Anomalous dynamic arrest in a mixture of large and small particles

Abstract

We present molecular dynamics simulations on the slow dynamics of a mixture of big and small soft-spheres with a large size disparity. Dynamics are investigated in a broad range of temperature and mixture composition. As a consequence of large size disparity, big and small particles exhibit very different relaxation times. As previously reported for simple models of short-ranged attractive colloids and polymer blends, several anomalous dynamic features are observed: i) sublinear behavior for mean squared displacements, ii) concave-to-convex crossover for density-density correlators, by varying temperature or wavevector, iii) logarithmic decay for specific wavevectors of density-density correlators. These anomalous features are observed over time intervals extending up to four decades, and strongly resemble predictions of the Mode Coupling Theory (MCT) for state points close to higher-order MCT transitions, which originate from the competition between different mechanisms for dynamic arrest. For the big particles we suggest competition between soft-sphere repulsion and depletion effects induced by neighboring small particles. For the small particles we suggest competition between bulk-like dynamics and confinement, respectively induced by neighboring small particles and by the slow matrix of big particles. By increasing the size disparity, a new relaxation scenario arises for the small particles. Self-correlators decay to zero at temperatures where densitydensity correlations are frozen. The behavior of the latters resembles features characteristic of type-A MCT transitions, defined by a zero value of the critical non-ergodicity parameter.

Atomic motions in the αβ-region of glass-forming polymers: molecular versus mode coupling theory approach

“…Values of λ approaching 1 point to a situation close to a MCT higher-order transition (λ = 1) [20]. Features associated to such transitions have been recently reported for attractive colloids [21] and a series of systems showing strong confinement effects [22]. The origin of the observed anomalous relaxation features is attributed to the presence of several competing mechanisms for dynamic arrest -steric repulsion and reversible bond formation in [21], bulk-like caging and confinement in [22].…”

“…Features associated to such transitions have been recently reported for attractive colloids [21] and a series of systems showing strong confinement effects [22]. The origin of the observed anomalous relaxation features is attributed to the presence of several competing mechanisms for dynamic arrest -steric repulsion and reversible bond formation in [21], bulk-like caging and confinement in [22]. In the case of the αβ-region of real polymers we may speculate that there also exist two active competing mechanisms leading to dynamic arrest: i) packing, of intermolecular character and present in all glass-forming systems; ii) barriers for conformational changes ("β-like"), of intramolecular origin, which are specific of macromolecular systems.…”

Glass formation in a mixture of hard disks and hard ellipses

“…44 For instance, multiple glasses occur even in simple binary hard-disk mixtures, as indicated by MCT calculations 62,63 and simulations. 64,65 Here, we demonstrate that the composition strongly influences the glassy dynamics in mixtures of hard disks and hard ellipses.…”

Complex dynamics induced by strong confinement – From tracer diffusion in strongly heterogeneous media to glassy relaxation of dense fluids in narrow slits

“…For example, for the alkali silica melts [26][27][28][29] the preferential diffusion pathways for the alkali ions emerge as a quasi-arrested defect structure of a tetrahedral network. Similarly, for size-disparate soft [31] or hard spheres [36] the interaction between the particles leads to strong short-range correlations which modify locally the structure of the pores. Nevertheless fingerprints of the anomalous dynamics as predicted for the Lorentz models are clearly found in all these systems.…”

“…with strong directional bonds yielding only a loosely packed host structure. However, computer simulations for strongly size-disparate mixtures interacting via soft-sphere repulsion [31,32] or repulsive Yukawa interactions [33] display the same phenomenology without invoking chemical effects or polymeric aspects [34]. Therefore one infers that the scenario of weakly coupled dynamics is generic.…”