Equilibrium gels of limited valence colloids

Gels are low-packing arrested states of matter which are able to support stress. On cooling, limited valence colloidal particles form open networks stabilized by the progressive increase of the interparticle bond lifetime. These gels, named equilibrium gels, are the focus of this review article. Differently from other types of colloidal gels, equilibrium gels do not require an underlying phase separation to form. Oppositely, they form in a region of densities deprived of thermodynamic instabilities. Limited valence equilibrium gels neither coarsen nor age with time

Tuning the liquid-liquid transition by modulating the hydrogen bond angular flexibility in a model for water

We propose a simple extension of the well known ST2 model for water [F.H. Stillinger and A. Rahman, J. Chem. Phys. {\bf 60}, 1545 (1974)] that allows for a continuous modification of the hydrogen bond angular flexibility. We show that the bond flexibility affects the relative thermodynamic stability of the liquid and of the hexagonal (or cubic) ice. On increasing flexibility, the liquid-liquid critical point, which in the original ST2 model is located in the no-man's land (i. e. the region where ice is the thermodynamically stable phase) progressively moves to a temperature where the liquid is more stable than ice. Our study definitively proves that the liquid-liquid transition in ST2 is a genuine phenomenon, of high relevance in all tetrahedral network-forming liquids, including water.Comment: Accepted in Phys. Rev. Let

On the possibility of extending the Nore-Frenkel generalized law of correspondent states to non-isotropic patchy interactions

Colloidal systems (and protein solutions) are often characterized by attractive interactions whose range is much smaller than the particle size. When this is the case and the interaction is spherical, systems obey a generalized law of correspondent states (GLCS), first proposed by Noro and Frenkel [ J.Chem.Phys. 113, 2941 (2000) ]. The thermodynamic properties become insensitive to the details of the potential, depending only on the value of the second virial coefficient B_2 and the density $\rho$. The GLCS does not generically hold for the case of non-spherical potentials. In this Letter we suggest that when particles interact via short-ranged small-angular amplitude patchy interactions (so that the condition of only one bond per patch is fulfilled) it is still possible to generalize the GLCS close to the liquid-gas critical point. Keywords: Colloids, Second Virial Coefficient, Proteins interactions, Short-ranged attractive attractions.Comment: 11 pages, 3 figures. Accepted for publication on J. Phys. Chem.

Extension of the Fluctuation-Dissipation theorem to the physical aging of a model glass-forming liquid

We present evidence in favor of the possibility of treating an out-of-equilibrium supercooled simple liquid as a system in quasi-equilibrium. Two different temperatures, one controlled by the external bath and one internally selected by the system characterize the quasi-equilibrium state. The value of the internal temperature is explicitly calculated within the inherent structure thermodynamic formalism. We find that the internal temperature controls the relation between the response to an external perturbation and the long-time decay of fluctuations in the liquid.Comment: 5 pages, 3 figure

Model for Assembly and Gelation of Four-Armed DNA Dendrimers

We introduce and numerically study a model designed to mimic the bulk behavior of a system composed of single-stranded DNA dendrimers. Complementarity of the base sequences of different strands results in the formation of strong cooperative intermolecular links. We find that in an extremely narrow temperature range the system forms a large-scale, low-density disordered network via a thermo-reversible gel transition. By controlling the strand length, the gel transition temperature can be made arbitrarily close to the percolation transition, in contrast with recent model systems of physical gelation. This study helps the understanding of self-assembly in this class of new biomaterials and provides an excellent bridge between physical and chemical gels

Slow dynamics in a primitive tetrahedral network model

We report extensive Monte Carlo and event-driven molecular dynamics simulations of the fluid and liquid phase of a primitive model for silica recently introduced by Ford, Auerbach and Monson [J. Chem. Phys. 17, 8415 (2004)]. We evaluate the iso-diffusivity lines in the temperature-density plane to provide an indication of the shape of the glass transition line. Except for large densities, arrest is driven by the onset of the tetrahedral bonding pattern and the resulting dynamics is strong in the Angell's classification scheme. We compare structural and dynamic properties with corresponding results of two recently studied primitive models of network forming liquids -- a primitive model for water and a angular-constraint free model of four-coordinated particles -- to pin down the role of the geometric constraints associated to the bonding. Eventually we discuss the similarities between "glass" formation in network forming liquids and "gel" formation in colloidal dispersions of patchy particles.Comment: 9 pages, 10 figure

Role of the unstable directions in the equilibrium and aging dynamics of supercooled liquids

The connectivity of the potential energy landscape in supercooled atomic liquids is investigated through the calculation of the instantaneous normal modes spectrum and a detailed analysis of the unstable directions in configuration space. We confirm the hypothesis that the mode-coupling critical temperature is the $T$ at which the dynamics crosses over from free to activated exploration of configuration space. We also report the observed changes in the local connectivity of configuration space sampled during aging, following a temperature jump from a liquid to a glassy state.Comment: 5 pages, 3 figures. Phys. Rev. Lett., in pres