Theory of slow coupled translational-rotational dynamics and viscoelasticity of nonspherical particle suspensions

摘要

The na??ve mode coupling theory (NMCT) for ideal kinetic arrest and the nonlinear Langevin equation (NLE) theory of activated single-particle barrier hopping dynamics are generalized to describe coupled center-of-mass (CM) translational and rotational motions of uniaxial particles within the interaction site formalism. The approach is based on the time-dependent scalar displacements of the particle CM and cumulative rotational angle as the relevant slow variables, and a two-dimensional dynamic free energy surface determined by equilibrium structure which quantifies localizing forces and torques. For hard-core uniaxial objects, three types of dynamic phases are predicted: fluid, plastic glass and double glass, the boundaries of which meet at a triple point corresponding to a most difficult to vitrify diatomic of aspect ratio ~ 1.43. The real space nature of the cage escape process is increasingly controlled by CM translation relative to rotational motion as the aspect ratio grows. The kinetic vitrification volume fraction and elastic shear modulus are nonmonotonic functions of aspect ratio. The first microscopic theory of the nonlinear viscoelasticity of dense fluids of nonspherical particles is presented. It provides a first principles explanation of the striking two-step yielding phenomenon experimentally observed in very concentrated low aspect ratio hard dicolloid suspensions. Stress induces a much stronger barrier softening effect on the second-step CM translation barrier than the first-step primarily rotational barrier. This barrier softening ???mismatch??? results in the prediction that for both absolute and dynamic yielding, the double yielding phenomena only occurs over a window of high volume fractions, which shifts to higher values for lower stress-sweep frequency. For large aspect ratio dicolloids, only one barrier exists for all stresses corresponding to a cooperative translation-rotation motion, and the translational motion is more dominant in the barrier hopping process as stress grows.For dicolloids that interact via short-range attractions, new complexity emerges due to the interplay of rotational degrees of freedom and bond formation. For large aspect ratio systems, translation and rotation are coupled in all activated regimes (repulsive glass, attractive glass and gel). Similar activated dynamic and shear elasticity properties are predicted for homogeneous and Janus dicolloids. For low aspect ratio suspensions, four activated regimes are possible (plastic glass, repulsive glass, attractive glass and gel). A ???plastic gel??? is not predicted. Attractions can greatly reduce translation-rotation decoupling. The no-fit-parameter NMCT-NLE theory calculations are in good agreement with experiments on the new nonspherical homogeneous colloid systems fabricated by Kramb and Zukoski. Modest shape anisotropy strongly delays kinetic arrest, and a re-entrant glass-to-fluid-to-gel transition as a function of ionic strength occurs for both spherical and nonspherical particles. The shear modulus grows roughly exponentially with volume fraction for all particle shapes and ionic strengths. For nearly hard core particles, a theoretically inspired universal master plot can be achieved for all shapes and repulsion strengths based on either the NMCT crossover or random close packing jamming as the relevant measure of crowding.