Dynamical heterogeneity and the freezing transition in hard-sphere suspensions: further analysis of the mean square displacement and the velocity autocorrelation function

Nanoparticle diffusion in polymer nanocomposites

Large-scale molecular dynamics simulations show that nanoparticle (NP) diffusivity in weakly interacting mixtures of NPs and polymer melts has two very different classes of behavior depending on their size. NP relaxation times and their diffusivities are completely described by the local, Rouse dynamics of the polymer chains for NPs smaller than the polymer entanglement mesh size. The motion of larger NPs, which are comparable to the entanglement mesh size, is significantly slowed by chain entanglements, and is not describable by the Stokes-Einstein relationship. Our results are in essentially quantitative agreement with a force-level generalized Langevin equation theory for all the NP sizes and chain lengths explored, and imply that for these lightly entangled systems, activated NP hopping is not important.

Aging in dense suspensions of soft thermosensitive microgel particles studied with particle-tracking microrheology

Using particle tracking microrheology, we studied the glass transition in dense suspensions of thermosensitive microgel particles. These suspensions can be tuned reversibly between the glass state at low temperature and the liquid state at high temperature. In the glass state, the ensemble averaged mean squared displacements (MSDs) of added fluorescent tracer particles depend on the age of the suspension. We also determine the local viscoelastic moduli, G' and G", from the MSDs using the Generalized Stokes-Einstein Relation and compare them to the bulk moduli, measured using conventional rheometry. With particle tracking, one probes the viscoelastic moduli in a lower frequency range than with macrorheology, which makes it possible to determine the mean relaxation time that is inaccessible with macrorheology. In the glass state, the mean relaxation time increases linearly with the age of the sample and the short time particle displacement distributions are non-Gaussian, indicating inhomogeneity of the system. The observed difference between conventional and microrheology is explained quantitatively assuming that the tracer particles are surrounded by a viscoelastic liquid shell, different from the bulk.

Scaling of the space-time correlation function of particle currents in a suspension of hard-sphere-like particles: exposing when the motion of particles is Brownian

The current correlation function is determined from dynamic light scattering measurements of a suspension of particles with hard spherelike interactions. For suspensions in thermodynamic equilibrium we find scaling of the space and time variables of the current correlation function. This finding supports the notion that the movement of suspended particles can be described in terms of uncorrelated Brownian encounters. However, in the metastable fluid, at volume fractions above freezing, this scaling fails.

Arrest of flow and emergence of activated processes at the glass transition of a suspension of particles with hard spherelike interactions

By combining aspects of the coherent and self-intermediate scattering functions, we show that the arrest of particle number density fluctuations spreads from the position of the main structure factor peak. We propose that this arrest impairs the system's ability to respond to diffusing momentum currents, leading to an enhanced resistance to flow. From the stretching of the coherent intermediate scattering functions in the glass, we read a manifestation of the undissipated thermal energy-the source of the ergodicity restoring processes that short-circuit the sharp transition to a perfect glass.

Slow dynamics and aging of a colloidal hard sphere glass

The intermediate scattering function (ISF) is measured for a colloidal hard-sphere glass as functions of the scattering vector and waiting time. For scattering vectors near the structure factor peak, we show that the ISF and the stretching index, defined at the crossover time between the fast and slow processes, depend algebraically on the waiting time. By contrast, the Debye-Waller factor is independent of the waiting time.

Comparison of dynamic light scattering measurements and mode-coupling theory for the tagged particle dynamics of a hard-sphere suspension

The mean-squared displacement, velocity autocorrelation function, and the non-Gaussian parameter, obtained by dynamic light scattering on suspensions of particles with hard-sphere interactions, are compared with the results of the idealized version of mode-coupling theory. Both leading order asymptotic and full numerical solutions of the mode-coupling equations are considered. Experiment and the full numerical results of the theory expose similar qualitative changes at the volume fraction of the first order freezing transition. In particular, the emergence of negative algebraic decays in the velocity autocorrelation function of the undercooled suspension suggest the emergence of clusters in which particles are trapped. Consistency of experiment, computer simulation, and theory in this regard suggests that, at particular strengths of the delayed, nonlinear feedback, contained in mode coupling theory, the latter predicts not only structural arrest which, as already established, is symptomatic of a glass transition, but also a more subtle change in dynamics that signals the onset of the first order transition.