Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y42: Physics and Applications of FoamsInvited Live Streamed
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Sponsoring Units: DPOLY DSOFT FIAP Chair: Kshitish Patankar, Dow, Inc. Room: McCormick Place W-375A |
Friday, March 18, 2022 8:00AM - 8:36AM |
Y42.00001: Finding the homogenous-nucleation "needle" in the polymer-foam "haystack" using theory as our guide Invited Speaker: Julie A Kornfield 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. |
Friday, March 18, 2022 8:36AM - 9:12AM |
Y42.00002: Retrograde vitrification equation of state for polymeric foaming Invited Speaker: Russell B Thompson A key to controlling the cell structure of foams is the interaction between the polymer and the blowing agent. Clearly, the pressure-volume-temperature equation of state relationship as expressed through the solubility of a blowing agent in a polymer melt is at the heart of the design and prediction of polymeric foams, particularly for micro- and nanocellular foams, where the voids are on the order of microns and nanometers, respectively. The Sanchez-Lacombe equation of state is a lattice-based choice that is widely used because it is a first principles rigorous approach while at the same time being coarse-grained with phenomenological parameters. This makes it, in many cases, qualitatively and quantitatively acceptable, provided the parameters are carefully calibrated while still being practical to implement in potentially complicated engineering situations. The Sanchez-Lacombe equation of state suffers from inconsistencies however, and I will discuss what these are and what can be done about them using an off-lattice approach. I will also discuss how finite flexibility of polymers can be incorporated into the off-lattice version in a way that also removes problems with the Gibbs-DiMarzio condition, so that the glass transition and retrograde vitrification in polymer solutions can be characterized. The latter has become particularly important in polymeric foaming. I will give some examples using common commodity polymers and blowing agents, and I'll discuss the range of applicability and limitations of the off-lattice model. |
Friday, March 18, 2022 9:12AM - 9:48AM |
Y42.00003: What Is in That Film? Invited Speaker: Yihan Liu The collapse of foam via liquid drainage and film rupture have been extensively studied. The current understanding is that many factors critically influence liquid drainage and film thinning including viscosity of the bulk liquid, surface viscosity and elasticity, surfactant transport, disjoining pressure (surface forces), and instability under hydrodynamic and capillary effects. Once the film is sufficiently thin (black film), thermal-induced fluctuation of surfactant concentration can lead to nucleation of a necking hole, the growth of which then completes the film rupture process. The literature on foam stability and its mechanism is voluminous. Many sophisticated measurement techniques have been developed. Most of the experimental data, however, were acquired on well-controlled foams, whereas foam conditions in practical applications are often too complicated to define. The common approach based on the viewpoint of the forces/pressures involved, is not very useful in practice because these various forces―Van der Waals and the electrostatic (treated by the DLVO theory), steric, capillary, etc.―are not easily accessible or known in the application. For a formulator in designing a foam, he/she needs to be able to predict foam stability based on the ingredients added by using a general understanding that can be checked with readily measurable properties. In this talk, a practical approach is presented. This approach is none other than the classical method of using phase behavior to predict film stability against rupture, a practice that has shown tremendous success in predicting liquid-liquid emulsion stability. It focuses on recognizing the structural entity that is most pertinent to the question at hand―the association structure of the surfactant. A case of aqueous foam in the presence of oil as relating to aqueous firefighting foams will be used as an example to illustrate the pragmatism of this approach. |
Friday, March 18, 2022 9:48AM - 10:24AM |
Y42.00004: Polymer Foams for Building Insulation (STYROFOAM and beyond) Invited Speaker: Stephane Costeux Thermoplastic foams have been used in buildings for over 75 years, since the initial introduction of STYROFOAM™ extruded polystyrene foam. Significant progress has been made to improve the product performance and its sustainability, through the use of more environmentally-friendly blowing agents and optimized foam structure. Yet, |
Friday, March 18, 2022 10:24AM - 11:00AM |
Y42.00005: Bubble Nucleation in Polymer–CO2 Mixtures Invited Speaker: Zhen-Gang Wang Using a PC-SAFT (perturbed-chain statistical associating fluid theory) based density functional theory (DFT) in combination with the string method, we investigate bubble nucleation in CO2-supersaturated polymers in both binary and ternary systems. Below the critical temperature of CO2, bubble nucleation can have two kinds of nuclei: vapor-like and liquid-liquid, with the liquid-like nuclei generally having a lower nucleation barrier. Nucleation in a two-component (polymer+ CO2) system generally requires very large supersaturation (high initial CO2 pressure). However, the addition of a third, volatile component (physical blowing agent) can significantly lower the nucleation barrier in the vicinity of a liquid–liquid–vapor 3-phase coexistence. We have also examined a polymer–CO2–surfactant ternary mixture using polypropylene glycol (polyol) as the polymer and polypropylene–poly dimethyl siloxane (PPO–PDMS) diblock copolymer as the surfactant. Adding CO2 can significantly decrease the critical micelle concentration of the surfactant, and adding surfactant to the polyol–CO2 can significantly reduce the bubble nucleation barrier, with complex nucleation pathways. |
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