Theory of gelation, vitrification, and activated barrier hopping in mixtures of hard and sticky spheres.
ABSTRACT Naive mode coupling theory (NMCT) and the nonlinear stochastic Langevin equation theory of activated dynamics have been generalized to mixtures of spherical particles. Two types of ideal nonergodicity transitions are predicted corresponding to localization of both, or only one, species. The NMCT transition signals a dynamical crossover to activated barrier hopping dynamics. For binary mixtures of equal diameter hard and attractive spheres, a mixture composition sensitive "glass-melting" type of phenomenon is predicted at high total packing fractions and weak attractions. As the total packing fraction decreases, a transition to partial localization occurs corresponding to the coexistence of a tightly localized sticky species in a gel-like state with a fluid of hard spheres. Complex behavior of the localization lengths and shear moduli exist because of the competition between excluded volume caging forces and attraction-induced physical bond formation between sticky particles. Beyond the NMCT transition, a two-dimensional nonequilibrium free energy surface emerges, which quantifies cooperative activated motions. The barrier locations and heights are sensitive to the relative amplitude of the cooperative displacements of the different species.
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ABSTRACT: We report a detailed experimental study of the structure and dynamics of glassy states in hard spheres with short-range attraction. The system is a suspension of nearly hard-sphere colloidal particles and nonadsorbing linear polymer which induces a depletion attraction between the particles. Observation of crystallization reveals a reentrant glass transition. Static light scattering shows a continuous change in the static structure factors upon increasing attraction. Dynamic light scattering results, which cover 11 orders of magnitude in time, are consistent with the existence of two distinct kinds of glasses, those dominated by interparticle repulsion and caging, and those dominated by attraction. Samples close to the "A3 point" predicted by mode coupling theory for such systems show very slow, logarithmic dynamics.Physical Review E 02/2004; 69(1 Pt 1):011503. · 2.31 Impact Factor
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ABSTRACT: Computer simulations were used to study the gel transition occurring in colloidal systems with short-range attractions. A colloid-polymer mixture was modeled and the results were compared with mode coupling theory (MCT) expectations and with the results for other systems (hard-spheres system and Lennard-Jones system). The self-intermediate scattering function and the mean squared displacement were used as the main dynamical quantities. Two different colloid packing fractions have been studied. For the lower packing fraction, alpha-scaling holds and the wave-vector analysis of the correlation function shows that gelation is a regular nonergodicity transition within MCT. The leading mechanism for the novel nonergodicity transition is identified as the bond formation caused by the short-range attraction. The time scale and diffusion coefficient also show qualitatively the expected behavior, although different exponents are found for the power-law divergences of these two quantities. The non-Gaussian parameter was also studied and a very large correction to Gaussian behavior was found. The system with higher colloid packing fraction shows indications of a nearby high-order singularity, causing alpha scaling to fail, but the general expectations for nonergodicity transitions still hold.Physical Review E 04/2003; 67(3 Pt 1):031406. · 2.31 Impact Factor
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ABSTRACT: The transition from a liquid to a glass in colloidal suspensions of particles interacting through a hard core plus an attractive square-well potential is studied within the mode-coupling-theory framework. When the width of the attractive potential is much shorter than the hard-core diameter, a reentrant behavior of the liquid-glass line and a glass-glass-transition line are found in the temperature-density plane of the model. For small well-width values, the glass-glass-transition line terminates in a third-order bifurcation point, i.e., in a A3 (cusp) singularity. On increasing the square-well width, the glass-glass line disappears, giving rise to a fourth-order A4 (swallow-tail) singularity at a critical well width. Close to the A3 and A4 singularities the decay of the density correlators shows stretching of huge dynamical windows, in particular logarithmic time dependence.Physical Review E 02/2001; 63(1 Pt 1):011401. · 2.31 Impact Factor