Equilibrium Cluster Phases and Low-Density Arrested Disordered States: The Role of Short-Range Attraction and Long-Range Repulsion

Sciortino, Francesco; Mossa, Stefano; Zaccarelli, Emanuela; Tartaglia, P.

doi:10.1103/physrevlett.93.055701

Cited by 476 publications

(648 citation statements)

References 27 publications

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“…The presence of a peak in the structure factor at length scales that are 5 times the nearest neighbor spacing has been observed in simulations involving short-range attractions and longrange repulsions [29]. This peak has been associated with the distance between neighboring clusters which are also observed in experiments.…”

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confidence: 49%

Low-Density Ordered Phase in Brownian Dipolar Colloidal Suspensions

Agarwal

Yethiraj

2009

Phys. Rev. Lett.

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We study the low volume fraction and electric field phase behavior of a Brownian colloidal suspension. On the application of a uniform ac field, we find a novel phase where chains of particles aggregate to form a well defined cellular network, consisting of particle-free ''voids'' surrounded by a percolating network of particle-rich walls. This cellular structure is stable to very long times, indicative of an equilibrium thermodynamic phase. The cell-cell spacing is not sensitive to the concentration of the sample but scales with sample thickness. Any self-consistent mechanism for the existence of this void phase must consist of long-ranged repulsions and shorter-ranged attractions. DOI: 10.1103/PhysRevLett.102.198301 PACS numbers: 47.57.JÀ, 64.70.pv Brownian colloidal suspensions with tunable interactions are important model systems for understanding phase transitions in atomic systems [1][2][3][4]. Real-space microscopy and light scattering experiments have been carried out on crystal nucleation dynamics [5,6] as well as structure and dynamics at the colloidal glass transition [7][8][9]. In the presence of an external electric field, Brownian colloidal spheres experience an anisotropic dipolar interaction and form chains (the ''string fluid'') and multichain aggregates [10,11], as well as body centered tetragonal (BCT) crystals [12][13][14]. Multiple competing interactions lead to richer phase behavior. In colloid-polymer mixtures both network-forming gel phases and glassy behavior can be observed by control of competing attractive and repulsive interactions [15,16]. Anisotropic interactions between colloids [17] in a liquid crystal medium also lead to a network of particle aggregates [18]. In dipolar colloids with added electrostatic repulsions, numerous crystalline phases are observed [19,20]. The dipolar interaction is of great interest because it consists of both competing on-axis (along the electric field) attractions and in-plane repulsions, and because it is an important driving force for nanoparticle selforganization [21].Simulations of spheres with both dipolar and van der Waals interactions have shown many variants (linear aggregates, droplets, columns) of phase separation into regions of higher and lower densities [22,23]. However, structure formation in the low-density regime of Brownian colloidal suspensions in an electric field has not attracted much experimental attention. Recently, interesting field induced cellular structures were reported in a granular medium [24], but the mechanism (and whether they represented equilibrium structures) was not clear.In this Letter, we report real-space confocal microscopy studies of the effect of an ac external electric field on the low-density structure of Brownian colloidal suspensions. We report the existence of a novel ''gel-like'' phase at low volume fractions ( < 4:0%) where chains of particles aggregate in time to form a cellular structure composed of ''voids''-particle-free domains enclosed completely by particle-rich walls. We propose that the co...

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confidence: 49%

Low-Density Ordered Phase in Brownian Dipolar Colloidal Suspensions

Agarwal

Yethiraj

2009

Phys. Rev. Lett.

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“…A peculiarity, which is often seen in gels of intermediate and lower densities, is a low-q peak in the static structure factor. It appears on the length scale of the voids in the structure when the sol-gel transition line is reached, increases towards the transition [6,47], and shifts to slightly lower q-values (cf. Fig.…”

Section: A Structure Factorsmentioning

confidence: 99%

Bond formation and slow heterogeneous dynamics in adhesive spheres with long-ranged repulsion: Quantitative test of mode coupling theory

et al. 2007

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A colloidal system of spheres interacting with both a deep and narrow attractive potential and a shallow long-ranged barrier exhibits a prepeak in the static structure factor. This peak can be related to an additional mesoscopic length scale of clusters and/or voids in the system. Simulation studies of this system have revealed that it vitrifies upon increasing the attraction into a gel-like solid at intermediate densities. The dynamics at the mesoscopic length scale corresponding to the prepeak represents the slowest mode in the system. Using mode coupling theory with all input directly taken from simulations, we reveal the mechanism for glassy arrest in the system at 40% packing fraction. The effects of the low-q peak and of polydispersity are considered in detail. We demonstrate that the local formation of physical bonds is the process whose slowing down causes arrest. It remains largely unaffected by the large-scale heterogeneities, and sets the clock for the slow cluster mode. Results from mode-coupling theory without adjustable parameters agree semi-quantitatively with the local density correlators but overestimate the lifetime of the mesoscopic structure (voids).

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“…x,y φ(x, y; t w )φ(x, y; t w ) (6) where t w is the waiting time, τ is the time lapse between the two density configurations and brackets stand for averaging over an ensemble of realizations. In figure 4, we show the correlation function corresponding to different waiting times t w (t w = 5 10 4 , red squares, t w = 2 10 5 , green circles and t w = 310 5 , blue triangles) for shear stress U 0 = 0.02.…”

Section: Pacs Numbersmentioning

confidence: 99%

“…Note that positive(negative) G code for repulsion(attraction) respectively. Our model is reminiscent of the potentials used to investigate arrested phase-separation and structural arrest in charged-colloidal systems [6], and also bears similarities to the NNN (next-to-nearest-neighbor) frustrated lattice spin models [7]. As compared with lattice spin models, in our case a high lattice connectivity is required to ensure compliance with macroscopic non-ideal hydrodynamics, particularly the isotropy of potential energy interactions, which lies at the heart of the complex rheology to be discussed in this work.…”

Section: Pacs Numbersmentioning

confidence: 99%

Mesoscopic Lattice Boltzmann Modeling of Flowing Soft Systems

2009

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A mesoscopic multi-component lattice Boltzmann model with short-range repulsion between different species and short/mid-ranged attractive/repulsive interactions between like-molecules is introduced. The interplay between these composite interactions gives rise to a rich configurational dynamics of the density field, exhibiting many features of disordered liquid dispersions (microemulsions) and soft-glassy materials, such as long-time relaxation due to caging effects, anomalous enhanced viscosity, ageing effects under moderate shear and flow above a critical shear rate. PACS numbers:The rheology of flowing soft systems, such as emulsions, foams, gels, slurries, colloidal glasses and related fluids, is a fast-growing sector of modern non-equilibrium thermodynamics, with many applications in material science,chemistry and biology [1].These materials exhibit a number of distinctive features, such as long-time relaxation, anomalous viscosity, aging behaviour, whose quantitative description is likely to require profound extensions of non-equilibrium statistical mechanics. The study of these phenomena sets a pressing challenge for computer simulation as well, since characteristic time-lenghts of disordered fluids can escalade tens of decades over the molecular time scales. To date, the most credited techniques for computational studies of these complex flowing materials are Molecular Dynamics and Monte Carlo simulations [2]. Molecular dynamics in principle provides a fully ab-initio description of the system, but it is limited to space-time scales significantly shorter than experimental ones. Monte Carlo methods are less affected by these limitations, but they are bound to deal with equilibrium states. As a result, neither MD nor MC can easily take into account the non-equilibrium dynamics of complex flowing materials, such as micro-emulsions, on space-time scales of hydrodynamic interest. In the last decade, a new class of mesoscopic methods, based on minimal lattice formulations of Boltzmann's kinetic equation, have captured significant interest as an efficient alternative to continuum methods based on the discretization of the Navier-Stokes equations for non-ideal fluids [3]. To date, a very popular such mesoscopic technique is the socalled pseudo-potential-Lattice-Boltzmann (LB) method, developed over a decade ago by Shan and Chen (SC) [4]. In the SC method, potential energy interactions are represented through a density-dependent mean-field pseudopotential, Ψ[ρ], and phase separation is achieved by imposing a short-range attraction between the light and dense phases. In this Letter, we provide the first numerical evidence that a suitably extended, two-species, mesoscopic lattice Boltzmann model is capable of reproducing many features of soft-glassy (micro-emulsions), such as structural arrest, anomalous viscosity, cage-effects and ageing under shear. The key feature of our model is the where f is is the probability of finding a particle of species s at site r and time t, moving along the ith lattice direction defi...

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Equilibrium Cluster Phases and Low-Density Arrested Disordered States: The Role of Short-Range Attraction and Long-Range Repulsion

Cited by 476 publications

References 27 publications

Low-Density Ordered Phase in Brownian Dipolar Colloidal Suspensions

Low-Density Ordered Phase in Brownian Dipolar Colloidal Suspensions

Bond formation and slow heterogeneous dynamics in adhesive spheres with long-ranged repulsion: Quantitative test of mode coupling theory

Mesoscopic Lattice Boltzmann Modeling of Flowing Soft Systems

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