Shear modulus of two-dimensional foams: The effect of area dispersity and disorder

We use the Surface Evolver to determine the shear modulus G of a dry 2D foam of 2500 bubbles, using both extensional and simple shear. We examine G for a range of monodisperse, bidisperse and polydisperse foams, and relate it to various measures of the structural disorder of each foam. In all cases, the shear modulus of a foam decreases with increasing disorder.     

Collapse dynamics of smectic-A bubbles

The collapse dynamics of smectic-A bubbles are analyzed experimentally and theoretically. Each bubble is expanded from a flat film stretched at the end of a hollow cylinder and deflated through a pressure release by means of a capillary tube. Its total collapse time can be varied between 0.1s and 20s by suitably choosing the length and the internal diameter of the capillary. This experiment allowed us to show that the collapse takes place in two steps: an initial one, which lasts a fraction of a second, where the meniscus destabilizes and fills up with focal conics, followed by a much longer period during which the bubble collapses and exchanges material with the meniscus. By measuring simultaneously the Laplace pressure and the internal pressure inside the bubble, we were able to fully characterize the shear-thinning behavior of the smectic phase within the meniscus. We emphasize that this method is generic and could be applied as well to other systems such as soap bubbles, on condition that inertial effects are negligible.     

Hydrogen Economy and Polymer Membranes

Summary: Hydrogen can be separated from its mixtures using polymer foams with closed cells. Each cell serves as a gas container which is filled through its walls – separation membranes. Foam, as a manifold membrane system, utilizes transient states of permeation and thus takes advantage of fastest diffusion of hydrogen. Large scale manufactured polystyrene foams were chosen to demonstrate the phenomenon. Novel proton conducting membranes containing ionic liquids are being developed. They can perform in intermediate-temperature fuel cells (FC).     

耐高温聚酰亚胺泡沫材料

聚酰亚胺泡沫具有低介电、隔热、吸声、高比强度以及高经济效益等诸多优点,因而近些年来在航空、航天、船舶航舰、能源与环境保护等领域有着广泛的应用。聚酰亚胺泡沫按照泡孔结构分为软质开孔泡沫和硬质闭孔泡沫两大类,其通常是由芳香族二酐与芳香族二胺通过缩聚反应制备得到分子量可控的聚酯铵盐,再将其作为前驱体经过热发泡制备得到最终的聚酰亚胺泡沫。前驱体的化学结构对最终的聚酰亚胺泡沫的机械性能和热性能都有非常显著的影响,同时前驱体的分子量也会对泡沫的密度、机械性能和热性能有非常显著的影响。聚酰亚胺泡沫的研究进展,特别是其化学结构、性能和应用都会在本文中逐一阐述。     

Boron/nitrogen flame retardant additives cross‐linked cellulose nanofibril/montmorillonite aerogels toward super‐low flammability and improved mechanical properties

Low‐flammability freeze‐dried cellulose nanofibril (CNF)/sodium montmorillonite (MMT) aerogels with improved mechanical properties were fabricated via a facile cross‐linking of boric acid (BA) and melamine‐formaldehyde (MF) resins. Scanning electron microscopy analysis showed that BA cross‐linking reduced the interspacing of layered CNF/MMT aerogels whereas introduction of MF formed polymeric fibrils that connected the layers. These changes on microstructures resulted in the improvement on compressive mechanical properties of the cross‐linked aerogels. Moreover, the boron (B)/nitrogen (N) containing flame retardant cross‐linkers greatly increased the limiting oxygen index values that could reach 85% and leveled the UL‐94 rating from no rating to V‐0. Cone calorimetric results suggested that BA and MF induced a synergistic effect on the flame retardant properties of the CNF/MMT aerogels. However, the thermal conductivity was little affected because pore structure and size was not substantially modified. This simple approach fabricated highly flame‐resistant and mechanically strong CNF‐based aerogels that could be used in various engineering fields.     

Microfluidic fabrication of polysiloxane/dimethyl methylphosphonate flame‐retardant microcapsule and its application in silicone foams

A novel and versatile route for fabricating flame‐retardant microcapsules via microfluidics technology is reported. The flame‐retardant microcapsules were prepared with a dimethyl methylphosphonate (DMMP) core and an ultraviolet‐curable (UV‐curable) polysiloxane shell. Furthermore, a UV‐curable polysiloxane was synthesized. The synthesis mechanism of UV‐curable polysiloxane and the curing mechanism of flame‐retardant microcapsules were analyzed. To verify that DMMP was encapsulated in the microcapsules, X‐ray fluorescence was used before and after microencapsulation. The resulting microcapsules were well monodispersed and exhibited a good spherical shape with a smooth surface. In addition, the size of the microcapsules decreased dramatically with an increasing flow‐rate ratio of the middle‐/inner‐phase or outer‐phase flow rate. The thermal stability of the microcapsules was worse than shell materials but superior to DMMP. Silicone foams (SiFs) with microcapsules prepared using a dehydrogenation method achieved a relatively higher limiting oxygen‐index value than the pure SiF, which indicated that the microcapsules could enhance the flame retardation of SiFs effectively. Because of the polysiloxane shell, the microcapsules had good compatibility with SiFs, and the influence of microcapsules on the mechanical properties of SiFs was unremarkable.     

Metallic foams: Radiative properties/comparison between different models

The aim of this study is to determine the radiative properties, which are the extinction coefficient, the scattering albedo and the scattering phase function, of highly porous open-cell aluminium foam, using more-or-less simple predictive models, and to compare all these models. The radiative properties are predicted using geometric optics laws to model the interaction of radiation with the particles forming the foam. Moreover, the particles forming the foam are large compared with the considered wavelength and are supposed to be sufficiently distant from each other to scatter radiation independently. Thus, the radiative characteristics of the foam can be determined by adding the contributions of each particle. A particular attention is paid on microstructure analysis and modelling. We considered different kinds of cell shapes and struts cross-section, using microscopic and tomographic analysis. Furthermore, a new phase function modelling is presented. Finally, we compare the results of each method with the radiative properties obtained from experimental measurements of directional and hemispherical transmittances and hemispherical reflectance.     

Mixing and sorting of bidisperse two-dimensional bubbles

We have examined a number of candidates for the minimum-surface-energy arrangement of two-dimensional clusters composed of N bubbles of area 1 and N bubbles of area λ ( λ≤1). These include hexagonal bubbles sorted into two monodisperse honeycomb tilings, and various mixed periodic tilings with at most four bubbles per unit cell. We identify, as a function of λ, the minimal configuration for N → ∞. For finite N, the energy of the external (i.e., cluster-gas) boundary and that of the interface between honeycombs in “phase-separated” clusters have to be taken into account. We estimate these contributions and find the lowest total energy configuration for each pair (N,λ). As λ is varied, this alternates between a circular cluster of one of the mixed tilings, and “partial wetting” of the monodisperse honeycomb of bubble area 1 by the monodisperse honeycomb of bubble area λ. Received 1 August 2002 RID="a" ID="a"e-mail: paulo@ist.utl.pt     

Second-Order Estimates for the Macroscopic Response and Loss of Ellipticity in Porous Rubbers at Large Deformations

This work presents the application of a recently proposed second-order homogenization method (Ponte Castañeda, 2002) to generate estimates for effective behavior and loss of ellipticity in hyperelastic porous materials with random microstructures that are subjected to finite deformations. The main concept behind the method is the introduction of an optimally selected linear thermoelastic comparison composite, which can then be used to convert available linear homogenization estimates into new estimates for the nonlinear hyperelastic composite. In this paper, explicit results are provided for the case where the matrix is taken to be isotropic and strongly elliptic. In spite of the strong ellipticity of the matrix phase, the homogenized second-order estimates for the overall behavior are found to lose ellipticity at sufficiently large compressive deformations corresponding to the possible development of shear band-type instabilities (Abeyaratne and Triantafyllidis, 1984). The reasons for this result have been linked to the evolution of the microstructure, which, under appropriate loading conditions, can induce geometric softening leading to overall loss of ellipticity. Furthermore, the second-order homogenization method has the merit that it recovers the exact evolution of the porosity under a finite-deformation history in the limit of incompressible behavior for the matrix. Mathematics Subject Classifications (2000) 49S05, 74B20, 74Q15, 74Q05.     

Stress-strain relations for elastomeric foams in uni-, bi- and tri-axial compression modes

Summary Based on suggestions given in [1], the uni-axial compressive stress-strain relations for elastomeric foams, which were developed in [2], have been extended to cover the bi- and tri-axial stress modes of compression for both open- and closed-cell elastomeric foams.The stress-strain relations for uni- and tri-axial stress compression modes were validated by comparing numerical predictions, which were based on them, to experimental results.