Finding the homogenous-nucleation "needle" in the polymer-foam "haystack" using theory as our guide*

Measurements of homogeneous bubble nucleation in polymers have been limited by stochasticity in space and time. To mitigate stochasticity in space, we localize supersaturation near the centerline of flow through a channel using a custom high-pressure (> 10 MPa) microfluidic focusing apparatus.  While nucleation remains stochastic in time, the linear decrease in pressure along the channel of the apparatus affords positions at which homogeneous nucleation occurs frequently (order once per second). Estimates of the nucleation barrier using the string method with DFT suggested homogeneous nucleation of CO₂ in polyol could occur in industrial high-pressure foaming and in our microfluidic focusing apparatus, e.g., for 25-30%w/w CO₂ (equilibrium pressure ca. 7 MPa) upon quenching to a pressure of ca. 1 MPa. Indeed, we observe the moment a CO₂ bubble becomes optically detectable (ca. 1 μm diameter) and its early growth (first few ms). However, bubble nucleation occurs on the nanoscale. To connect optical measurements of bubble size to the moment of nucleation, we extrapolated backwards in time using a model and measurements of CO₂ diffusivity. Despite the inherent uncertainties, the extrapolations place a bound on the lag time between nucleation and first detection, typically < 200 μs. The ability to observe homogeneous nucleation in CO₂/polyol sets the stage for studies of the influence of surfactants and blowing agents with the potential to improve thermally-insulating foams.