Bubble-bubble pinch-off in symmetric and asymmetric microfluidic expansion channels for ordered foam generation.

By incorporating the techniques of geometrically mediated splitting and bubble-bubble breakup, the present work offers a novel microfluidic foam generation system via production of segregated, mono- or bidisperse bubbles at capacities exceeding 10 000 bubbles per second. Bubble-bubble pinch-off is precise at high capillary numbers (Ca > 0.065), generating monodisperse or bidisperse daughter bubbles for a symmetric or an asymmetric expansion respectively. Bi- or tridisperse foam is produced as pinch-off perfectly alternates such that the system contains twice the number of fragmented bubbles as intact bubbles. A relationship between the upstream bubble extension and the capillary number demarcates the different regimes of pinch-off defined with respect to frequency and precision: non-splitting, irregular, polydisperse, and monodisperse (or bidisperse for an asymmetric expansion). For tridisperse foam generation via a fixed asymmetric expansion geometry, the wall bubble confinement can be tuned to adjust the pinch-off accuracy in order to access a spectrum of fragmented bubble size ratios. The simplicity in operating and characterizing our system will enable studies on dynamic bubble interactions and ordered, wet foam applications.