Morphologies of Low-Density Cellular Materials
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ECEMBER1990
A reticulate foam is an open-cell foam with a strutlike structure. The term should be restricted to require firmly joined, reasonably straight struts. One idéal version is derived from a froth precursor (Figure 2.1b). The faces of the froth polyhedra are destroyed (e.g., by combustion), leaving the accumulated material along junction arcs mentioned previously. The final struts tend to be fairly straight and meet tetrahedrally, but there usually are substantial déviations from this, particularly when the cell size is small. We are unaware of any LDMMs with this idéal structure. Some model geometries fall into the reticulate category, and theoretical studies of différences in their mechanical properties hâve proved illuminating. 2 Tetrahedral reticulate foams are relatively stiff. A syntactic foam consists of hollow shells (typically spherical) b o n d e d together. Thèse foams hâve an open/ closed-cell structure, the shells being closed cells and space between them forming an open network of connected channels. Epoxy-bonded carbon shells and lightly sintered glass microballoons are examples. With additional sintering, the microballoon structure can evolve into a froth. We are unaware of any syntactic LDMMs. What we call a spinodal foam morphology is an open, bifurcated morphology. Ideally, the structure would consist of a highly bifurcated sheet structure with holes, conceptually approximating a nonintersecting periodic minimal surface. A periodic minimal surface is doubly curved everywhere so that its mean curvature is zéro everywhere. 4 Closely related are analogous surfaces of constant (nonzero) mean curvature. This morphology is theoretically associated with foams derived by thermally induced phase séparation of a polymer dissolved in a low molecular weight solvent (see the section on Synthesis). In practice, LDMMs formed this way only rarely display anything resembling the idéal spinodal morphology, presumably because structures readily coarsen and, thereby, change in appearance. (Figure 2.1c). Some foams made by replication do exhibit a spinodal type of morphology; replica carbon made from well-sintered sait approximates it (see the section on Carbon LDMMs), because the surface of
the porous sait tends toward a constant mean curvature. A nearly idéal example is a silicone foam made from a sea urchin spine (Figure 2.1d). Foams of this morphology hâve relatively low stiffness. The inverse emulsion m o r p h o l o g y arises when the material distribution is more or less concentrated where poreformers (liquid droplets or spherical solid particles) neck or are nearly in contact. The idéal version consists of fairly thick rings of material at the necks with rather thin films covering the rest of the droplet surface (Figure 2.1e). The relative thicknesses of the rings and films dépends on process détails, such as surfactant concentration, etc. Conceptually, the inverse emulsion morphology might be an open or a closed-cell one, but in practice there are usually enough holes in the rings or films to make t
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