Deformation Dynamics of Wet Foams and Bubbles in Wide Microfluidic Channels
Vecchiolla, Daniel J
Biswal, Sibani Lisa
Doctor of Philosophy
A prominent feature of microfluidics is the ability to generate monodisperse bubbles (or droplets) of tunable volumes and densities, which rapidly self-order under the confinement of the channels. The geometric and hydrodynamic controllability of these systems enables the precise handling and manipulation of the fluids to foster the plastic deformation of crystalline foam and bubble-bubble pinch-off using wide (>1000 μm) channels. Densely packed wet foam was subjected to expansion-contraction flow to study localized plastic deformation events from extensional and compressive stresses within the monodisperse bubble matrix. Dislocations cyclically reflected in tension or compression with disparate mechanisms in two independent rearrangement zones. The relationship between structures resembling the Inverse-Stone-Wales (ISW) defect and the partially dissociated ISW defect of graphene was examined in the model foam. An extended expansion region to force the flowing crystal out of long-range order to investigate 2-D phase transitions (i.e. melting and recrystallization) could be a promising area of future research. Symmetric and asymmetric expansions were utilized for promoting systematic bubble-bubble pinch-off to produce segregated, mono- and bidisperse bubbles at capacities exceeding 10,000 bubbles per second. The pinch-off dynamics demonstrate that bubbles split from the confinement of a “pincher” bubble and “wall” bubble, in connection with pore-level breakup mechanisms previously discovered by our lab. The wall bubble was shown to modulate the fluidic resistance in an asymmetric expansion, allowing the fragmented bubble size ratio to be adjusted by tuning the size of the bubbles formed upstream. Consequently, the system acts as a generator of ordered bi- or tridisperse foam that can be employed to study dynamic bubble interactions (e.g. coarsening) and ordered, multidisperse foam deformation. Cross-shaped surface energy wells with comparable in-plane dimensions to the initially trapped bubbles were employed to study the interactions between a large trapped bubble and the smaller monodisperse bubbles of the surrounding foam including diffusive gas exchange, bubble-bubble breakup and large bubble migration.