arXiv subjects

Emanuela Zaccarelli

Publications and source records attributed to Emanuela Zaccarelli.

At least 91 records · Page 5Linked to original sources

arXiv2016

Anomalous dynamics of intruders in a crowded environment of mobile obstacles

Many natural and industrial processes rely on constrained transport, such as proteins moving through cells, particles confined in nanocomposite materials or gels, individuals in highly dense collec- tives and vehicular traffic conditions. These are examples of motion through crowded environments, in which the host matrix may retain some glass-like dynamics. Here we investigate constrained transport in a colloidal model system, in which dilute small spheres move in a slowly rearranging, glassy matrix of large spheres. Using confocal differential dynamic microscopy and simulations, we discover a critical size asymmetry at which anomalous collective transport of the small particles appears, manifested as a logarithmic decay of the density autocorrelation functions. We demonstrate that the matrix mobility is central for the observed anomalous behaviour. These results, crucially depending on size-induced dynamic asymmetry, are of relevance for a wide range of phenomena ranging from glassy systems to cell biology.

cond-mat.soft↗

arXiv2016

The Physics of Protein Self-Assembly

Understanding protein self-assembly is important for many biological and industrial processes. Proteins can self-assemble into crystals, filaments, gels, and other amorphous aggregates. The final forms include virus capsids and condensed phases associated with diseases, such as amyloid fibrils. Although seemingly different, these assemblies all originate from fundamental protein interactions and are driven by similar thermodynamic and kinetic factors. Here we review recent advances in understanding protein self-assembly through a soft condensed matter perspective with an emphasis on three specific systems: globular proteins, viruses and amyloid fibers. We conclude with a discussion of unanswered questions in the field.

cond-mat.soft↗

arXiv2015

Validity of Stokes-Einstein Relation in Soft Colloids up to the Glass Transition

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors S(Q, t) on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

cond-mat.soft↗

arXiv2014

How soft repulsion enhances the depletion mechanism

We investigate binary mixtures of large colloids interacting through soft potentials with small, ideal depletants. We show that softness has a dramatic effect on the resulting colloid-colloid effective potential when the depletant-to-colloid size ratio $q$ is small, with significant consequences on the colloidal phase behaviour. We also provide an exact relation that allows us to obtain the effective pair potential for {\it any} type of colloid-depletant interactions in the case of ideal depletants, without having to rely on complicated and expensive full-mixture simulations. We also show that soft repulsion among depletants further enhances the tendency of colloids to aggregate. Our theoretical and numerical results demonstrate that --- in the limit of small $q$ --- soft mixtures cannot be mapped onto hard systems and hence soft depletion is not a mere extension of the widely used Asakura-Oosawa potential.

cond-mat.soft↗

arXiv2014

Multiple higher-order singularities and iso-dynamics in a simple glass-former model

We investigate the slow dynamics of a colloidal model with two repulsive length scales, whose interaction potential is the sum of a hard-core and a square shoulder. Despite the simplicity of the interactions, Mode-Coupling theory predicts a complex dynamic scenario: a fluid-glass line with two reentrances and a glass-glass line ending with multiple higher-order ($A_3$ or $A_4$) singularities. In this work we verify the existence of the two $A_4$ points by numerical simulations, observing subdiffusive behaviour of the mean-square displacement and logarithmic decay of the density correlators. Surprisingly, we also discover a novel dynamic behaviour generated by the competition between the two higher-order singularities. This results in the presence of special loci along which the dynamics is identical \textit{at all} length and time scales.

cond-mat.soft↗

arXiv2014

Casimir-like forces at the percolation transition

Percolation and critical phenomena show common features such as scaling and universality. Colloidal particles, immersed in a solvent close to criticality, experience long-range effective forces, named critical Casimir forces. %These originate from the confinement of the solvent critical fluctuations between the colloids. Building on the analogy between critical phenomena and percolation, we explore the possibility of observing long-range forces near a percolation threshold. To this aim we numerically evaluate the effective potential between two colloidal particles dispersed in a chemical sol and we show that it becomes attractive and long-ranged on approaching the sol percolation transition. We develop a theoretical description based on a polydisperse Asakura-Oosawa model which captures the divergence of the interaction range, allowing us to interpret such effect in terms of depletion interactions in a structured solvent. Our results provide the geometric analogue of the critical Casimir force, suggesting a novel way for tuning colloidal interactions by controlling the clustering properties of the solvent.

cond-mat.soft↗

arXiv2014

Avalanches mediate crystallization in a hard-sphere glass

By molecular-dynamics simulations, we have studied the devitrification (or crystallization) of aged hard-sphere glasses. First, we find that the dynamics of the particles are intermittent: Quiescent periods, when the particles simply "rattle" in their nearest-neighbor cages, are interrupted by abrupt "avalanches," where a subset of particles undergo large rearrangements. Second, we find that crystallization is associated with these avalanches but that the connection is not straightforward. The amount of crystal in the system increases during an avalanche, but most of the particles that become crystalline are different from those involved in the avalanche. Third, the occurrence of the avalanches is a largely stochastic process. Randomizing the velocities of the particles at any time during the simulation leads to a different subsequent series of avalanches. The spatial distribution of avalanching particles appears random, although correlations are found among avalanche initiation events. By contrast, we find that crystallization tends to take place in regions that already show incipient local order.

cond-mat.soft↗

arXiv2013

On polydispersity and the hard-sphere glass transition

We simulate the dynamics of polydisperse hard spheres at high packing fractions, $ϕ$, with an experimentally-realistic particle size distribution (PSD) and other commonly-used PSDs such as gaussian or top hat. We find that the mode of kinetic arrest depends on the PSD's shape and not only on its variance. For the experimentally-realistic PSD, the largest particles undergo an ideal glass transition at $ϕ\sim 0.588$ while the smallest particles remain mobile. Such species-specific localisation was previously observed only in asymmetric binary mixtures. Our findings suggest that the recent observation of ergodic behavior up to $ϕ\sim 0.6$ in a hard-sphere system is not evidence for activated dynamics, but an effect of polydispersity.

cond-mat.soft↗

arXiv2013

Critical Casimir forces and colloidal aggregation: A numerical study

We present a numerical study of the effective potential $V_\mathrm{eff}$ between two hard-sphere colloids dispersed in a solvent of interacting particles, for several values of temperature and solvent density, approaching the solvent gas-liquid critical point. We investigate the stability of a system of particles interacting via $V_\mathrm{eff}$ to evaluate the locus of colloidal aggregation in the solvent phase-diagram, and its dependence on the colloid size. We assess how the excluded volume depletion forces are modified by solvent attraction and discuss under which conditions solvent critical fluctuations, in the form of critical Casimir forces, can be used to effectively manipulate colloidal aggregation.

cond-mat.soft↗

arXiv2013

Unveiling the complex glassy dynamics of square shoulder systems: simulations and theory

We performed extensive molecular dynamics (MD) simulations, supplemented by Mode Coupling Theory (MCT) calculations, for the Square Shoulder (SS) model, a purely repulsive potential where the hard-core is complemented by a finite shoulder. For the one-component version of this model, MCT predicted [Sperl {\it et al.} Phys. Rev. Lett. {\bf 104}, 145701 (2010)] the presence of diffusion anomalies both upon cooling and upon compression and the occurrence of glass-glass transitions. In the simulations, we focus on a non-crystallising binary mixture, which, at the investigated shoulder width, shows a non-monotonic behaviour of the diffusion upon cooling but not upon isothermal compression. In addition, we find the presence of a disconnected glass-glass line in the phase diagram, ending in two higher-order singularities. These points generate a logarithmic dependence of the density correlators as well as a subdiffusive behaviour of the mean squared displacement, although with the interference of the nearby liquid-glass transition. We also perform novel MCT calculations using as input the partial structure factors obtained within MD, confirming the simulation results. The presence of two hard sphere glasses, differing only in their hard core length, is revealed, showing that the simple competition between the two is sufficient for creating a rather complex dynamical behaviour.

cond-mat.soft↗

arXiv2012

Tuning effective interactions close to the critical point in colloidal suspensions

We report a numerical investigation of two colloids immersed in a critical solvent, with the aim of quantifying the effective colloid-colloid interaction potential. By turning on an attraction between the colloid and the solvent particles we follow the evolution from the case in which the solvent density close to the colloids changes from values smaller than the bulk to values larger than the bulk. We thus effectively implement the so-called $(+,+)$ and $(-,-)$ boundary conditions defined in field theoretical approaches focused on the description of critical Casimir forces. We find that the effective potential at large distances decays exponentially, with a characteristic decay length compatible with the bulk critical correlation length, in full agreement with theoretical predictions. We also investigate the case of $(+,-)$ boundary condition, where the effective potential becomes repulsive. Our study provides a guidance for a design of the interaction potential which can be exploited to control the stability of colloidal systems.

cond-mat.soft↗

arXiv2012

From compact to fractal crystalline clusters in concentrated systems of monodisperse hard spheres

We address the crystallization of monodisperse hard spheres in terms of the properties of finite- size crystalline clusters. By means of large scale event-driven Molecular Dynamics simulations, we study systems at different packing fractions ϕ ranging from weakly supersaturated state points to glassy ones, covering different nucleation regimes. We find that such regimes also result in different properties of the crystalline clusters: compact clusters are formed in the classical-nucleation-theory regime (ϕ \leq 0.54), while a crossover to fractal, ramified clusters is encountered upon increasing packing fraction (ϕ \geq 0.56), where nucleation is more spinodal-like. We draw an analogy between macroscopic crystallization of our clusters and percolation of attractive systems to provide ideas on how the packing fraction influences the final structure of the macroscopic crystals. In our previous work (Phys. Rev. Lett., 106, 215701, 2011), we have demonstrated how crystallization from a glass (at ϕ > 0.58) happens via a gradual (many-step) mechanism: in this paper we show how the mechanism of gradual growth seems to hold also in super-saturated systems just above freezing showing that static properties of clusters are not much affected by dynamics.

cond-mat.soft↗

arXiv2011

Crystallization Mechanism of Hard Sphere Glasses

In supercooled liquids, vitrification generally suppresses crystallization. Yet some glasses can still crystallize despite the arrest of diffusive motion. This ill-understood process may limit the stability of glasses, but its microscopic mechanism is not yet known. Here we present extensive computer simulations addressing the crystallization of monodisperse hard-sphere glasses at constant volume (as in a colloid experiment). Multiple crystalline patches appear without particles having to diffuse more than one diameter. As these patches grow, the mobility in neighbouring areas is enhanced, creating dynamic heterogeneity with positive feedback. The future crystallization pattern cannot be predicted from the coordinates alone: crystallization proceeds by a sequence of stochastic micro-nucleation events, correlated in space by emergent dynamic heterogeneity.

physics.chem-ph↗

arXiv2010

From caging to Rouse dynamics in polymer melts with intramolecular barriers: a critical test of the Mode Coupling Theory

By means of computer simulations and solution of the equations of the Mode Coupling Theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of non-entangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed devations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and non-monotonous wavelength dependence of the mode correlators.

cond-mat.soft↗

arXiv2009

Disconnected Glass-Glass Transitions and Diffusion Anomalies in a Model with two Repulsive Length Scales

Building on mode-coupling-theory calculations, we report a novel scenario for multiple glass transitions in a purely repulsive spherical potential: the square-shoulder. The liquid-glass transition lines exhibit both melting by cooling and melting by compression as well as associated diffusion anomalies, similar to the ones observed in water. Differently from all previously investigated models, here for small shoulder widths a glass-glass line is found that is disconnected from the liquid phase. Upon increasing the shoulder width such a glass-glass line merges with the liquid-glass transition lines, featuring two distinct endpoint singularities that give rise to logarithmic decays in the dynamics. These findings can be explained analytically by the interplay of different repulsive length scales.

cond-mat.stat-mech↗

arXiv2009

Colloidal systems with competing interactions: from an arrested repulsive cluster phase to a gel

We report an extensive numerical study of a charged colloidal system with competing short-range depletion attraction and long-range electrostatic repulsion. By analizing the cluster properties, we identify two distinct regions in the phase diagram: a state composed of stable finite-size clusters, whose relative interactions are dominated by long-range repulsion, and a percolating network. Both states are found to dynamically arrest at low temperatures, providing evidence of the existence of two distinct non-ergodic states in these systems: a Wigner glass of clusters and a gel.

cond-mat.soft↗

arXiv2009

Connecting Irreversible to Reversible Aggregation: Time and Temperature

We report molecular dynamics simulations of a gel-forming mixture of ellipsoidal patchy particles with different functionality. We show that in this model, which disfavors the formation of bond-loops, elapsed time during irreversible aggregation -- leading to the formation of an extended network -- can be formally correlated with equilibrium temperature in reversible aggregation. We also show that it is possible to develop a parameter-free description of the self-assembly kinetics, bringing reversible and irreversible aggregation of loopless branched systems to the same level of understanding as equilibrium polymerization.

cond-mat.dis-nn↗

arXiv2008

Multiple glass transitions in star polymer mixtures: Insights from theory and simulations

The glass transition in binary mixtures of star polymers is studied by mode coupling theory and extensive molecular dynamics computer simulations. In particular, we have explored vitrification in the parameter space of size asymmetry $δ$ and concentration $ρ_2$ of the small star polymers at fixed concentration of the large ones. Depending on the choice of parameters, three different glassy states are identified: a single glass of big polymers at low $δ$ and low $ρ_2$, a double glass at high $δ$ and low $ρ_2$, and a novel double glass at high $ρ_2$ and high $δ$ which is characterized by a strong localization of the small particles. At low $δ$ and high $ρ_2$ there is a competition between vitrification and phase separation. Centered in the $(δ, ρ_2)$-plane, a liquid lake shows up revealing reentrant glass formation. We compare the behavior of the dynamical density correlators with the predictions of the theory and find remarkable agreement between the two.

cond-mat.soft↗