Wigner crystals for a planar, equimolar binary mixture of classical, charged particles

We have investigated the ground state configurations of an equimolar, binary mixture of classical charged particles (with nominal charges $Q_1$ and $Q_2$) that condensate on a neutralizing plane. Using efficient Ewald summation techniques for the calculation of the ground state energies, we have identified the energetically most favourable ordered particle arrangements with the help of a highly reliable optimization tool based on ideas of evolutionary algorithms. Over a large range of charge ratios, $q = Q_2 / Q_1$, we identify six non-trivial ground states, some of which show a remarkable and unexpected structural complexity. For $0.59 \lesssim q < 1$ the system undergoes a phase separation where the two charge species populate in a hexagonal arrangement spatially separated areas.Comment: 14 pages, 8 figure

The Hierarchical Reference Theory as applied to square well fluids of variable range

Continuing our investigation into the numerical properties of the Hierarchical Reference Theory, we study the square well fluid of range lambda from slightly above unity up to 3.6. After briefly touching upon the core condition and the related decoupling assumption necessary for numerical calculations, we shed some light on the way an inappropriate choice of the boundary condition imposed at high density may adversely affect the numerical results; we also discuss the problem of the partial differential equation becoming stiff for close-to-critical and sub-critical temperatures. While agreement of the theory's predictions with simulational and purely theoretical studies of the square well system is generally satisfactory for lambda greater than about 2, the combination of stiffness and the closure chosen is found to render the critical point numerically inaccessible in the current formulation of the theory for most of the systems with narrower wells. The mechanism responsible for some deficiencies is illuminated at least partially and allows us to conclude that the specific difficulties encountered for square wells are not likely to resurface for continuous potentials.Comment: 15 pages LaTeX/RevTeX, 3 tables, 4 figures. Also see http://purl.oclc.org/NET/a-reiner/sci/texts/20011127-0

Impact of random obstacles on the dynamics of a dense colloidal fluid

Using molecular dynamics simulations we study the slow dynamics of a colloidal fluid annealed within a matrix of obstacles quenched from an equilibrated colloidal fluid. We choose all particles to be of the same size and to interact as hard spheres, thus retaining all features of the porous confinement while limiting the control parameters to the packing fraction of the matrix, {\Phi}m, and that of the fluid, {\Phi}f. We conduct detailed investigations on several dynamic properties, including the tagged-particle and collective intermediate scattering functions, the mean-squared displacement, and the van Hove function. We show the confining obstacles to profoundly impact the relaxation pattern of various quantifiers pertinent to the fluid. Varying the type of quantifier (tagged-particle or collective) as well as {\Phi}m and {\Phi}f, we unveil both discontinuous and continuous arrest scenarios. Furthermore, we discover subdiffusive behavior and demonstrate its close connection to the matrix structure. Our findings partly confirm the various predictions of a recent extension of mode-coupling theory to the quenched-annealed protocol.Comment: 16 pages, 20 figures, minor revision

Dynamic arrest of colloids in porous environments: disentangling crowding and confinement

Using numerical simulations we study the slow dynamics of a colloidal hard-sphere fluid adsorbed in a matrix of disordered hard-sphere obstacles. We calculate separately the contributions to the single-particle dynamic correlation functions due to free and trapped particles. The separation is based on a Delaunay tessellation to partition the space accessible to the centres of fluid particles into percolating and disconnected voids. We find that the trapping of particles into disconnected voids of the matrix is responsible for the appearance of a nonzero long-time plateau in the single-particle intermediate scattering functions of the full fluid. The subdiffusive exponent $z$, obtained from the logarithmic derivative of the mean-squared displacement, is observed to be essentially unaffected by the motion of trapped particles: close to the percolation transition, we determined $z \simeq 0.5$ for both the full fluid and the particles moving in the percolating void. Notably, the same value of $z$ is found in single-file diffusion and is also predicted by mode-coupling theory along the diffusion-localisation line. We also reveal subtle effects of dynamic heterogeneity in both the free and the trapped component of the fluid particles, and discuss microscopic mechanisms that contribute to this phenomenon.Comment: 18 pages, 12 figures, minor change

Hopping and microscopic dynamics of ultrasoft particles in cluster crystals

We have investigated the slow dynamics of ultrasoft particles in crystalline cluster phases, where point particles interact through the generalized exponential potential u(r) = \epsilon \exp[-(r/\sigma)^n], focusing on the cluster fcc phase of this model with n=4. In an effort to elucidate how the mechanisms of mass transport depend on the microscopic dynamics and in order to mimic a realistic scenario in a related experiment we have performed molecular dynamics, Brownian dynamics, and Monte Carlo simulations. In molecular dynamics simulations the diffusion of particles proceeds through long-range jumps, guided by strong correlations in the momentum direction. In Monte Carlo and Brownian dynamics simulations jump events are short-ranged, reflecting the purely configurational nature of the dynamics. In contrast to what was found in models of glass-forming liquids, the effect of Newtonian and stochastic microscopic dynamics on the long-time relaxation cannot be accounted for by a temperature-independent rescaling of the time units. From the obvious qualitative discrepancies in the short time behavior between the three simulation methods and the non-trivial difference in jump length distributions, long time relaxation, and dynamic heterogeneity, we learn that a more complex modeling of the dynamics in realistic systems of ultrasoft colloids is required.Comment: 12 pages, 18 figures, added results of Brownian dynamics simulation

Clustering, conductor-insulator transition and phase separation of an ultrasoft model of electrolytes

We investigate the clustering and phase separation of a model of ultrasoft, oppositely charged macroions by a combination of Monte Carlo and Molecular Dynamics simulations. Static and dynamic diagnostics, including the dielectric permittivity and the electric conductivity of the model, show that ion pairing induces a sharp conductor-insulator transition at low temperatures and densities, which impacts the separation into dilute and concentrated phases below a critical temperature. Preliminary evidence is presented for a possible tricritical nature of the phase diagram of the model.Comment: 5 pages, 5 figure

Antinematic local order in dendrimer liquids

We use monomer-resolved numerical simulations to study the positional and orientational structure of a dense dendrimer solution, focusing on the effects of dendrimers' prolate shape and deformability on the short-range order. Our results provide unambiguous evidence that the nearest-neighbor shell of a tagged particle consists of a mixture of crossed, side-by-side, side-to-end, and end-to-end pair configurations, imposing antinematic rather than nematic order observed in undeformable rodlike particles. This packing pattern persists even at densities where particle overlap becomes sizable. We demonstrate that the antinematic arrangement is compatible with the A15 crystal lattice reported in several dendrimer compounds.Comment: 6 pages, 3 figure