation studies limited to single-bubble breakup are insufficient to fully reveal the actual mechanisms and application laws of passive breakup in microfluidic devices. This study, based on three-dimensional numerical simulations, systematically investigates for the first time the effect of bubble spacing on the breakup characteristics of bubbles in T-shaped microchannels with a square cross-section. By adjusting the inlet boundary conditions, precise control over the length and spacing of consecutive bubbles was achieved. Six typical flow patterns were identified, of which the irregular breakup (IB) and the symmetrical breakup after merging patterns (BM) were observed for the first time. New characteristic parameters were defined to quantitatively describe the three-dimensional breakup dynamics of bubbles in the junction region, based on which the evolution patterns of different flow regimes were analyzed. A flow pattern transition map was constructed based on the Capillary number ( Ca ) and bubble spacing. The results indicate that the minimum bubble spacing required for symmetric breakup gradually decreases with increasing Ca number. For short bubbles, the minimum relative bubble spacing required for symmetric breakup is typically greater than 1, especially at low Ca numbers. As the bubble length increases, the proportion of the symmetric breakup region gradually expands, and the bubble spacing has a markedly greater influence on breakup than either the Ca number or bubble length. A novel flow pattern transition correlation is proposed, which first quantitatively characterizes the relationship between bubble spacing and the transition between symmetric and asymmetric breakup patterns.