arXiv subjects

Emanuela Zaccarelli

Publications and source records attributed to Emanuela Zaccarelli.

108 records · Page 6Linked to original sources

arXiv2008

Gelation as arrested phase separation in short-ranged attractive colloid-polymer mixtures

We present further evidence that gelation is an arrested phase separation in attractive colloid-polymer mixtures, based on a method combining confocal microscopy experiments with numerical simulations recently established in {\bf Nature 453, 499 (2008)}. Our results are independent of the form of the interparticle attractive potential, and therefore should apply broadly to any attractive particle system with short-ranged, isotropic attractions. We also give additional characterization of the gel states in terms of their structure, inhomogeneous character and local density.

cond-mat.soft↗

arXiv2008

Numerical Investigation of Glassy Dynamics in Low Density Systems

Vitrification in colloidal systems typically occurs at high densities driven by sharply varying, short-ranged interactions. The possibility of glassy behavior arising from smoothly varying, long-ranged particle interactions has received relatively little attention. Here we investigate the behavior of screened charged particles, and explicitly demonstrate that these systems exhibit glassy properties in the regime of low temperature and low density. Properties close to this low density (Wigner) glass transition share many features with their hard-sphere counterparts, but differ in quantitative aspects that may be accounted for via microscopic theoretical considerations.

cond-mat.dis-nn↗

arXiv2008

A molecular dynamics study of chemical gelation in a patchy particle model

We report event-driven molecular dynamics simulations of the irreversible gelation of hard ellipsoids of revolution containing several associating groups, characterizing how the cluster size distribution evolves as a function of the extent of reaction, both below and above the gel point. We find that in a very large interval of values of the extent of reaction, parameter-free mean-field predictions are extremely accurate, providing evidence that in this model the Ginzburg zone near the gel point, where non-mean field effects are important, is very limited. We also find that the Flory's hypothesis for the post-gelation regime properly describes the connectivity of the clusters even if the long-time limit of the extent of reaction does not reach the fully reacted state. This study shows that irreversibly aggregating asymmetric hard-core patchy particles may provide a close realization of the mean-field model, for which available theoretical predictions may help control the structure and the connectivity of the gel state. Besides chemical gels, the model is relevant to network-forming soft materials like systems with bioselective interactions, functionalized molecules and patchy colloids.

cond-mat.soft↗

arXiv2008

Theoretical and numerical study of the phase diagram of patchy colloids: ordered and disordered patch arrangements

We report theoretical and numerical evaluations of the phase diagram for a model of patchy particles. Specifically we study hard-spheres whose surface is decorated by a small number f of identical sites ("sticky spots'') interacting via a short-range square-well attraction. We theoretically evaluate, solving the Wertheim theory, the location of the critical point and the gas-liquid coexistence line for several values of f and compare them to results of Gibbs and Grand Canonical Monte Carlo simulations. We study both ordered and disordered arrangements of the sites on the hard-sphere surface and confirm that patchiness has a strong effect on the phase diagram: the gas-liquid coexistence region in the temperature-density plane is significantly reduced as f decreases. We also theoretically evaluate the locus of specific heat maxima and the percolation line.

cond-mat.soft↗

arXiv2007

A spherical model with directional interactions: I. Static properties

We introduce a simple spherical model whose structural properties are similar to the ones generated by models with directional interactions, by employing a binary mixture of large and small hard spheres, with a square-well attraction acting only between particles of different size. The small particles provide the bonds between the large ones. With a proper choice of the interaction parameters, as well as of the relative concentration of the two species, it is possible to control the effective valence. Here we focus on a specific choice of the parameters which favors tetrahedral ordering and study the equilibrium static properties of the system in a large window of densities and temperatures. Upon lowering the temperature we observe a progressive increase in local order, accompanied by the formation of a four-coordinated network of bonds. Three different density regions are observed: at low density the system phase separates into a gas and a liquid phase; at intermediate densities a network of fully bonded particles develops; at high densities -- due to the competition between excluded volume and attractive interactions -- the system forms a defective network. The very same behavior has been previously observed in numerical studies of non-spherical models for molecular liquids, such as water, and in models of patchy colloidal particles. Differently from these models, theoretical treatments devised for spherical potentials, e.g. integral equations and ideal mode coupling theory for the glass transition can be applied in the present case, opening the way for a deeper understanding of the thermodynamic and dynamic behavior of low valence molecules and particles.

cond-mat.dis-nn↗

arXiv2007

Colloidal Gels: Equilibrium and Non-Equilibrium Routes

We attempt a classification of different colloidal gels based on colloid-colloid interactions. We discriminate primarily between non-equilibrium and equilibrium routes to gelation, the former case being slaved to thermodynamic phase separation while the latter is individuated in the framework of competing interactions and of patchy colloids. Emphasis is put on recent numerical simulations of colloidal gelation and their connection to experiments. Finally we underline typical signatures of different gel types, to be looked in more details in experiments.

cond-mat.soft↗

arXiv2007

Viscoelasticity and Stokes-Einstein relation in repulsive and attractive colloidal glasses

We report a numerical investigation of the visco-elastic behavior in models for steric repulsive and short-range attractive colloidal suspensions, along different paths in the attraction-strength vs packing fraction plane. More specifically, we study the behavior of the viscosity (and its frequency dependence) on approaching the repulsive glass, the attractive glass and in the re-entrant region where viscosity shows a non monotonic behavior on increasing attraction strength. On approaching the glass lines, the increase of the viscosity is consistent with a power-law divergence with the same exponent and critical packing fraction previously obtained for the divergence of the density fluctuations. Based on mode-coupling calculations, we associate the increase of the viscosity with specific contributions from different length scales. We also show that the results are independent on the microscopic dynamics by comparing newtonian and brownian simulations for the same model. Finally we evaluate the Stokes-Einstein relation approaching both glass transitions, finding a clear breakdown which is particularly strong for the case of the attractive glass.

cond-mat.soft↗

arXiv2006

Phase diagram of patchy colloids: towards empty liquids

We report theoretical and numerical evaluations of the phase diagram for patchy colloidal particles of new generation. We show that the reduction of the number of bonded nearest neighbours offers the possibility of generating liquid states (i.e. states with temperature $T$ lower than the liquid-gas critical temperature) with a vanishing occupied packing fraction ($ϕ$), a case which can not be realized with spherically interacting particles. Theoretical results suggest that such reduction is accompanied by an increase of the region of stability of the liquid phase in the ($T$-$ϕ$) plane, possibly favoring the establishment of homogeneous disordered materials at small $ϕ$, i.e. stable equilibrium gels.

cond-mat.soft↗

arXiv2006

Modeling Equilibrium Clusters in Lysozyme Solutions

We present a combined experimental and numerical study of the equilibrium cluster formation in globular protein solutions under no-added salt conditions. We show that a cluster phase emerges as a result of a competition between a long-range screened Coulomb repulsion and a short-range attraction. A simple effective potential, in which only depth and width of the attractive part of the potential are optimized, accounts in a remarkable way for the wavevector dependence of the X-ray scattering structure factor.

cond-mat.soft↗

arXiv2006

Gel to glass transition in simulation of a valence-limited colloidal system

We numerically study a simple model for thermo-reversible colloidal gelation in which particles can form reversible bonds with a predefined maximum number of neighbors. We focus on three and four maximally coordinated particles, since in these two cases the low valency makes it possible to probe, in equilibrium, slow dynamics down to very low temperatures $T$. By studying a large region of $T$ and packing fraction $ϕ$ we are able to estimate both the location of the liquid-gas phase separation spinodal and the locus of dynamic arrest, where the system is trapped in a disordered non-ergodic state. We find that there are two distinct arrest lines for the system: a {\it glass} line at high packing fraction, and a {\it gel} line at low $ϕ$ and $T$. The former is rather vertical ($ϕ$-controlled), while the latter is rather horizontal ($T$-controlled) in the $(ϕ-T)$ plane. Dynamics on approaching the glass line along isotherms exhibit a power-law dependence on $ϕ$, while dynamics along isochores follow an activated (Arrhenius) dependence. The gel has clearly distinct properties from those of both a repulsive and an attractive glass. A gel to glass crossover occurs in a fairly narrow range in $ϕ$ along low $T$ isotherms, seen most strikingly in the behavior of the non-ergodicity factor. Interestingly, we detect the presence of anomalous dynamics, such as subdiffusive behavior for the mean squared displacement and logarithmic decay for the density correlation functions in the region where the gel dynamics interferes with the glass dynamics.

cond-mat.soft↗

arXiv2005

One-dimensional cluster growth and branching gels in colloidal systems with short-range depletion attraction and screened electrostatic repulsion

We report extensive numerical simulations of a simple model for charged colloidal particles in suspension with small non-adsorbing polymers. The chosen effective one-component interaction potential is composed of a short-range attractive part complemented by a Yukawa repulsive tail. We focus on the case where the screening length is comparable to the particle radius. Under these conditions, at low temperature, particles locally cluster into quasi one-dimensional aggregates which, via a branching mechanism, form a macroscopic percolating gel structure. We discuss gel formation and contrast it with the case of longer screening lengths, for which previous studies have shown that arrest is driven by the approach to a Yukawa glass of spherical clusters. We compare our results with recent experimental work on charged colloidal suspensions [A. I. Campbell {\it et al.} cond-mat/0412108, Phys. Rev. Lett. in press].

cond-mat.soft↗

arXiv2005

Evidence of a higher-order singularity in dense short-ranged attractive colloids

We study a model in which particles interact through a hard-core repulsion complemented by a short-ranged attractive potential, of the kind found in colloidal suspensions. Combining theoretical and numerical work we locate the line of higher-order glass transition singularities and its end-point -- named $A_4$ -- on the fluid-glass line. Close to the $A_4$ point, we detect logarithmic decay of density correlations and sub linear power-law increase of the mean square displacement, for time intervals up to four order of magnitudes. We establish the presence of the $A_4$ singularity by studying how the range of the potential affects the time-window where anomalous dynamics is observed.

cond-mat.dis-nn↗

arXiv2004

Static and Dynamic Anomalies in a Repulsive Spherical Ramp Liquid: Theory and Simulation

We compare theoretical and simulation results for static and dynamic properties for a model of particles interacting via a spherically symmetric repulsive ramp potential. The model displays anomalies similar to those found in liquid water, namely, expansion upon cooling and an increase of diffusivity upon compression. In particular, we calculate the phase diagram from the simulation and successfully compare it with the phase diagram obtained using the Rogers-Young (RY) closure for the Ornstein-Zernike equation. Both the theoretical and the numerical calculations confirm the presence of a line of isobaric density maxima, and lines of compressibility minima and maxima. Indirect evidence of a liquid-liquid critical point is found. Dynamic properties also show anomalies. Along constant temperature paths, as the density increases, the dynamics alternates several times between slowing down and speeding up, and we associate this behavior with the progressive structuring and de-structuring of the liquid. Finally we confirm that mode coupling theory successfully predicts the non-monotonic behavior of dynamics and the presence of multiple glass phases, providing strong evidence that structure (the only input of mode coupling theory) controls dynamics.

cond-mat.stat-mech↗

arXiv2004

Viscoelastic properties of attractive and repulsive colloidal glasses

We report a numerical study of the shear viscosity and the frequency dependent elastic moduli close to dynamical arrest for a model of short-range attractive colloids, both for the repulsive and the attractive glass transition. Calculating the stress autocorrelation functions, we find that density fluctuations of wavevectors close to the first peak in the structure factor control the viscosity rise on approaching the repulsive glass, while fluctuations of larger wavevectors control the viscosity close to the attractive glass. On approaching the glass transition, the viscosity diverges with a power law with the same exponent as the density autocorrelation time.

cond-mat.soft↗

arXiv2004

Equilibrium cluster phases and low-density arrested disordered states: The role of short-range attraction and long-range repulsion

We study a model in which particles interact with short-ranged attractive and long-ranged repulsive interactions, in an attempt to model the equilibrium cluster phase recently discovered in sterically stabilized colloidal systems in the presence of depletion interactions. At low packing fraction particles form stable equilibrium clusters which act as building blocks of a cluster fluid. We study the possibility that cluster fluids generate a low-density disordered arrested phase, a gel, via a glass transition driven by the repulsive interaction. In this model the gel formation is formally described with the same physics of the glass formation.

cond-mat.soft↗

arXiv2004

Numerical study of the glass-glass transition in short-ranged attractive colloids

We report extensive numerical simulations in the {\it glass} region for a simple model of short-ranged attractive colloids, the square well model. We investigate the behavior of the density autocorrelation function and of the static structure factor in the region of temperatures and packing fractions where a glass-glass transition is expected according to theoretical predictions. We strengthen our observations by studying both waiting time and history dependence of the numerical results. We provide evidence supporting the possibility that activated bond-breaking processes destabilize the attractive glass, preventing the full observation of a sharp glass-glass kinetic transition.

cond-mat.soft↗

arXiv2003

Activated bond-breaking processes preempt the observation of a sharp glass-glass transition in dense short-ranged attractive colloids

We study -- using molecular dynamics simulations -- the temperature dependence of the dynamics in a dense short-ranged attractive colloidal glass to find evidence of the kinetic glass-glass transition predicted by the ideal Mode Coupling Theory. According to the theory, the two distinct glasses are stabilized one by excluded volume and the other by short-ranged attractive interactions. By studying the density autocorrelation functions, we discover that the short-ranged attractive glass is unstable. Indeed, activated bond-breaking processes slowly convert the attractive glass into the hard-sphere one, preempting the observation of a sharp glass-glass transition.

cond-mat.dis-nn↗

arXiv2001

Phase equilibria and glass transition in colloidal systems with short-ranged attractive interactions. Application to protein crystallization

We have studied a model of a complex fluid consisting of particles interacting through a hard core and a short range attractive potential of both Yukawa and square-well form. Using a hybrid method, including a self-consistent and quite accurate approximation for the liquid integral equation in the case of the Yukawa fluid, perturbation theory to evaluate the crystal free energies, and mode-coupling theory of the glass transition, we determine both the equilibrium phase diagram of the system and the lines of equilibrium between the supercooled fluid and the glass phases. For these potentials, we study the phase diagrams for different values of the potential range, the ratio of the range of the interaction to the diameter of the repulsive core being the main control parameter. Our arguments are relevant to a variety of systems, from dense colloidal systems with depletion forces, through particle gels, nano-particle aggregation, and globular protein crystallization.

cond-mat.soft↗