Charged soft-matter systems-such as colloidal dispersions and charged polymers-are dominated by attractive forces between constituent like-charged particles when neutralizing counterions of high charge valency are introduced. Such counter-intuitive effects indicate strong electrostatic coupling between like-charged particles, which essentially results from electrostatic correlations among counterions residing near particle surfaces. In this paper, the attraction mechanism and the structure of counterionic correlations are discussed in the limit of strong coupling based on recent numerical and analytical investigations and for various geometries (planar, spherical and cylindrical) of charged objects.

Above a certain density threshold, suspensions of rodlike colloidal particles form system-spanning networks. Using Monte Carlo simulations, we investigate how the depletion forces caused by spherical particles affect these networks in isotropic suspensions of rods. Although the depletion forces are strongly anisotropic and favor alignment of the rods, the percolation threshold of the rods decreases significantly. The relative size of the effect increases with the aspect ratio of the rods. The structural changes induced in the suspension by the depletant are characterized in detail and the system is compared to an ideal fluid of freely interpenetrable rods. DOI: 10.1103/PhysRevLett.98.108303 PACS numbers: 82.70.Dd, 61.20.Ja, 64.60.Ak Networks of electrically conducting carbon nanotubes (CNTs) have important applications in the development of lightweight conducting composites and films [1][2][3][4][5]. At equilibrium, colloidal suspensions of CNTs contain clusters with a distribution of sizes. Contiguous conducting paths first appear on a macroscopic scale when the CNT density crosses the percolation threshold, at which the average cluster size diverges. The sample's conductivity rises sharply by several orders of magnitude at this point [1][2][3][4][5]. The large aspect ratio of CNTs means that, for a disordered distribution of positions and orientations, a single particle provides connectivity over a longer distance than several more spherical particles with the same total volume. Hence, an insulating matrix can be loaded with a smaller volume fraction of conducting filler, leading to a lighter material.The percolation threshold of the CNTs depends both on their aspect ratio, and on the interactions between them. Recent studies show that the addition of surfactant to a suspension of CNT bundles (CNTBs) substantially lowers the percolation threshold [6]. At sufficiently high concentrations, the surfactant self-assembles into micelles, which induce depletion attraction between the bundles [7]. Since the attraction is weak, percolation is reversible and the system remains at thermodynamic equilibrium, in contrast to strongly interacting particles, which may form structurally arrested gels [8].The depletion forces arise from competition between the translational and orientational entropy of the CNTBs on the one hand, and the translational entropy of the micelles on the other. If two bundles approach closely, the volume available to micelles increases, inducing an effective attraction between bundles. The effect is highly anisotropic, since the volume excluded to the depletant depends on the relative orientations of the bundles [9,10]. The excluded volume is minimized in parallel arrangements of neighboring rods, but alignment also reduces the spatial extent of a cluster, and should therefore suppress rather than enhance the formation of percolating networks [11]. Even in systems of spheres, it is possible for attraction to raise the percolation threshold [12]. Nevertheless, recent analytic work on mixtures of ro...

There has been increasing interest in ice nucleation research in the last decade. To identify important gaps in our knowledge of ice nucleation processes and their impacts, two international workshops on ice nucleation were held in Vienna, Austria in 2015 and 2016. Experts from these workshops identified the following research needs: (1) uncovering the molecular identity of active sites for ice nucleation; (2) the importance of modeling for the understanding of heterogeneous ice nucleation; (3) identifying and quantifying contributions of biological ice nuclei from natural and managed environments; (4) examining the role of aging in ice nuclei; (5) conducting targeted sampling campaigns in clouds; and (6) designing lab and field experiments to increase our understanding of

Transition interface path sampling combined with straightforward molecular dynamics simulation was applied to study the mechanism and kinetics of the crystallization of an undercooled 3:1 binary Lennard-Jones mixture with diameter ratio 0.85 and equal interaction strengths. We find that this mixture freezes via the formation of crystalline clusters consisting of a fcc-rich core and a bcc-rich surface layer, with an excess of large particles and particle species distributed randomly. A detailed comparison reveals that the transition mechanism is similar to that of the pure fluid but occurs with much smaller nucleation rates even at comparable degrees of undercooling. Also, the growth of the crystalline cluster in the mixture proceeds at a pace about 1 order of magnitude slower than in the pure system. Possibly, this slow dynamics of the mixture is related to the occurrence and subsequent relaxation of icosahedral structures in the growing crystal as well as in the liquid surrounding it.

We investigated the impact of rotational diffusion on the process of irreversible nanoparticle aggregation.

We report on numerical simulations of a monolayer of charge-stabilized colloids driven over a substrate potential by an external dc force acting along a symmetry axis of the monolayer. Using overdamped Langevin dynamics, we studied the sliding transition for various inter-particle interaction strengths as a function of the driving force. For weak interactions, the diffusion of individual defects is responsible for the motion of the monolayer. As the interaction strength is increased, sliding is induced by distinct density compression and decompression zones. For very strong interactions, a type of stick-slip mechanism emerges, in which the sliding of the monolayer is mediated by the propagation of collective distortion waves. Our predictions can be tested experimentally with two-dimensional arrangements of colloidal particles exposed to periodic light fields and our work shows that the inter-particle interaction strength tunes the degree of correlation in the sliding mechanism adopted by a monolayer driven over a commensurate substrate.

Crystallization, a prototypical self-organization process during which a disordered state spontaneously transforms into a crystal characterized by a regular arrangement of its building blocks, usually proceeds by nucleation and growth. In the initial stages of the transformation, a localized nucleus of the new phase forms in the old one due to a random fluctuation. Most of these nuclei disappear after a short time, but rarely a crystalline embryo may reach a critical size after which further growth becomes thermodynamically favorable and the entire system is converted into the new phase. In this article, we will discuss several theoretical concepts and computational methods to study crystallization. More specifically, we will address the rare event problem arising in the simulation of nucleation processes and explain how to calculate nucleation rates accurately. Particular attention is directed towards discussing statistical tools to analyze crystallization trajectories and identify the transition mechanism.

When non-adsorbing polymers are added to an isotropic suspension of rod-like colloids, the colloids effectively attract each other via depletion forces. We performed Monte Carlo simulations to study the phase diagram of such rod-polymer mixture. The colloidal rods were modeled as hard spherocylinders; the polymers were described as spheres of the same diameter as the rods. The polymers may overlap with no energy cost, while the overlap of polymers and rods is forbidden. Large amounts of depletant cause phase separation of the mixture. We estimated the phase boundaries of isotropic-isotropic coexistence both in the bulk and in confinement. To determine the phase boundaries we applied the grand canonical ensemble using successive umbrella sampling ͓J. Chem. Phys. 120, 10925 ͑2004͔͒, and we performed a finite size scaling analysis to estimate the location of the critical point. The results are compared with predictions of the free volume theory developed by Lekkerkerker and Stroobants ͓Nuovo Cimento D 16, 949 ͑1994͔͒. We also give estimates for the interfacial tension between the coexisting isotropic phases and analyze its power-law behavior on the approach of the critical point.