Elasticity of Arrested Short-Ranged Attractive Colloids: Homogeneous and Heterogeneous Glasses

Abstract

We evaluate the elasticity of arrested short-ranged attractive colloids by combining an analytically solvable elastic model with a hierarchical arrest scheme into a new approach, which allows to discriminate the microscopic (primary particle-level) from the mesoscopic (cluster-level) contribution to the macroscopic shear modulus. The results quantitatively predict experimental data in a wide range of volume fractions and indicate in which cases the relevant contribution is due to mesoscopic structures. On this basis we propose that different arrested states of short-ranged attractive colloids can be meaningfully distinguished as homogeneous or heterogeneous colloidal glasses in terms of the length-scale which controls their elastic behavior.PACS numbers: 82.70.Dd,81.40.Jj Solutions of short-ranged attractive colloidal particles are the object of intense study due to their technological applications (proteins, paints etc.), as the constituent blocks of nanomaterials, as well as model systems to investigate phase behavior and dynamical arrest of condensed matter [1]. A landscape of phases has been observed upon varying the volume fraction φ or the interaction parameters [2], and, as a matter of fact, extended regions of the phase diagram are still poorly understood. In very dense suspensions (φ > 0.5) the arrested states are spatially homogeneous (i.e. the typical linear size of structural heterogeneity is smaller than the particle diameter R 0 ). Particles are immobilized within the range of attraction, giving rise to bonds that are persistent under strain in the linear regime, and the high density leads to (attractive) glassy states [3]. For 0.2 < φ < 0.5, the situation is complicated: arrested states can only occur thanks to pronounced structural heterogeneities, typically on length scales larger than R 0 . Therefore, they are more related to gelation [4], rather than to the caging typical of crowded random media [5]. However, such arrested states are also hardly classifiable as classic network gels in view of the different morphology and stress-bearing mechanisms. For them, different microscopic phenomena should be considered and a new theoretical framework, able to account for the strong spatial heterogeneities and currently still missing, would be desirable. A crucial point, mostly neglected in recent studies, is that arrested states occurring at different volume fractions and attraction strengths do display dramatically diverse mechanical and rheological properties [6]. That is why, their characterization is of true interest in technological applications and material design.In this Letter we propose a new, more down to earth, approach to characterize arrested short-ranged attractive colloids, based on their mechanical response. Being the problem extremely hard to tackle, we rely on a simplified picture: We combine an analytically solvable elastic model with a hierarchical arrest scheme and provide a first attempt to discriminate the microscopic (primary particle-level) from the mesoscopic (cluster-level)...

Microdynamics and arrest of coarsening during spinodal decomposition in thermoreversible colloidal gels

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Superposition rheology and anisotropy in rheological properties of sheared colloidal gels

“…Various approaches have been proposed to explain a) Author to whom correspondence should be addressed; electronic mail: jan.vermant@mat.ethz.ch these properties, with most theories focusing on the structure at rest. For strong gels, fractal scalings [7,8] and poroelastic models [9] have been proposed linking the elasticity of the gel network to the rigidity of intra-or interfloc links [7,10]. In weaker gels with more of a bicontinuous, disordered morphology, it has been suggested that the elasticity is determined by the localization of particles in the gel, which slows down the Brownian relaxation processes.…”

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