Anisotropic porous structures

The structure of the so-called "composite" membranes used in reverse osmosis is also much more complex than the conventional, simplistic description of the ultrathin semipermeable film deposited on and supported by a porous substrate. Most of these membranes which exhibit high flux and separation are composed of an anisotropic, porous substrate topped by an anisotropic, ultrathin permselective dense layer which is either highly crosslinked, or exhibits a progressively decreased hydrophilicity toward the surface. The basic difference between the conventional anisotropic (asymmetric) membrane and the thin film composite is that the latter might be... 

The highly anisotropic porous texture of alumina membranes containing monodispersed cylindrical pores has been characterised using a combination of three techniques SEM, mercury porosimetry and SANS. The SANS technique is a promising and very sensitive method for the analysis of such anisotropic pore structures. Further quantitative analysis of these membranes, which contain uniform macropores, will require SANS measurements at much lower Q. 

In studies of a completely different type of porous structure, Lipshitz and Etheredge [24] showed that articular cartilage is anisotropic in flow of interstitial fluid and that its properties are a function of the impedence to flow during and following compression. 

For porous electrodes, an additional frequency dispersion appears. First, it can be induced by a non-local effect when a dimension of a system (for example, pore length) is shorter than a characteristic length (for example, diffusion length), i.e. for diffusion in finite space. Second, the distribution characteristic may refer to various heterogeneities such as roughness, distribution of pores, surface disorder and anisotropic surface structures. De Levie used a transmission-line-equivalent circuit to simulate the frequency response in a pore where cylindrical pore shape, equal radius and length for all pores were assumed [14]. 

In the case of filler particles in shape of fibers or platelets (as, e.g., organoclays), oriented randomly, the distribution of voids created at their interfaces on loading can be not homogeneous enough to produce a uniform porous structure needed for successful toughening. Therefore, toughening with anisotropic rigid particles seems more difficult than with semi-equiaxed ones. 

Bruschke, M.V. Advani, S.G. A finite element/control volume approach to mold filling in anisotropic porous media. Polym. Compos. 1990, 11, 398-405. Bruschke, M.V. Advani, S.G. RTM Filling simulation of complex three-dimensional shell-like structures. SAMPE Q. 1991, 22, 2-11. 

TOF-NIRS was introduced to clarify the optical characteristics of wood having a cellular or porous structure as an anisotropic medium. The combined effects of the wood structure, the wavelength of the laser beam, and the detection position of transmitted light on the time-resolved profiles were investigated. In this study, the time-resolved profile of a sample with thickness of 1 mm was taken as a reference. 

Matz, R. (1972). The structure of cellulose acetate membranes 1. The development of porous structures in anisotropic membranes. Desalination 10, 1. 

We have overviewed some strategies for the surface-mediated fabrication of metal and alloy nanoscale wires and particles in mesoporous space, and their structural characterization and catalytic performances. Extension of the present approaches for metal/alloy nanowires may lead to the realization of the prospechve tailored design of super active, selective and stable catalysts applicable in industrial processes. The organometallic clusters and nanowires offer exciting and prospechve opportunities for the creahon of new catalysts for industry. Various metal/ alloy nanowires and nanoparhcles in the anisotropic arrangement in porous supports would help in understanding the unexpected electronic and optic properties due to the quantum effect, which are relevant to the rational design of advanced electronic and optic devices. 

The membrane cast from chloroform-formic acid mixtures had an anisotropic structure with a 0.9-1.2 p active layer and a 40 p porous support layer. At a water flux of 139 1/m2 day (kg/cm2 at 20 °C), the membrane showed 99.4 % rejection of cytochrome C and 72.7% of Vitamine B12. At 3980 1/m2 day water flux level, the rejection for bovine serum hemoglobin (MW, 66000 68000), cytochrome C, and Vitamine Bl2 were, 95.6, 79.4, and 39.8%, respectively. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

---