The Physics of Complex Materials: Macroscopic Approaches
- PDF / 1,092,138 Bytes
- 3 Pages / 604.8 x 806.4 pts Page_size
- 95 Downloads / 175 Views
IN/JANUARY1991
form, and we shall see how they are used to quickly and simply visualize difficult nonequilibrium, nonlinear dynamic phenomena driven by electric fields and température gradients.
Complexity générâtes rules of its own on scales much larger than the microscopic scale of the molécules involved. P.G. de Gennes is well-known as a major contributor to many aspects of the physics of condensed Systems: superconductivity, liquid crystals, polymers, magnetism, and phase transitions. He was the first person to teach and apply the unifying power of a macroscopic physical approach to advancing our understanding of complex Systems. In his article, he p r o p o s e s a " n o n c o n formist" approach coupled with his reptation idea adopted by polymer theorists such as Doi, Edwards and others. The mean field of ail the polymer chains interacting with a particular chain is modeled by a tube surrounding it. A particular chain r a n d o m l y wriggles around, or reptates like a snake, inside its oversized tube. What happens when the polymer is sheared? This, of course, is a problem of immense practical importance for our "plastic âge," surrounded as we are by products formed by pouring
polymers into molds, by extrusion, or some other shearing process. Doi and Edwards predicted that after shear is applied, the distance spanned by the molécule in the direction of the shear gradient shrinks as the molécule retracts within its tube. In neutron scattering experiments, no shrinkage has been seen. De Gennes' article explains the lack of shrinkage by a novel "dissipation argument," which, when coupled with his reptation idea, predicts no retraction during shear. The implication is that viscosity is independent of shear rate when the shear rate is large. In "Polymer-Dispersed Liquid Crystals: Boojums at Work," J.W. Doane describes an impressive application that results when monomers and nematic liquid crystals (LCs), characterized by long-range orientational order, are mixed to form a dispersion of fine LC droplets. Then the monomers are rapidly polymerized to form a solid matrix around the droplets. The resuit is a new génération of liquid crystal télévision projection displays with greater brightness, color, and speed than conventional liquid crystal télévision displays. Thèse displays make use of a topological theorem: you cannot stroke a spherical cat (hère, the surfaces of the nematic droplets in contact with the polymer) so that its fur (the direction of orientational order) is everywhere flat on its body. Somewhere on the surface there will be a fur tuft (a point defect in the directional order). Inspired by Lewis Carroll's poem, "The Hunting of the Snark," N.D. Mermin has called point defects on a surface boojums: a boojum remains while other entities "softly and suddenly vanish away." Nematic liquid crystals can use nonlinear elasticity to escape symmetry imposed by certain boundary conditions that lead to Une defects in linear elasticity. Thus, some line defects can "softly and suddenly vanish away" in the bulk of t
Data Loading...
