We introduce an ultrasoft core model of interpenetrating polycations and polyanions, with continuous Gaussian charge distributions, to investigate polyelectrolyte aggregation in dilute and semi-dilute salt-free solutions. The model is studied by a combination of approximate theories (random phase approximation and hypernetted chain theory) and numerical simulations. The calculated pair structure, thermodynamics, phase diagram, and polyion dynamics of the symmetric version of the model (the "ultrasoft restricted primitive model" or UPRM) differ from the corresponding properties of the widely studied "restricted primitive model" (RPM) where ions have hard cores. At sufficiently low temperatures and densities, oppositely charged polyions form weakly interacting, polarizable neutral pairs. The clustering probabilities, dielectric behavior, and electrical conductivity point to a line of sharp conductor-insulator transitions in the density-temperature plane. At very low temperatures, the conductor-insulator transition line terminates near the top of a first order coexistence curve separating a high-density liquid phase from a low-density vapor phase. The simulation data hint at a tricritical behavior, reminiscent of that observed for the two-dimensional Coulomb gas, which contrasts with the Ising criticality of its three-dimensional counterpart, the RPM.

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 essentially unaffected by the motion of trapped particles: close to the percolation transition, we determined z similar or equal to 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-localization 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.

Part of Special section on complex dynamics of fluids in disordered and crowded environmentsOver the past two decades, the dynamics of fluids under nanoscale confinement has attracted much attention. Motivation for this rapidly increasing interest is based on both practical and fundamental reasons. On the practical and rather applied side, problems in a wide range of scientific topics, such as polymer and colloidal sciences, rheology, geology, or biophysics, benefit from a profound understanding of the dynamical behaviour of confined fluids. Further, effects similar to those observed in confinement are expected in fluids whose constituents have strong size or mass asymmetry, and in biological systems where crowding and obstruction phenomena in the cytosol are responsible for clear separations of time scales for macromolecular transport in the cell. In fundamental research, on the other hand, the interest focuses on the complex interplay between confinement and structural relaxation, which is responsible for the emergence of new phenomena in the dynamics of the system: in confinement, geometric constraints associated with the pore shape are imposed to the adsorbed fluids and an additional characteristic length scale, i.e. the pore size, comes into play.For many years, the topic has been mostly experimentally driven. Indeed, a broad spectrum of systems has been investigated by sophisticated experimental techniques, while theoretical and simulation studies were rather scarce due to conceptual and computational issues. In the past few years, however, theory and simulations could largely catch up with experiments. On one side, new theories have been put forward that duly take into account the porosity, the connectivity, and the randomness of the confinement. On the other side, the ever increasing available computational power now allows investigations that were far out of reach a few years ago. Nowadays, instead of isolated state points, systematic investigations on the dynamics of confined fluids, covering a wide range of system parameters, can be realized. In fact, theory and simulations were recently able to predict new and surprising dynamical features, such as the occurrence of sub-diffusive laws, which result from the trapping due to the geometric and topological constraints and/or quenched disorder, the presence of both continuous and discontinuous glass transitions, and diffusion-localization transitions. Together, theory and simulations are thus able to contribute to a deeper insight into the complex dynamical behaviour of fluids in disordered confinement. Still, many yet unsolved problems remain.The fact that theoretical and simulation approaches have caught up with experimental investigations, has motivated us to organize a workshop on the dynamics of fluids confined in disordered environments, so as to bring together the different communities working in this field: theory and simulations, with their recent developments based on the mode-coupling theory of the glass transition, and experiments, with particular emphasis on colloidal systems and novel techniques. In an effort to give credit to recent developments in related problems of biophysical relevance, an entire session of the programme was dedicated to anomalous diffusion in crowded environments. The workshop was thus aimed at providing a deeper understanding of the complex dynamics of fluids in confinement as well as up-to-date perspectives on the interdisciplinary applications of this field of research.We are proud to say that all 32 contacted speakers accepted our invitation. Additional participants were attracted by our scientific programme, contributing poster presentations to the workshop. In total, close to 50 participants were registered, arriving from 11 different countries (including the US, Japan, and Mexico). Thus we conclude that the workshop indeed addressed a highly topical scientific field. From the scientific point of view a broad range of problems was covered, ranging from biophysics over soft matter to fermion systems. From the vivid discussions it became evident that the close scientific contact between theory, simulation and experiment brought along a fruitful and mutually inspiring atmosphere. On the theoretical side, these discussions have allowed clarification of several connections between the dynamics of models of fluids in porous media (quenched-annealed, pinned particles models), those of well-known limiting cases (Lorentz gas), of realistic models of liquids with strong dynamic asymmetry (asymmetric size and mass mixtures, sodium silicates, polymers blends) and even of bulk glass-formers. On the experimental side, it appeared that soft matter systems may provide an excellent test-bed to verify the theoretical predictions. From the concluding discussion it was also clear that addressing related issues relevant to biology still remains an open challenge for the future. In view of all this, it was concluded that within a short time period a workshop with analogous scope should be organized to address the progress made on both fundamental and interdisciplinary aspects.The realization of this workshop was made possible by generous financial support from CECAM, Centre Blaise Pascal-ENS de Lyon, and the ESF network 'Molecular Simulations in Biosystems and Material Science' (SimBioMa).

The influence of static correlations beyond the pair level on the dynamics of selected model glass formers is investigated. The pair structure, angular distribution functions, and statistics of Voronoi polyhedra of two well-known Lennard-Jones mixtures as well as of the corresponding Weeks-Chandler-Andersen variants, in which the attractive part of the potential is truncated, are compared. By means of the Voronoi construction, the atomic arrangements corresponding to the locally preferred structures of the models are identified. It is found that the growth of domains formed by interconnected locally preferred structures signals the onset of the slow-dynamics regime and allows the rationalization of the different dynamic behaviors of the models. At low temperature, the spatial extension of the structurally correlated domains, evaluated at fixed relaxation time, increases with the fragility of the models and is systematically reduced by truncating the attractions. In view of these results, proper inclusion of many-body static correlations in theories of the glass transition appears crucial for the description of the dynamics of fragile glass formers.

We perform a comparative simulation study of the dynamical properties of ultrasoft particles in cluster phases. We focus on the generalized exponential model (GEM), a prototypical model for ultrasoft colloids, such as dendrimers and microgels. In the GEM of order n, particles interact through the potential u(r) = exp [−(r/σ)^n ], where σ and are length and energy scales, respectively. For n > 2 the GEM displays crystalline phases with multiply-occupied sites, i.e., it forms cluster crystals. We present extensive simulation results on the hopping dynamics of the GEM with n = 4 in the fcc cluster phase and report the first simulation evidence of cluster glasses in a bidisperse version of the model. The dynamics of the model in the fcc cluster phase is investigated through a combination of molecular dynamics (MD), Brownian dynamics (BD) and Monte Carlo (MC) simulations. We study the activated dynamics of particles as they hop from site to site in the cluster structure, and analyze the statistics of jump events. We find that the diffusion mechanism depends sensitively on the microscopic dynamics. In MD simulations particles can jump over several cluster site in a correlated fashion, leading to a broad distribution of jump lengths P_jump (r). In MC and BD simulations, by contrast, particles hop only to nearest neighbor sites. The underlying distributions of jump lengths affect the long time dynamics of the particles, giving rise, for instance, to qualitatively different shapes of the van Hove correlation functions. We attribute these differences to the suppression of momentum correlation in the two stochastic methods. The agreement between MC and BD simulations support the view that MC dynamics effectively incorporate solvent effects in a simulation of model colloids. We then extend our investigations to a binary mixture of particles with n = 4 and size ratio σ_11/σ_22 = 1.4, simulated over a wide range of densities. As the fluid is slowly quenched from high T, we observe the formation of stable clusters, whose centers of mass eventually arrest into a disordered configuration. The location of the cluster glass transition does not depend sensitively on the quench rate and is associated to a fragile-to-strong crossover in the T-dependence of the partial diffusion coefficients. In the cluster glass, particles hop from cluster to cluster. Unlike in cluster crystals, however, long range jumps are suppressed in MD simulations due to the disorder of the underlying cluster structure. We attribute the ability to form amorphous cluster phases to the size bidispersity of the particles. Clusters formed at low T are mostly homo-coordinated, leading to an effective binary mixture of clusters. This, in turn, frustrates crystallization of the clusters' centers of mass. This interpretation is supported by the comparison with a variant of the model with continuous size polydispersity. Our observations thus suggest a viable route to cluster glass formation in a more ample class of colloidal fluids, such as systems with competing interactions.

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.