The “Little” Bang: Tracing Bubble Growth Back to Nucleation in Polymer Foaming*

Bubble nucleation sets the structure of polymer foams well before bubbles grow to their final size, but it is more difficult to measure than growth. Despite the many models of bubble nucleation, they lack experimental validation, often due to the challenge of observing fleeting nanoscopic nuclei. Here, we estimate the nucleation time of bubbles by first modeling their growth after they reach an optically observable size, then extrapolating backwards in time to their expected size at nucleation. We examined bubbles of CO2 in polyol as they underwent a rapid (< 1 s) depressurization from as much as 10 MPa to atmospheric pressure along a custom-built microfluidic channel. Bubbles were recorded with high-speed optical microscopy as short as tens of microseconds after nucleation. We modeled their growth with an Epstein-Plesset model modified to account for depressurization, using thermophysical properties of polyol and CO2 mixtures measured with the Di Maio group (U Naples) as model parameters. We compared our results to predictions of nucleation energy barriers made using the string method applied to density functional theory developed by the Wang group (Caltech).