Dynamic interplay between flow and bubbles in polymer foaming drives surprising consequences for nucleation and ripening*

Polymer foams are often born through high supersaturations of dissolved gas and high shear stresses from the nozzle. Just as high supersaturation nucleates bubbles that reduce local viscosity and drive flow, so can high shear stresses mix dissolved gas back into depleted regions and drive bubble nucleation. This dynamic interplay between bubbles and flow has a significant impact on foam structure but lacks experimental study due to the stochasticity of homogeneous nucleation and challenge of observing such short time and length scales.

Here, we share surprising observations of nucleation and subsequent ripening, coalescence, and breakup of carbon dioxide (CO2) bubbles in supersaturated polyol under high shear. We capture these behaviors in a custom, high-pressure microfluidic flow-focusing device using high-speed microscopy. In particular, we show that large bubbles can entrain CO2 into an extended wake, which becomes fertile ground to nucleate more bubbles. These bubbles are strongly deformed by the flow in the wake as they grow and encounter each other, leading to unexpected “winners” in the competition for CO2 during ripening. Most bubbles coalesce into a larger bubble which, once again, entrains CO2 in an extended wake, repeating the cycle.