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Register a new userNetwork formation and relaxation dynamics in a new model for colloidal gelation
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We investigate the gel formation from the equilibrium sol phase in a simple model that has the characteristics of (colloidal) gel-forming systems at a finite temperature. At low volume fraction and low temperatures, particles are linked by long-living bonds and form an open percolating network. By means of molecular dynamics simulations, we study the lifetime of bonds and nodes of the gel network in order to relate these quantities to the complex relaxation dynamics observed.
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We study a lattice model of attractive colloids. It is exactly solvable on sparse random graphs. As the pressure and temperature are varied it reproduces many characteristic phenomena of liquids, glasses and colloidal systems such as ideal gel formation, liquid-glass phase coexistence, jamming, or the reentrance of the glass transition.
We use numerical simulations and an athermal quasi-static shear protocol to investigate the yielding of a model colloidal gel. Under increasing deformation, the elastic regime is followed by a significant stiffening before yielding takes place. A space-resolved analysis of deformations and stresses unravel how the complex load curve observed is the result of stress localization and that the yielding can take place by breaking a very small fraction of the network connections. The stiffening corresponds to the stretching of the network chains, unbent and aligned along the direction of maximum extension. It is characterized by a strong localization of tensile stresses, that triggers the breaking of a few network nodes at around 30% of strain. Increasing deformation favors further breaking but also shear-induced bonding, eventually leading to a large-scale reorganization of the gel structure at the yielding. At low enough shear rates, density and velocity profiles display significant spatial inhomogeneity during yielding in agreement with experimental observations.
A number of general trends are known to occur in systems displaying secondary processes in glasses and glass formers. Universal features can be identified as components of large and small cooperativeness whose competition leads to excess wings or apart peaks in the susceptibility spectrum. To the aim of understanding such rich and complex phenomenology we analyze the behavior of a model combining two apart glassy components with a tunable different cooperativeness. The model salient feature is, indeed, based on the competition of the energetic contribution of groups of dynamically relevant variables, e.g., density fluctuations, interacting in small and large sets. We investigate how the model is able to reproduce the secondary processes physics without further ad hoc ingredients, displaying known trends and properties under cooling or pressing.
Colloidal particles with strong, short-ranged attractions can form a gel. We simulate this process without and with hydrodynamic interactions (HI), using the lattice-Boltzmann method to account for presence of a thermalized solvent. We show that HI speed up and slow down gelation at low and high volume fractions, respectively. The transition between these two regimes is linked to the existence of a percolating cluster shortly after quenching the system. However, when we compare gels at matched structural age, we find nearly indistinguishable structures with and without HI. Our result explains longstanding, unresolved conflicts in the literature.
In this letter, we investigate several aspects related to the effect of hydrodynamics interactions on phase separation-induced gelation of colloidal particles. We explain physically the observation of Tanaka and Araki[Phys. Rev. Lett. {bf 85}, 1338 (2000)] of hydrodynamic stabilization of cellular network structures in two dimensions. We demonstrate that hydrodynamic interactions have only a minor quantitative influence on the structure of transient gels in three dimensions. We discuss some experimental implications of our results.
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