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...

We review some results on the dynamics of gelation phenomena, obtained via a lattice model and via molecular dynamics using a DLVO potential. This study allowed us to make a connection between classical gelation and the phenomenology of colloidal systems, suggesting that gelation phenomena in attractive colloids at low temperature and low volume fraction can be described in terms of a two-line scenario.

The present paper proposes a Statistical Mechanics approach to the inherent states of glassy systems and granular materials, following the original ideas developed by Edwards for granular materials. Two lattice models, a diluted Spin Glass and a system of hard-spheres under gravity, introduced in the context of glassy systems and granular materials, are evolved using a "tap dynamics" analogous to that of experiments on granular materials. The asymptotic macrostates, reached by the system, are shown to be described by a single thermodynamical parameter, and this parameter to coincide with the temperature, called the "configurational temperature", predicted assuming that the distribution among the inherent states satisfies the principle of maximum entropy.The thermodynamics of macroscopic systems evolving at equilibrium is well described by Statistical Mechanics. However there are many systems, typically found in "frozen states", where they do not evolve at all. These are, for example, supercooled liquids quenched at zero temperature in states, called inherent states [1,2], corresponding to the local minima of the potential energy in the 3N-dimensional configuration space of particle coordinates. Granular materials [3] at rest are another important example of system frozen in mechanically stable microstates. Grains are "frozen" because, due to their large masses [3], the thermal kinetic energy is negligible compared to the gravitational energy; thus the external bath temperature, T bath , can be considered equal to zero (by analogy with supercooled liquids, we call these mechanically stable configurations inherent states).In this paper, following the original ideas by Edwards for granular materials [4] we attempt to develop a unified Statistical Mechanics approach for the inherent states of glassy systems and granular materials along the line of Ref. [5]. The connection between Edwards approach and recent developments on glass theory has received much attention [6][7][8][9][10][11][12].The first step is to introduce a suitable dynamics which allows to explore the configurations of the inherent states. In granular materials the dynamics, from one stable microstate to another, can be induced by sequences of "taps", in which the energy is pumped into the system in pulses. Due to inelastic collisions the kinetic energy is totally dissipated after each tap, and the system is again frozen in one of its inherent states [13]. Similarly, in glass formers at zero temperature the dynamics, from one inherent state to another, can be induced by sequences of taps, where each tap consists in raising the bath temperature and, after a lapse of time τ 0 , quenching it back to zero. By repeating the process cyclically the system explores the space of the inherent states [5,[7][8][9]14,15]. For a tap of infinite length (τ 0 → ∞) the way to explore the inherent states coincides with the one used in [1,2] for a system of Lennard Jones mixture. In the approach of Barrat et al.[6] the system instead evolves in an out of equilibrium quasi-...

We study the structure and the dynamics in the formation of irreversible gels by means of molecular dynamics simulation of a model system where the gelation transition is due to the random percolation of permanent bonds between neighboring particles. We analyze the heterogeneities of the dynamics in terms of the fluctuations of the self-intermediate scattering functions: in the sol phase close to the percolation threshold, we find that this dynamic susceptibility increases with the time until it reaches a plateau. At the gelation threshold this plateau scales as a function of the wave vector k as k(eta-2), with eta being related to the decay of the percolation pair connectedness function. At the lowest wave vector, approaching the gelation threshold it diverges with the same exponent gamma as the mean cluster size. These findings suggest an alternative way of measuring critical exponents in a system undergoing chemical gelation.

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We here discuss the results of three-dimensional Monte Carlo simulations of a minimal lattice model for gelling systems. We focus on the dynamics investigated by means of the time autocorrelation function of the density fluctuations and the particle mean-square displacement. We start from the case of chemical gelation, i.e., with permanent bonds, and characterize the critical dynamics as determined by the formation of the percolating cluster, as actually observed in polymer gels. By opportunely introducing a finite bond lifetime tau(b), the dynamics displays relevant changes and eventually the onset of a glassy regime. This has been interpreted in terms of a crossover to dynamics more typical of colloidal systems and a connection between classical gelation and recent results on colloidal systems is suggested. By systematically comparing the results in the case of permanent bonds to finite bond lifetime, the crossover and the glassy regime can be understood in terms of effective clusters.

The transport properties at finite temperature of crystalline organic semiconductors are investigated, within the Su-Schrieffer-Heeger model, by combining exact diagonalization technique, Monte Carlo approaches, and maximum entropy method. The temperature-dependent mobility data measured in single crystals of rubrene are successfully reproduced: a crossover from super-to subdiffusive motion occurs in the range 150 ≤ T ≤ 200 K, where the mean free path becomes of the order of the lattice parameter and strong memory effects start to appear. We provide an effective model which can successfully explain low frequencies features of the absorption spectra. The observed response to slowly varying electric field is interpreted by means of a simple model where the interaction between the charge carrier and lattice polarization modes is simulated by a harmonic interaction between a fictitious particle and an electron embedded in a viscous fluid.PACS numbers: 72.80. Le, 78.40.Me Small molecule organic semiconductors, crystals of small molecules held together by van der Waals forces, are the focus of an intensive research activity being the material basis for the organic electronics, and in particular for the plastic electronics, a rapidly developing field [1]. Because of the weak van der Waals intermolecular bonding, there is a small overlap between the electronic orbitals of these small molecules leading to narrow electronic bands: the transfer integral t turns out to be about 100 meV [2,3]. At the same time the electronphonon interaction (EPI) plays a crucial role [4] and, now, it is well established that it stems from Peierls's coupling mechanism [5]. EPI exhibits a strong momentum dependence, and, given the pronounced anisotropy of these compounds [6], typically the coupling of the electrons with the lattice vibrations is described by using a one-dimensional tight-binding model involving Einstein phonons with the lattice displacements affecting the electronic hopping integral [2,7] (Su-Schrieffer-Heeger coupling [8]). However the charge transport understanding in organic semiconductors remains limited. Indeed from an experimental point of view ultrapure crystals of pentacene or rubrene exhibit: 1) actived transport at low temperatures[6, 9, 10] (up to ≃ 160 K); 2) a band-like mobility up to room temperature, i.e. the mobility decreases as T −α with α ≃ 2[9, 11]. At the same time, at room temperature, optical absorption spectra are characterized by a broad peak centered around 40 meV [12], reminiscent of disordered systems in the insulating phase. It has been shown that the rapid drop of the mobility below 160 K is due to the crossover to the trap dominated regime (extrinsic disorder). Measurements of the transverse Hall conductivity [9,13] allowed to extract the intrinsic, trapfree mobility that increases always with cooling (shallow traps do not contribute to the Hall voltage since the Lorentz force is zero for these charge carriers). It remains to explain the puzzle regarding the simultaneous presence of the signat...

We investigate the slow dynamics in gelling systems by means of MonteCarlo simulations on the cubic lattice of a minimal statistical mechanics model. By opportunely varying some model parameter we are able to describe a crossover from the chemical gelation behaviour to dynamics more typical of colloidal systems. The results suggest a novel connection linking classical gelation, as originally described by Flory, to more recent results on colloidal systems. PACS: 82.70.Gg, 83.10.Nn The chemical gelation transition, as it is typically observed in polymer systems, transforms a solution of polymeric molecules, the sol, from a viscous fluid to an elastic disordered solid, the gel. The viscosity coefficient grows with a power law behaviour, as function of the relative difference from the critical polymer concentration, characterized by a critical exponent k. The onset of the elastic response in the system, as function of the same control parameter, displays a power law increasing of the elastic modulus described by a critical exponent f [1]. This corresponds to the constitution inside the sol of a macroscopic polymeric structure [2], that characterizes the gel phase. As implicitly suggested in the work of Flory and Stockmayer [2], the percolation model is considered as the basic model for the chemical gelation transition and the macromolecular stress-bearing structure in these systems is a percolating network [3][4][5]. Moreover slow dynamics characterize the gelling solution: the relaxation functions in the experiments display a long time stretched exponential decay ∼ e −(as the gelation threshold is approached. The relaxation process becomes critically slow at the gel point, where the onset of a power law decay is typically observed [6]. Gelation phenomena are also observed in colloidal systems, that are suspensions of mesoscopic particles interacting via short range attraction: due to aggregation phenomena at low density (colloidal gelation) these systems display gel states with a power law behaviour of the viscosity coefficient and of the elastic modulus [7], as in chemical gelation. Yet the aggregation process gives rise to cluster-cluster aggregation producing a spanning cluster with a fractal dimensionality smaller than the random percolation case [8][9][10][11]. On the other hand with a weaker attraction at higher density a gelation characterized by a glass-like kinetic arrest [12,13] may be observed. The relaxation patterns closely recall the ones observed in glassy systems and are well fitted by the modecoupling theory [14] predictions for supercooled liquids approaching the glass transition [13]. On the theoretical side the application of the mode-coupling theory to systems with short range attractive interaction [15][16][17] (attractive glasses) has been recently considered and the connection with the colloidal glass transition has been proposed. The question, that we want to investigate in this paper, is whether and to what extent colloidal gelation, colloidal glass transition and chemical gelation ar...