Repulsive Glass Research Articles

Low temperature behavior and glass line in the symmetrical colloidal electrolyte

We report on the low temperature behavior of the colloidal electrolyte by means of molecular dynamics simulations, where the electrostatic interactions were modeled using effective screened interactions. As in previous works, we have found a region of gas-liquid coexistence located in the low T-low rho region. At temperatures much lower than the critical one, the system cannot reach equilibrium, that is, the gas-liquid transition is arrested. Two different mechanisms have been identified to cause arrest: crowding at intermediate T values, associated with the crossing point between the binodal and the glass line, and a gelationlike arrest at very low T. To test the latter, the dynamics of the colloidal electrolyte near this crossing point has been computed and compared to the universal predictions of the ideal mode-coupling theory. As in other glass-forming liquids, we found good agreement between this mean field theory and the dynamics of this complex system, although it fails just at the transition. Interestingly, in this region we found that the dynamics of this system is driven mainly by the steric interactions, showing all the typical properties of a repulsive colloidal glass. Finally, the isodiffusivity lines show that in this system with short-range attractions, there is no reentrant glass phenomenon as opposed to monocomponent attractive systems.

Kinetic arrest and glass-glass transition in short-ranged attractive colloids

A thermally reversible repulsive hard-sphere to sticky-sphere transition was studied in a model colloidal system over a wide volume fraction range. The static microstructure was obtained from high resolution small angle x-ray scattering, the colloid dynamics was probed by dynamic x-ray and light scattering, and supplementary mechanical properties were derived from bulk rheology. At low concentration, the system shows features of gas-liquid type phase separation. The bulk phase separation is presumably interrupted by a gelation transition at the intermediate volume fraction range. At high volume fractions, fluid-attractive glass and repulsive glass-attractive glass transitions are observed. It is shown that the volume fraction of the particles can be reliably deduced from the absolute scattered intensity. The static structure factor is modeled in terms of an attractive square-well potential, using the leading order series expansion of Percus-Yevick approximation. The ensemble-averaged intermediate scattering function shows different levels of frozen components in the attractive and repulsive glassy states. The observed static and dynamic behavior are consistent with the predictions of a mode-coupling theory and numerical simulations for a square-well attractive system.

Rotational arrest in a repulsive colloidal glass

The rotational dynamics of aqueous colloidal dispersions of magnetic nanoparticles isexamined whilst increasing the volume fraction up to the formation of a macroscopic solid.The freezing of the rotation is studied by magneto-optical relaxation experiments andrelated to small-angle neutron-scattering results showing that the structure, albeit imposedby the Coulombic repulsion, remains amorphous because of the size polydispersity. Thevolume fraction at which the freezing occurs is noticeably dependent on the salinity of thesuspension. The latter point is discussed together with the microscopic mechanism of thearrest.

Mode-coupling theory predictions for a limited valency attractive square wellmodel

Recently we have studied, using numerical simulations, a limited valencymodel, i.e. an attractive square well model with a constraint on the maximumnumber of bonded neighbours. Studying a large region of temperaturesT and packingfractions ϕ, we have estimated the location of the liquid–gas phase separation spinodaland the loci of dynamic arrest, where the system is trapped in a disorderednon-ergodic state. Two distinct arrest lines for the system are present in thesystem: a (repulsive) glass line at high packing fraction, and a gel line at lowϕ andT. The former isessentially vertical (ϕ controlled), while the latter is rather horizontal(T controlled)in the (ϕ–T) plane. We here complement the molecular dynamics results with mode coupling theorycalculations, using the numerical structure factors as input. We find that the theorypredicts a repulsive glass line—in satisfactory agreement with the simulationresults—and an attractive glass line, which appears to be unrelated to the gel line.

Dynamical heterogeneities in an attraction driven colloidal glass

The dynamical heterogeneities (DH) in non-ergodic states of an attractive colloidal glass are studied, as a function of the waiting time. Whereas the fluid states close to vitrification showed strong DH, the distribution of squared displacements of the glassy states studied here only present a tail of particles with increased mobility for the lower attraction strength at short waiting times. These particles are in the surface of the percolating cluster that comprises all of the particles, reminiscent of the fastest particles in the fluid. The quench deeper into the attractive glass is dynamically more homogeneous, in agreement with repulsive glasses (i.e. Lennard-Jones glass).

Direct imaging of repulsive and attractive colloidal glasses

Coherent anti-Stokes Raman scattering microscopy is performed on glassy systems of poly(methylmethacrylate) colloidal particles in density- and refractive-index-matched solvents. Samples are prepared with varying amounts of linear polystyrene, which induces a depletion driven attraction between the nearly hard-sphere particles. Images collected over several hours confirm the existence of a reentrant glass transition. The images also reveal that the dynamics of repulsive and attractive glasses are qualitatively different. Colloidal particles in repulsive glasses exhibit cage rattling and escape, while those in attractive glasses are nearly static while caged but exhibit large displacements upon (infrequent) cage escape.

Microstructure and Rheology near an Attractive Colloidal Glass Transition

Microstructure and rheological properties of a thermally reversible short-ranged attractive colloidal system are studied in the vicinity of the attractive glass transition line. At high volume fractions, the static structure factor changes very little but the low frequency shear moduli varies over several orders of magnitude across the transition. From the frequency dependence of shear moduli, fluid-attractive glass and repulsive glass-attractive glass transitions are identified.

Computer Simulation of Macroion Layering in a Wedge Film

The layering of macroions confined to a wedge slit formed by two uncharged hard walls is studied using a canonical Monte Carlo method combined with a simulation cell that contains both wedge-shaped slit and bath regions. The macroion solution is modeled within a one-component fluid approach that in an effective way incorporates the double layer repulsion due to simple electrolyte ions as well as the discrete nature of an aqueous solvent. The layer formation under a wedge confinement is analyzed by carrying out separate simulation runs for a set of consecutive wedge segments designed to represent a single wedge slit. As the wedge thickness progressively increases, the sequence of regions along the wedge film with distinct features of macroion layering has been established. This sequence comprises (i) a wedge region of the thickness smaller than the macroion diameter that is free of macroions; (ii) a region with a one-dimensional macroion chain along the wedge corner at a wedge thickness of a one macroion diameter; (iii) a region comprising a low-ordered macroion monolayer that extends until the wedge thickness slightly above two macroion diameters; (iv) a region comprising a pair of well-defined two-dimensional configurations of macroions segregated on each of the wedge walls; and (v) a free-of-macroions wedge region between two surface monolayers that now originates from an electrostatic repulsion imposed by the surface macroions, which is followed by (vi) a well-defined macroion monolayer film between two surface monolayers, a less defined bilayer film, a three-layer film, and so on up to the bulk solution. Once formed, the macroion surface monolayers persist for all remaining wedge thicknesses up to the bulk, forming in such a way effective charged wedge boundaries. Such a formation of the macroion surface monolayers on the uncharged confining walls is related to the haloing mechanism for regulating the stability in colloidal suspensions [Tohver et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8951] and is discussed as well. Finally, the estimated boundary of the free of macroion region between surface monolayers correlates well with the location of the boundary of the so-called "vacuum" phase that has been observed experimentally in an aqueous suspension of charged polystyrene spheres bounded by electrostatically repulsive glass walls [Pieransky et al. Phys. Rev. Lett. 1983, 50, 900].

Kinetics of ergodic-to-nonergodic transitions in charged colloidal suspensions: Aging and gelation

There are two types of isotropic disordered nonergodic states in colloidal suspensions: colloidal glasses and gels. In a recent paper [H. Tanaka, J. Meunier, and D. Bonn, Phys. Rev. E 69, 031404 (2004)], we discussed the static aspect of the differences and the similarities between the two. In this paper, we focus on the dynamic aspect. The kinetics of the liquid-glass transition is called "aging," while that of the sol-gel transition is called "gelation." The former is primarily governed by repulsive interactions between particles, while the latter is dominated by attractive interactions. Slowing down of the dynamics during aging reflects the increasing cooperativity required for the escape of a particle from the cage formed by the surrounding particles, while that during gelation reflects the increase in the size of particle clusters towards the percolation transition. Despite these clear differences in the origin of the slowing down of the kinetics between the two, it is not straightforward experimentally to distinguish them in a clear manner. For an understanding of the universal nature of ergodic-to-nonergodic transitions, it is of fundamental importance to elucidate the differences and the similarities in the kinetics between aging and gelation. We consider this problem, taking Laponite suspension as an explicit example. In particular, we focus on the two types of nonergodic states: (i) an attractive gel formed by van der Waals attractions for high ionic strengths and (ii) a repulsive Wigner glass stabilized by long-range Coulomb repulsions for low ionic strengths. We demonstrate that the aging of colloidal Wigner glass crucially differs not only from gelation, but also from the aging of structural and spin glasses. The aging of the colloidal Wigner glass is characterized by the unique cage-forming regime that does not exist in the aging of spin and structural glasses.

Equilibrium cluster formation in concentrated protein solutions and colloids

Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels). The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid-polymer systems and concentrated protein solutions. However, intriguing problems like the frequent occurrence of cluster phases remain. Here we report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid-polymer mixtures. We demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions.

Studies of structural arrest transition inL64/D2O micellar solutions

We summarize the results of analyses of a series of small angle neutron scattering (SANS)experiments on dense L64 copolymer micellar solutions in heavy water. The system ismodelled as a suspension of micellar particles that interact among themselves with a hardcore plus a short range square well attractive interaction. The observed phasebehaviour in the plane of temperature and polymer concentration is described.Characteristic features such as the existence of the glass-to-liquid-to-glass re-entrancetransition and the line of glass-to-glass transition with its associated end point(A3 singularity) are found which are quantitatively consistent with the recent mode couplingtheory calculations using the same model potential. Supplementary to SANS experiments,photon correlation spectroscopy experiments on the same system have been performed.They show that although the Debye–Waller factors (the long time limits of the coherentintermediate scattering functions) of the two glasses do indeed become identical at theA3 point as predicted, the intermediate scattering functions exhibit distinctlydifferent intermediate time relaxations. Furthermore, we show thatthe Debye–Waller factors obtained from volume fractions beyond theA3 point have the same magnitude as for the repulsive glass obtained before theA3 point. In addition, the effect of applied pressure on the kinetic glass transition(KGT) is explored for the first time. We observe that, for the KGT inL64/D2O micellar system, increasing applied pressure at constant temperature has a similar effect toreducing temperature at constant pressure. The preliminary observation of the ageing effecton quenching into the attractive glass state with the SANS technique is reported as well.

Aging in short-ranged attractive colloids: a numerical study.

We study the aging dynamics in a model for dense simple liquids, in which particles interact through a hard-core repulsion complemented by a short-ranged attractive potential, of the kind found in colloidal suspensions. In this system, at large packing fractions, kinetically arrested disordered states can be created both on cooling (attractive glass) and on heating (repulsive glass). The possibility of having two distinct glasses, at the same packing fraction, with two different dynamics offers the unique possibility of comparing-within the same model-the differences in aging dynamics. We find that, while the aging dynamics of the repulsive glass is similar to the one observed in atomic and molecular systems, the aging dynamics of the attractive glass shows novel unexpected features.

Nonergodic states of charged colloidal suspensions: repulsive and attractive glasses and gels.

Two types of isotropic disordered nonergodic states exist in colloidal suspensions: glasses and gels. The difference between the two is that the nonergodicity, or elasticity, of gel stems from the existence of a percolated network, while that of glass stems from caging effects. Despite this clear difference in the origin of nonergodicity, it is not straightforward to distinguish the two states in a clear manner. Taking a Laponite suspension as an explicit example, we propose a general phase diagram for charged colloidal systems. It follows that a transition from the glass to the gel state can be induced by changing the interparticle interactions from predominantly repulsive to attractive. This originates from the competition between electrostatic Coulomb repulsion and van der Waals attraction. If the repulsion dominates, the system forms a Wigner glass, while in a predominantly attractive situation it forms a gel. In the intermediate region, where both repulsive and attractive interactions play roles, it may form an attractive glass.

Neutron- and light-scattering studies of the liquid-to-glass and glass-to-glass transitions in dense copolymer micellar solutions

Recent mode coupling theory (MCT) calculations show that if a short-range attractive interaction is added to the pure hard sphere system, one may observe a new type of glass originating from the clustering effect (the attractive glass) as a result of the attractive interaction. This is in addition to the known glass-forming mechanism due to the cage effect in the hard sphere system (the repulsive glass). The calculations also indicate that if the range of attraction is sufficiently short compared to the diameter of the particle, within a certain interval of volume fractions where the two glass-forming mechanisms nearly balance each other, varying the external control parameter, the effective temperature, makes the glass-to-liquid-to-glass reentrance and the glass-to-glass transitions possible. Here we present experimental evidence of both transitions, obtained from small-angle neutron-scattering and photon correlation measurements taken from dense L64 copolymer micellar solutions in heavy water. Varying the temperature in certain predicted volume fraction range triggers a sharp transition between these two different types of glass. In particular, according to MCT, there is an end point (called A3 singularity) of this glass-to-glass transition line, beyond which the long-time dynamics of the two glasses become identical. Our findings confirm this theoretical prediction. Surprisingly, although the Debye-Waller factors, the long-time limit of the coherent intermediate scattering functions, of these two glasses obtained from photon correlation measurements indeed become identical at the predicted volume fraction, they exhibit distinctly different intermediate time relaxation. Furthermore, our experimental results obtained from volume fractions beyond the end point are characterized by the same features as the repulsive glass obtained before the end point. A complete phase diagram giving the boundaries of the structural arrest transitions for L64 micellar system is given.

Observation of Liquid-to-Glass and Glass-to-Glass Transitions in L64/D2O Triblock Copolymer Micellar System

We report photon correlation spectroscopy (PCS), low shear viscosity and small angle neutron scattering (SANS) studies of a Pluronic L64 micellar system in aqueous solutions at high polymer concentrations (wt%), 35.0%<c<54.4%, in a temperature range where a kinetic glass transition (KGT) has been observed. We analyze SANS intensity distributions in the liquid state assuming that the micelles are spherical and interact among themselves via an effective pair-potential consisting of a hard core plus a narrow attractive square-well, the depth of which is temperature dependent. Comparison of the potential parameters so obtained with the phase diagram predicted recently by mode coupling theory (MCT) allows us to verify the existence of the so-called “liquid to attractive glass” KGT line. We then use scaling plots of SANS intensity distributions to show the existence of another branch of the KGT line where transitions from a repulsive glass to an attractive glass occur. In particular, we show an evidence of the existence of the so-called A3 singularity end point where two glasses become identical in local structure [Sow-Hsin Chen, Wei-Ren Chen, Francesco Mallamace, “The glass–glass transition and its end point in a co-polymer micellar system”, to be published].

Pair correlations of a dilute charged colloidal fluid near a glass wall.

Using confocal microscopy we examine the static structure of low density, highly charged colloidal suspensions near a repulsive glass boundary. We find no sign of an interparticle attraction of the magnitude noted previously.

Competition between crystallization and glassification for particles with short-ranged attraction. Possible applications to protein crystallization

We discuss the phase behaviour of spherical hard-core particles, with an attractive potential, as described by a hard-core Yukawa model. The ratio of the range of the attraction to the diameter of the particles is an important control parameter of the problem. Upon decreasing the range of the attraction, the phase diagram changes quite significantly, with the liquid–gas transition becoming metastable, and the crystal being in equilibrium with the fluid, with no intervening liquid. We also study the glass transition lines and, crucially, find that the situation, being very simple for pure repulsive potentials, becomes much richer in competition between glass and crystal phases for short-range attractions. Also a transition between attractive and repulsive glass appears somewhat in analogy with the isostructural equilibrium transition between two crystals.

Mode-coupling theory of colloids with short-range attractions

Within the framework of the mode-coupling theory of super-cooledliquids, we investigate new phenomena in colloidal systems on approachto their glass transitions. When the inter-particle potentialcontains an attractive part, besides the usual repulsive hard core,two intersecting liquid-glass transition lines appear, one of whichextends to low densities, while the other one, at high densities,shows a re-entrant behaviour. In the glassy region a newtype of transition appears between two different types ofglasses. The complex phenomenology can be described in terms of higherorder glass transition singularities. The various glass phases arecharacterized by means of their viscoelastic properties.The glass driven by attractions has been associatedwith particle gels, andthe other glass is the well known repulsive colloidal glass. Thesecorrespondences, in association with the new predictions of glassybehaviour, mean that such phenomena may be expected in colloidal systemswith, for example, strong depletion or other short-ranged attractivepotentials.

The vibrational motions of particle gels

A simple discussion of the nature of repulsive and attractive glasses is given, and results presented in the framework of idealized mode coupling theory (MCT). The attractive glass is considered to be equivalent to the particle gel state observed in colloidal science, and the repulsive glass is equivalent to the dense colloidal glass that has recently been studied in colloidal science. Given the equivalence between a particle gel and a glass, it is possible to use MCT to calculate the dynamical structure factor, and thereby the spectrum of all vibrational motions in the glass. We present such spectra for a more and less dense particle gel state, and interpret the motions predicted by the theory. It is pointed out that such motions should resolve between what we call the particle gel, and the traditional repulsive colloidal glass, and as such are perhaps a most promising manner in which to study particle gels in the future.

Dynamical phasetransitions in glasses induced by the ruggedness of the free-energylandscape

We propose damage spreading (DS) as a tool for investigating thetopological features related to the ruggedness of the free-energylandscape. We argue that DS measures the positiveness of the largestLyapunov exponent associated with the basins of attraction visited bythe system during its dynamical evolution. We discuss recent resultsobtained in the framework of mode-coupling theory and comment on possible extensions to the study of realistic glasses. Preliminary results arepresented for purely repulsive soft-sphere glasses.