Diffusion and Relaxation Dynamics in Cluster Crystals

For a large class of fluids exhibiting ultrasoft bounded pair potentials, particles form crystals consisting of clusters located in the lattice sites, with a density-independent lattice constant. Here we present an investigation on the dynamic features of a representative example of this class. It is found that particles can diffuse between lattice sites, maintaining the lattice structure, through an activated hopping mechanism. This feature yields finite values for the diffusivity and full relaxation of density correlation functions. Simulations suggest the existence of a localization transition which is avoided by hopping, and a dynamic decoupling between self- and collective correlations.Comment: 4 pages, 7 figure

Multiblob coarse-graining for mixtures of long polymers and soft colloids

Soft nanocomposites represent both a theoretical and an experimental challenge due to the high number of the microscopic constituents that strongly influence the behaviour of the systems. An effective theoretical description of such systems invokes a reduction of the degrees of freedom to be analysed, hence requiring the introduction of an efficient, quantitative, coarse-grained description. We here report on a novel coarse graining approach based on a set of transferable potentials that quantitatively reproduces properties of mixtures of linear and star-shaped homopolymeric nanocomposites. By renormalizing groups of monomers into a single effective potential between a $f$-functional star polymer and an homopolymer of length $N_0$, and through a scaling argument, it will be shown how a substantial reduction of the to degrees of freedom allows for a full quantitative description of the system. Our methodology is tested upon full monomer simulations for systems of different molecular weight, proving its full predictive potential

Soft self-assembled nanoparticles with temperature-dependent properties

The fabrication of versatile building blocks that are reliably self-assemble into desired ordered and disordered phases is amongst the hottest topics in contemporary material science. To this end, microscopic units of varying complexity, aimed at assembling the target phases, have been thought, designed, investigated and built. Such a path usually requires laborious fabrication techniques, especially when a specific funcionalisation of the building blocks is required. Telechelic star polymers, i.e., star polymers made of a number $f$ of di-block copolymers consisting of solvophobic and solvophilic monomers grafted on a central anchoring point, spontaneously self-assemble into soft patchy particles featuring attractive spots (patches) on the surface. Here we show that the tunability of such a system can be widely extended by controlling the physical and chemical parameters of the solution. Indeed, at fixed external conditions the self-assembly behaviour depends only on the number of arms and/or on the ratio of solvophobic to solvophilic monomers. However, changes in temperature and/or solvent quality makes it possible to reliably change the number and size of the attractive patches. This allows to steer the mesoscopic self-assembly behaviour without modifying the microscopic constituents. Interestingly, we also demonstrate that diverse combinations of the parameters can generate stars with the same number of patches but different radial and angular stiffness. This mechanism could provide a neat way of further fine-tuning the elastic properties of the supramolecular network without changing its topology.Comment: 8 pages, 7 figures. Submitted to Nanoscal

Crystallization of magnetic dipolar monolayers: a density functional approach

We employ density functional theory to study in detail the crystallization of super-paramagnetic particles in two dimensions under the influence of an external magnetic field that lies perpendicular to the confining plane. The field induces non-fluctuating magnetic dipoles on the particles, resulting into an interparticle interaction that scales as the inverse cube of the distance separating them. In line with previous findings for long-range interactions in three spatial dimensions, we find that explicit inclusion of liquid-state structural information on the {\it triplet} correlations is crucial to yield theoretical predictions that agree quantitatively with experiment. A non-perturbative treatment is superior to the oft-employed functional Taylor expansions, truncated at second or third order. We go beyond the usual Gaussian parametrization of the density site-orbitals by performing free minimizations with respect to both the shape and the normalization of the profiles, allowing for finite defect concentrations.Comment: 23 pages, 18 figure

Effect of bending rigidity on the knotting of a polymer under tension

A coarse-grained computational model is used to investigate how the bending rigidity of a polymer under tension affects the formation of a trefoil knot. Thermodynamic integration techniques are applied to demonstrate that the free-energy cost of forming a knot has a minimum at non-zero bending rigidity. The position of the minimum exhibits a power-law dependence on the applied tension. For knotted polymers with non-uniform bending rigidity, the knots preferentially localize in the region with a bending rigidity that minimizes the free-energy.Comment: 15 pages, 6 figures. Corrected problem with references to equation