Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S33: Physics of Foams: Fundamentals and ApplicationsFocus
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Sponsoring Units: DPOLY FIAP DSOFT GSNP Chair: Kshitish Patankar, Dow Room: 505 |
Thursday, March 5, 2020 11:15AM - 11:51AM |
S33.00001: Bubble Nucleation in Polymer-CO2 Mixtures Invited Speaker: Zhen-Gang Wang Bubble nucleation is a critical first step in the foaming process, but its theoretical understanding is still rather incomplete. While the classical nucleation theory (CNT) offers a qualitative picture of the process, more advanced theories are needed to better describe molecular details. Here, we combine the string method with the state-of-the-art density functional theory (DFT) to investigate bubble nucleation in CO2-supersaturated polymers. Nucleation in our study is initiated by saturating the polymer liquid with high pressure CO2 gas and subsequently reducing the pressure to ambient condition. The string method is used to find the minimum free energy path on the free energy landscape that connects the metastable CO2-supersaturated state with a well-developed bubble. The nucleation pathway, i.e., the evolution of the density profiles in the formation of the nuclei, as well as the nucleation barrier, are obtained as a function of the temperature and the initial pressure. We present an incipient-phase analysis which allows identification of phases that can form from the metastable CO2-supersaturated parent phase. As result of an underlying metastable transition from an incipient CO2-rich-vapor phase to an incipient CO2-rich-liquid phase, there can be two kinds of nucleating bubbles, a liquid-like bubble and a vapor-like bubble, which can differ substantially in their nucleation barrier. We apply our theory to three common polymers: polystyrene, polymethylmethacrylate, and a commercial polyol. We also examine the effects of adding a third volatile component to the polymer-CO2 mixture. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S33.00002: Equilibrium coexistence between polyol, CO2, and a physical blowing agent at elevated pressures Huikuan Chao, Andrew Ylitalo, Julie Kornfield, Valeriy Ginzburg, Weijun Zhou, Thomas Fitzgibbons, Zhen-Gang Wang Rigid polyurethane (PU) foams are widely used as thermal and acoustic insulation materials. The insulation properties depend critically on the size and distribution of cells from nucleation and growth of small-molecule bubbles during foaming. To achieve desired cell structures, the foaming recipe needs to be formulated. Besides reactants(polyols, isocyanates and water), the recipe typically contains physical blowing agents (PBAs) to increase the nucleation rate of bubbles. However, due to complex chemical and physical reactions, how PBAs affect the bubble formation are not understood. As a fist step to approach the issue, we develop a liquid-state theory of a ternary foaming system (PBA, CO2 and polyols) and study the phase behaviors of the system at manufacture-relevant conditions. Our results show a transition in the system from a liquid-vapor into a liquid-liquid coexistence via an intermediate triple-phase coexistence with increasing amount of PBA. When the polyol-rich phase in the coexistence is destabilized (e.g. upon rapid decompression), we find the incipient phase that metastablly coexists with the polyol-rich phase changes its nature from a vapor-like to a liquid-like PBA/CO2-rich phase with increasing weight fraction PAB in the polyol-rich phase. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S33.00003: Microfluidic Approach to Study Bubble Nucleation in Polymeric Foams for the Development of DFT-based Models Andrew Ylitalo, Huikuan Chao, Thomas Fitzgibbons, Valeriy Ginzburg, Weijun Zhou, Zhen-Gang Wang, Julie Kornfield Bubble nucleation sets the structural foundation of a foam, yet remains poorly understood. Single-component classical nucleation theory predicts erroneous nucleation barriers, particularly when applied to foaming of multicomponent liquids used to produce lightweight, functional materials. Multicomponent theories are limited by lack of (1) thermodynamic and (2) nucleation rate data for validation. For the first, we used gravimetry-axisymmetric drop shape analysis (G-ADSA) in the Di Maio lab (U Naples) to measure equilibrium compositions and interfacial tension of polymer-CO2 mixtures up to 8MPa, which validated PC-SAFT and DFT models from the Wang group (Caltech) that enable string method calculations of the nucleation barrier. For the second, we developed a microfluidic hydrodynamic-focusing apparatus that linearly decreases pressure from 10MPa to as low as 0.1MPa within 100ms while observing bubble nucleation and growth. The instrument, designed for use with optical microscopy, X-ray scattering, thermography and FTIR, will be described. This approach to validating multicomponent theories of bubble nucleation can be extended to study complex effects of additives on foaming and enable design of foam composition and processing conditions to achieve desired material properties. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S33.00004: An Off-Lattice Sanchez-Lacombe Related Equation of State Extensible to Polymeric Foams Hassan Alam, Chul B. Park, Russell B Thompson We have extended an off-lattice model of the Sanchez-Lacombe equation of state for pure polymeric systems by including internal degrees of freedom to polymer chains. The extension allows us to predict glass transition temperatures of pure polymeric systems at different pressures. To find unknown parameters of the model only two experimental values are required, namely, the glass transition temperature of the pure polymer at atmospheric pressure and the change in isobaric heat capacity of the pure polymer across the glass transition. The model is also successful in predicting the behavior of isobaric heat capacities of polymer melts as a function of temperature. The model shows that the Gibbs-DiMarzio criterion for the glass transition is invalid for the Sanchez-Lacombe equation of state. Thus, we proposed a new criterion for glass transition temperature calculations. Predictions of the model are compared with a lattice-based version of the Sanchez-Lacombe equation of state for several pure polymeric systems. The proposed model can be extended to multi-component polymeric systems and thus to retrograde vitrification in polymeric foams. |
Thursday, March 5, 2020 12:27PM - 1:03PM |
S33.00005: Experimental Test of the Border-Crossing Model of Diffusive Coarsening in Wet Foams Invited Speaker: Douglas Durian For dry foams, the transport of gas from small high-pressure bubbles to large low-pressure bubbles is dominated by diffusion across the thin soap films separating neighboring bubbles. For wetter foams, the film areas become smaller as the Plateau borders and vertices inflate with liquid. So-called ``border-blocking" models can explain some features of wet-foam coarsening based on the presumption that the inflated borders totally block the gas flux; however, this approximation dramatically fails in the wet/unjamming limit where the bubbles become close-packed spheres. Recently we accounted for the ever-present border-crossing flux by a new length scale defined by the average gradient of gas concentration inside the borders that is proportional to the geometric average of film and border thicknesses. We also showed how the well-known dA/dt=K(n−6) von Neumann law is modified by the appearance of terms that depend on bubble size and bubble shape as well as the concentration gradient length scale. Here we extend this theory and describe on-going experiments to test it by measurement of area changes for six-sided bubbles, which would neither grow nor shrink if perfectly dry. Indeed we find six-sided bubbles that change area with time, and we find that they do so in good accord with our prediction based on their size and shape and degree of wetness. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S33.00006: Experimentally Testing a Generalized Coarsening Model for Quasi-Two-Dimensional Wet Foams Anthony Chieco, Douglas Durian In dry foams, a bubble's area grows or shrinks according only to its number of sides, dA/dt=K0(n-6). While exact for a purely two dimensional foam with no liquid content this von Neumann law is increasingly violated for wetter quasi-2d foams. These latter foams have Plateau borders that are inflated with liquid and extend into the z-plane. Accounting for the size of the Plateau borders and gas that diffuses through both the Plateau Borders and thin films separating two bubbles, we modify von Neumann's law to a no-fit general coarsening equation where bubble size and shape now matter. To test this experimentally, we measure the growth rate of individual bubbles in quasi-2d foams of variable wetness confined between parallel plates. Interestingly, some 6-sided bubbles grow and others shrink - in direct violation of the usual von Neumann law but in agreement with our generalized version. We will show the coarsening of 6-sided bubbles and other violations of von Neumann's law for n-sided bubbles are driven predominantly by the bubble shape which is a key ingredient in our model. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S33.00007: Intelligent Design of Cellular Solids for Impact Mitigation Marcos Reyes-Martinez, Meng Shen, Edwin P Chan, Christopher Soles, Nidhi Pashine, Sidney Robert Nagel, Heinrich M. Jaeger, Juan De Pablo Disordered networks, comprised of random arrangements of bonds and nodes, offer degrees of mechanical tunability not accessible to periodic systems. These networks have emerged as the basis of a new class of metamaterials allowing for precise control over density (ρ), bulk (K), shear (μ) modulus, and Poisson’s ratio (ν). Here, we demonstrate bond pruning and global node position optimization as effective routes for designing cellular materials with isotropic auxetic properties, and for independently controlling μ and K. We show how bond pruning controls ρ, μ and K. Node position optimization allows for the tunability of ν from ∼0.3 to −0.5, control over two orders of magnitude of K with almost no change to μ and minimal change in foam density. We use multi-resin 3D-printing to extract the 2D and 3D disordered networks out of the simulation box and systematically study how the tunability of elastic constants affect their dynamic deformation behavior. We explore how node bending stiffness can affect global elastic properties and quantify how it affects the range of elastic tunability. This study demonstrates the power of top-down design of disordered network metamaterials and how such level of tunabililty can affect the next generation of materials for impact mitigation. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S33.00008: Heat transfer of low-density styrenic foam Anson Wong, Mark Rickard The heat transfer characteristics of extruded styrenic foams with various carbonaceous additives were reported. The effects of cellular morphology and type of additives on the thermal conductivity of the foams were analyzed with a predictive model. The total thermal conductivity as well as hemispherical reflectance and transmittance were measured. Comparison between the theoretical and experimental results was made to assess the performance of the predictive model. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S33.00009: Human Comfort and the Physics of Foams Kaoru Aou, Wenbo Xu, Laura Dietsche, Douglas Brune, Manoj Thota Human Comfort and the Physics of Foams is a current topic in the bedding and furniture industry. Polyurethane (PU) foams are thermally insulating, cushioning materials. Flexible open cell PU foams are used in padding materials for furnitures and bed mattresses, where insulation can be useful, but depending on the local climate, insulation can lead to localized heating that can lead to discomfort. In order to maintain comfort, two factors are needed: (1) preventing overheating, and (2) good cushioning (absence of pressure points). Thermal comfort simulation was conducted using the ASHRAE Human Comfort model, coupled with a multiphysics model taking into account natural convection of air, thermal conduction and mass (water vapor) diffusion phenomena. It is demonstrated that effective diffusivity of water vapor through foam is the critical parameter that determines whether the human body is able to regulate its skin temperature. Pressure relief measurements and simulations were also conducted, using an indenter held at a constant temperature of 23 and 34 °C. The relative insensitivity of the contact pressure to indenter temperature is reported. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S33.00010: Foam formation in microfluidic EDGE devices: tuning the bubble size Boxin Deng, Karin Schroën, Jolet De Ruiter The stability and flow properties of foams crucially depend on the bubble size and its distribution. Microfluidic devices are capable of producing extremely monodisperse yet small foam bubbles at low energetic cost. Upscaling towards an array of parallelized pores allows to increase the throughput of the device, but increases complexity of bubble formation due to possible pore-pore and bubble-bubble interactions. We use so-called partitioned-EDGE microfluidic chips with which we can produce bubbles in an array of rectangular pores. Pore height is minimized to (sub)micron size in order to produce bubbles down to 20 micron in diameter. The size of a single bubble formed at a pore is independent of pressure for the lower range of applied gas pressures (spontaneous formation mechanism), followed by bubble size increase (blow-up) at higher gas pressures. Here we discuss the final bubble size obtained for high gas flow rate through an array of parallel pores, which induces additional bubble-bubble interactions that can be modified by shear flow rate and stabilizer (protein) concentration. Surprisingly, monodisperse foams with a coefficient of variation around 10% can be produced, irrespective of these additional complexities. |
Thursday, March 5, 2020 2:03PM - 2:15PM |
S33.00011: Phoamtonic designs yield sizeable 3D photonic band gaps Michael Andreas Klatt, Paul J Steinhardt, Salvatore Torquato We show that it is possible to construct foam-based heterostructures with complete photonic band gaps. Three-dimensional foams are promising candidates for the self-organization of large photonic networks with combinations of physical characteristics that may be useful for applications. The largest band gap found is based on 3D Weaire-Phelan foam, a structure that was originally introduced as a solution to the Kelvin problem of finding the 3D tessellation comprised of equal-volume cells that has the least surface area. The photonic band gap has a maximal size of 16.9% (at a volume fraction of 21.6% for a dielectric contrast of 13) and a high degree of isotropy, properties that are advantageous in designing photonic waveguides and circuits. We also present results for two other foam-based heterostructures based on Kelvin and C15 foams that have somewhat smaller but still significant band gaps. |
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