Résumé:

Pinning a fraction of particles in a liquid has been recently proposed as a means to probe high-order static and dynamics correlations. In particular, the response of model supercooled liquids to the external field exerted by a set of pinned particles has allowed to reveal growing point-to-set length scales [1] and the non-monotonic temperature dependence of a dynamic length scale [2]. To test the universality of these phenomena, we explore by numerical simulations the effect of different pinning geometries on various model liquids. We relate the observation of non-monotonic dynamic length scales in a weakly fragile, close-packed liquid to the suppression of static correlations reflecting the preferred local order of the liquid. The simulation results obtained by random pinning are discussed within the frameworks of the random first order transition theory and of the mode-coupling theory, and are compared to those obtained by studying finite size effects [3]. [1] G. M. Hocky, T. E. Markland, D. R. Reichman, Phys. Rev. Lett. 108, 225506 (2012) [2] W. Kob, S. Roldan-Vargas, L. Berthier, Nature Phys. 8, 697 (2012) [3] L. Berthier, G. Biroli, D. Coslovich, W. Kob, C. Toninelli, Phys. Rev. E 86, 031502 (2012)

Ref HAL: hal-00753725_v1
PMID 23163391
Ref Arxiv: 1209.1471
DOI: 10.1063/1.4765704
WoS: 000311317800035
Ref. & Cit.: NASA ADS
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22 Citations
Résumé:

We present molecular dynamics (MD) simulations results for dense fluids of ultrasoft, fully-penetrable particles. These are a binary mixture and a polydisperse system of particles interacting via the generalized exponential model, which is known to yield cluster crystal phases for the corresponding monodisperse systems. Because of the dispersity in the particle size, the systems investigated in this work do not crystallize and form disordered cluster phases. The clustering transition appears as a smooth crossover to a regime in which particles are mostly located in clusters, isolated particles being infrequent. The analysis of the internal cluster structure reveals microsegregation of the big and small particles, with a strong homo-coordination in the binary mixture. Upon further lowering the temperature below the clustering transition, the motion of the clusters' centers-of-mass slows down dramatically, giving way to a cluster glass transition. In the cluster glass, the diffusivities remain finite and display an activated temperature dependence, indicating that relaxation in the cluster glass occurs via particle hopping in a nearly arrested matrix of clusters. Finally we discuss the influence of the microscopic dynamics on the transport properties by comparing the MD results with Monte Carlo simulations.

Ref HAL: hal-00705279_v1
Ref Arxiv: 1203.3392
DOI: 10.1103/PhysRevE.86.031502
WoS: 000308530600006
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
68 Citations
Résumé:

We present a comprehensive theoretical study of finite size effects in the relaxation dynamics of glass-forming liquids. Our analysis is motivated by recent theoretical progress regarding the understanding of relevant correlation length scales in liquids approaching the glass transition. We obtain predictions both from general theoretical arguments and from a variety of specific perspectives: mode-coupling theory, kinetically constrained and defect models, and random first order transition theory. In the latter approach, we predict in particular a non-monotonic evolution of finite size effects across the mode-coupling crossover due to the competition between mode-coupling and activated relaxation. We study the role of competing relaxation mechanisms in giving rise to non-monotonic finite size effects by devising a kinetically constrained model where the proximity to the mode-coupling singularity can be continuously tuned by changing the lattice topology. We use our theoretical findings to interpret the results of extensive molecular dynamics studies of four model liquids with distinct structures and kinetic fragilities. While the less fragile model only displays modest finite size effects, we find a more significant size dependence evolving with temperature for more fragile models, such as Lennard-Jones particles and soft spheres. Finally, for a binary mixture of harmonic spheres we observe the predicted non-monotonic temperature evolution of finite size effects near the fitted mode-coupling singularity, suggesting that the crossover from mode-coupling to activated dynamics is more pronounced for this model. Finally, we discuss the close connection between our results and the recent report of a non-monotonic temperature evolution of a dynamic length scale near the mode-coupling crossover in harmonic spheres.

Commentaires: 19 pages, 10 figures

Ref HAL: hal-00668236_v1
DOI: 10.1080/00268976.2011.621459
WoS: 000299109300026
Exporter : BibTex | endNote
6 Citations
Résumé:

We generalize the familiar effective DLVO (Derjaguin-Landau-Verwey-Overbeek) pair potential between charged, hard core colloidal particles to the case of solutions of oppositely charged, penetrable polyelectrolyte coils in the presence of microions, within the framework of classical Density Functional Theory. The limiting behaviour of the effective potentials is derived in the limits of weak and strong microion screening; in the latter regime the effective potentials are shown to go over to a universal Gaussian form, multiplied by the square of the microion Debye screening length. The physical implications of screening on polyelectrolyte aggregation are discussed and illustrated by preliminary Monte Carlo simulations and the results of fluid integral equations for the polyelectrolyte pair structure.

Résumé:

We discuss the relationship between many-body static correlations and fragility in model viscous liquids. To probe static correlations beyond the pair level we analyze the spatial extension of clusters of interconnected locally preferred structures [1]. We also extract the point-to-set correlation lengths by randomly pinning the positions of finite fractions of particles. As a general rule, we find that many-body static correlations of LJ mixtures starts growing markedly upon cooling below the onset temperature T_O. Deep in the slow dynamics regime, the thermal rate of growth of static correlations is higher the more fragile the liquid. These two observations allow us to rationalize the large discrepancies of dynamic behavior between LJ and WCA liquids reported in [2]. Eventually, we compare our results for close-packed liquids to those for a model of strong, tetrahedral liquid. In this latter case, the growth of point-to-set correlation lengths by decreasing temperature appears weaker than in the LJ liquids. Our results thus indicate that inclusion of many-body static correlations in theories of the glass transition should be most crucial for the description of fragile glass-formers. [1] D. Coslovich, PRE 83, 051505 (2011) [2] L. Berthier and G. Tarjus PRL 103, 170601 (2009)

Résumé:

We will review recent results on primitive models of oppositely charged polyions and present a new, ultrasoft core model of interpenetrating polycations and polyanions with continuous Gaussian charge distributions. The model aims at investigating the aggregation process ("complex coacervation") of ultrasoft polyelectrolytes in dilute and semi-dilute solutions, in the absence and presence of added salt. In the salt-free case, the effective interaction between the polyions is given by a bounded potential at short distances and a long-range Coulomb interaction. By means of numerical simulations, we show that the topology of the phase diagram of the symmetric version of the model (the ``ultrasoft restricted primitive model'') differ from that of the widely studied ``restricted primitive model'' (RPM), where ions have hard cores. At sufficiently low temperatures and densities, oppositely charged polyions form weakly interacting, polarizable neutral pairs, leading to a sharp conductor-insulator transition. The conductor-insulator transition line terminates near the top of a first order coexistence curve separating a high-density liquid phase from a low-density vapor phase. The simulation data thus hint at a tricritical behavior, reminiscent of that observed in the two-dimensional Coulomb Gas, which contrasts with the Ising criticality of its three-dimensional counterpart, the RPM. The effect of salt addition on the physical properties of the model and the possible aggregation patterns in the asymmetric version of the model will be briefly discussed.