Interplay of flow and microstructure in complex fluids: Semiflexible polymer solutions and emulsions
Doctor of Philosophy
This thesis investigates the interactions between inicrostructural properties and flow conditions in polymeric solutions and in emulsions. An efficient algorithm is presented for Brownian dynamics simulations of semiflexible bead-rod chains with configuration-dependent anisotropic bead friction tensor. The algorithm for the dynamics and the stress generalizes an earlier one restricted to isotropic, configuration-independent friction (Grassia and Hinch 1996); it is based on Morse's (2004) theory of constrained Brownian motion. The dynamics and the non linear viscoelasticity of dilute solutions of semiflexible polymers in shear flow are studied. The effects of chain stiffness and flow strength on the configurational and material properties of semiflexible polymers are discussed. It is shown that such molecules shrink in shear flow and assume highly bent configurations, due to a buckling instability. A theory to predict the onset of strong buckling is presented; according to this theory, worm-like polymer molecules transition from rod-like to flexible-like behavior. The theory is verified by Brownian dynamics, which show a decrease of the average end-to-end distance in flow and a transition from rod-like to coil-like of the shear thinning power law exponent. The dynamics of the collapse of a single, semiflexible polymer in a poor solvent are investigated via Brownian dynamics simulations. The rate of decay of uncollapsed states, the preferential pathways of condensation, and the likelihood and lifespan of the different metastable states are analyzed. The collapsed states are described quantitatively through the spatial correlation of tangent vectors along the chain. The viscoelastic behavior of attractive and repulsive emulsions is compared, to understand the effect of the interaction forces between droplets, and the interplay between microstructural features and rheological properties. The salient finding is that caged attractive emulsions show a viscosity plateau at intermediate shear rate, and the first normal stress difference N1 transitions sharply from nearly zero to negative in this region. Correspondingly, cylindrical flocs form, align along the vorticity, and undergo a logrolling movement. An analysis of the interplay between steric constraints, attractive forces, and composition explains this behavior.
Chemical engineering; Condensed matter physics