Alejandro Rey

Computational Rheology of Carbonaceous Mesophases

A review of flow and texture modeling of liquid crystalline materials with emphasis on carbonaceous mesophases is presented.  Two models of nematodynamics are presented and discussed  in terms of their ability to resolve  time and length scales likely to arise in typical rheological and processing flows.   Defect physics  and rheophysics are integrated with nematodynamics and specific mechanisms of defect nucleation and annihilation are used to derive texture scale power laws. The integrated nematodynamics models specialized to carbonaceous mesophases are used to analyze: (i) linear and nonlinear viscoelasticity, (ii) rheological flows, and (iii) carbon fiber and flow-induced  textures. The linear and nonlinear viscoelasticity reveals the essential nature of these materials : coupling between flow-induced orientation and orientation-induced flow , elastic storage through orientation gradients, and anisotropy.  The rheological flow simulations, shown to be in excellent agreement  with experimental data, reveal several liquid crystal specific rheological characteristics including shear thinning due to anisotropic viscosities and flow-induced orientation, and negative first normal stress difference due to orientation nonlinearities in the shear stress. Nematodynamic predictions are shown to follows a Carreau-Yasuda liquid crystal equation.  Nematodynamics predictions rationalize shear-induced texture refinement in terms of defect nucleation and coarsening mechanisms and are used to derive texture scaling relations  in terms of  macroscopic, molecular, and flow time scales.  This knowledge is then condensed into a generic texture-flow  diagram that specifies the required temperature and Deborah number required to produce well oriented monodomain materials.  The fine details of mesophase structuring by flow through screens are shown to be captured by nematostatic simulations.  Finally the mechanisms behind the carbon fiber textures produced by melt spinning of carbonaceous mesophases are elucidated.  The proven range and predictive accuracy of nematodynamics to simulate flows of textured mesophases and the ever-growing industrial interest in lower cost high performance super-fibers and functional materials will fuel the evolution of liquid crystal rheology and processing science for years to come.