Anisotropy Cellular

Mechanical Properties and Structural Performance. As a result of the manufacturing process, some cellular plastics have an elongated cell shape and thus exhibit anisotropy in mechanical, thermal, and expansion properties (35,36). Efforts are underway to develop manufacturing techniques that reduce such anisotropy and its effects. In general, higher strengths occur for the paraHel-to-rise direction than in the perpendicular-to-rise orientation. Properties of these materials show variabiUty due to specimen form and position in the bulk material and to uncertainty in the axes with respect to direction of foam rise. Expanded and molded bead products exhibit Httie anisotropy. 

Kinsey, ST Locke, BR Penke, B Moerland, TS, Diffusional Anisotropy Is Induced by Sub-cellular Barriers in Skeletal Muscle, NMR in Biomedicine 12, 1, 1999. 

Free diffusion of molecules in solution is characteristically a haphazard process with net directionality determined only by solute gradients and diffusion coefficients. Within cellular and extracellular spaces, however, diffusion can be strongly influenced by noncovalent interactions of solvent and solute molecules with membranes as well as the cellular and extracellular matrix. Channels and orifices can also alter the movement of solute and solvent molecules. These interactions can greatly alter the magnitude of the diffusion coefficient for a molecule from its isotropic value D in water to apparent diffusion coefficient D (which often can be directionally resolved into D, Dy, and D ). The parameter A, known as the tortuosity, equals DID y. In principle, A has X, y, and z components that need not be equal if there is any anisotropy in the local electrical fields or porosity of the matrix. 

Various other instances of hydrodynamic and electrohydrodynamic instabilities in nematic and, to a lesser extent, smectic liquid crystals have been investigated. No attempt is made here to review this work. For the present discussion, it is sufficient to note that (a) most of the work has dealt with oriented layers having anisotropic properties, and (b) some interesting instabilities arise in oriented layers which do not occur for isotropic materials. An example of the latter is cellular convection in a fluid layer confined between horizontal plates maintained at different temperatures. With an isotropic fluid, convection can arise only if the lower plate is hotter than the upper plate. Then, fluid near the lower plate is less dense and tends to rise while fluid near the upper plate is denser and tends to sink. With an oriented layer, however, convection can arise even when the upper plate is hotter if the anisotropy of thermal conduction properties is of a particular type (8). 

P.-C. Lin, U. Kreutzer, and T. Jue. Anisotropy and temperature dependence of myoglobin translational diffusion in myocardium implication for oxygen transport and cellular architecture../. Am. Chem. Soc., 56 658-666, 1934. 

Figure 10.15. The expression of anisotropy in puffed extruded cellular solids. Notice the qualitative as well as the quantitative differenee in the force-displacement curves when determined in the longitudinal and transversal direetions.

This survey deals with the fundamental morphological parameters of foamed polymers including size, shape and number of cells, closeness of cells, cellular structure anisotropy, cell size distribution, surface area etc. The methods of measurement and calculation of these parameters are discussed. Attempts are made to evaluate the effect and the contribution of each of these parameters to the main physical properties of foamed polymers namely apparent density, strength and thermoconductivity. The cellular structure of foamed polymers is considered as a particular case of porous statistical systems. Future trends and tasks in the study of the morphology and cellular structure-properties relations are discussed. 

The cell aspect ratio, which is characteristic for foam anisotropy, can be determined by microscopically measuring the linear cell dimensions on sections cut from different portions of a foam block. The results of such measurements can only be representative of the morphology of certain portions of a cellular structure. A statistical analysis of samples cut of different block portions and in different directions with respect to that of expansion is a very difficult task. 

The cellular anisotropy of plastic foams may be evaluated by e.g. an anisotropy coefficient q which is equal to the ratio between average cell dimensions along the major symmetry axes of the respective model. An isotropic material Is characterized by only one anisotropy coefficient (q = 1), a transversally anisotropic material by two(qj = q and q ) and an orthotropic medium by three coefficients (q q q ). 

Cationic lipids can destabilize a cellular membrane because of its intrinsic detergent property. Therefore, destabilization of endosomal and/or lysosomal membrane may be a contribution from the cationic lipids itself In the same context, it was shown that the cationic lipid/DOPE or cationic lipid/cholesterol liposome formulation exhibit surface anisotropies in terms of increased liposomal surface pH (161,162). The surface pH of the liposomal formulations exhibits at least two pH units higher than the pH of the solution at which they are made. Therefore, a liposomal solution made at physiological pH may in reality exhibit a surface pH > 9, which is detrimental for both the stability of endosome and activity of lysosomal enzymes. Endosomal disruptions were also done with fusogenic peptides, which promote pH-dependent fusion of small liposomes when associated with lipid bilayer. When these peptides were co-delivered with lipid/DNA complex, they imparted formidable endosomal disruption by changing its usual random coil conformation into amphipathic a-helix conformation at lower pH, resulting in consequent cytoplasmic delivery of DNA (163). 

Favier used the USMP-1 opportunity to explore the interfacial breakdown in Bi-doped Sn, which, like most metals, solidifies as a plane front with little kinetic undercooling. His U.S. co-investigator, Abbaschian, investigated interfacial stability on the other side of the phase diagram i.e., Sn-doped Bi, which solidifies with a faceted interface. The purpose was to test the extension of the Mullins-Sekerka stability criterion to include the effects of anisotropy, which acts to stabilize the interface against breakdown into cellular and dendritic growth. " ... 

The situation is altered profoundly in the case of a nematic because of its anisotropic transport properties. Dubois-Violette was the first to give an approximate theoretical treatment of thermal convection in a planar (homogeneously aligned) nematic and to show by consideration of torques that such a system will be unstable against cellular flow when the film is heated from below if > 0, or when it is heated from above if < 0, where = K —K is the anisotropy of thermal conductivity (which is positive for all known nematics )- Dubois-Violette also showed that the critical temperature gradient fi (=ATJd) should be much less than that... 

In particular, A is directly proportional to the thermal conductivity of the foam and inversely proportional to the cellular anisotropy T) according to the formula [9] ... 

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