In colloidal suspensions, at low volume fraction and temperature, dynamical arrest occurs via the growth of elongated structures, that aggregate to form a connected network at gelation. Here we show that, in the region of parameter space where gelation occurs, the stable thermodynamical phase is a crystalline columnar one. Near and above the gelation threshold, the disordered spanning network slowly evolves and finally orders to form the crystalline structure. At higher volume fractions the stable phase is a lamellar one, that seems to have a still longer ordering time.PACS numbers: 82.70. Dd, 64.60.Ak, 82.70.Gg In colloidal suspensions solid (or liquid) mesoscopic particles are dispersed in another substance. These systems, like blood, proteins in water, milk, black ink or paints, are important in our everyday lives, in biology and industry [1,2]. It is crucial, for example, to control the process of aggregation in paint and paper industries [3], or to favour the protein crystallization in the production of pharmaceuticals and photonic crystals [4,5].A practical and exciting feature of colloidal suspensions is that the interaction energy between particles can be well controlled [6][7][8]. In fact particles can be coated and stabilized leading to a hard sphere behaviour, and an attractive depletion interaction can be brought out by adding some non-adsorbing polymers. The range and strength of the potential are controlled respectively by the size and concentration of the polymer [8,9]. Recent experimental works highlighted the presence of a net charge on colloidal particles [7,10] giving rise to a long range electrostatic repulsion in addition to the depletion attraction.The competition between attractive and repulsive interactions produces a rich phenomenology and a complex behavior as far as structural and dynamical properties are concerned. For particular choices of the interaction parameters, the aggregation of particles is favoured but the liquid-gas phase transition can be avoided and the cluster size can be stabilized at an optimum value [11]. Experimentally, such a cluster phase made of small equilibrium monodisperse clusters is observed using confocal microscopy at low volume fraction and low temperature (or high attraction strength) [7,10,12]. Increasing the volume fraction, the system is transformed from an ergodic cluster liquid into a nonergodic gel [10,12], where structural arrest occurs. Using molecular dynamic simulations, we showed that such structural arrest is crucially related to the formation of a long living spanning cluster, providing evidence for the percolation nature of the colloidal gel transition at low volume fraction and low temperature [13,14]. This scenario was confirmed by recent experiments [10] and molecular dynamics simulations [15], where it was shown that increasing the volume fraction clusters coalesce into elongated structures eventually forming a disordered spanning network. A realistic framework for the modelization of these systems is represented by DLVO interaction p...

A micro-mechanical study of coarsening and rheology of colloidal gels: Cage building, cage hopping, and Smoluchowski's ratchet Journal of Rheology 58, 1121 (2014) SynopsisWe use numerical simulations and an athermal quasistatic 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. V C 2014 The Society of Rheology.[http://dx

Strength and other mechanical properties of cement and concrete rely upon the formation of calcium-silicate-hydrates (C-S-H) during cement hydration. Controlling structure and properties of the C-S-H phase is a challenge, due to the complexity of this hydration product and of the mechanisms that drive its precipitation from the ionic solution upon dissolution of cement grains in water.Departing from traditional models mostly focused on length scales above the micrometer, recent research addressed the molecular structure of C-S-H. However, small-angle neutron scattering, electron-microscopy imaging, and nanoindentation experiments suggest that its mesoscale organization, extending over hundreds of nanometers, may be more important. Here we unveil the C-S-H mesoscale texture, a crucial step to connect the fundamental scales to the macroscale of engineering properties. We use simulations that combine information of the nanoscale building units of C-S-H and their effective interactions, obtained from atomistic simulations and experiments, into a statistical physics framework for aggregating nanoparticles. We compute small-angle scattering intensities, pore size distributions, specific surface area, local densities, indentation modulus, and hardness of the material, providing quantitative understanding of different experimental investigations. Our results provide insight into how the heterogeneities developed during the early stages of hydration persist in the structure of C-S-H and impact the mechanical performance of the hardened cement paste. Unraveling such links in cement hydrates can be groundbreaking and controlling them can be the key to smarter mix designs of cementitious materials.U pon dissolution of cement powder in water, calcium-silicate-hydrates (C-S-H) precipitate and assemble into a cohesive gel that fills the pore space in the cement paste over hundreds of nanometers and binds the different components of concrete together (1). The mechanics and microstructure are key to concrete performance and durability, but the level of understanding needed to design new, more performant cements and have an impact on the CO 2 footprint of the material is far from being reached (2).Most of the experimental characterization and models used to predict and design cement performance have been developed at a macroscopic level and hardly include any material heterogeneity over length scales smaller than micrometers (3). However, EM imaging, nanoindentation tests, X-rays and neutron scattering, and NMR analysis as well as atomistic simulations have now elucidated several structural and mechanical features concentrated within a few nanometers (4-8). The hygrothermal behavior of cement suggests a hierarchical and complex pore structure that develops during hydration and continues to evolve (1, 9-11). NMR and small-angle neutron scattering (SANS) studies of hardened C-S-H identified distinctive features of the complex pore network and detected significant structural heterogeneities spanning length scales between tens and hu...

Soft solids with tunable mechanical response are at the core of new material technologies, but a crucial limit for applications is their progressive aging over time, which dramatically affects their functionalities. The generally accepted paradigm is that such aging is gradual and its origin is in slower than exponential microscopic dynamics, akin to the ones in supercooled liquids or glasses. Nevertheless, time- and space-resolved measurements have provided contrasting evidence: dynamics faster than exponential, intermittency and abrupt structural changes. Here we use 3D computer simulations of a microscopic model to reveal that the timescales governing stress relaxation, respectively, through thermal fluctuations and elastic recovery are key for the aging dynamics. When thermal fluctuations are too weak, stress heterogeneities frozen-in upon solidification can still partially relax through elastically driven fluctuations. Such fluctuations are intermittent, because of strong correlations that persist over the timescale of experiments or simulations, leading to faster than exponential dynamics.