Ronald Blaak

Crystal Nucleation of Colloidal Suspensions under Shear

We use Brownian dynamics simulations in combination with the umbrella sampling technique to study the effect of shear flow on homogeneous crystal nucleation. We find that a homogeneous shear rate leads to a significant suppression of the crystal nucleation rate and to an increase of the size of the critical nucleus. A simple, phenomenological extension of classical nucleation theory accounts for these observations. The orientation of the crystal nucleus is tilted with respect to the shear direction.

Telechelic Star Polymers as Self-Assembling Units from the Molecular to the Macroscopic Scale

By means of multiscale molecular simulations, we show that telechelic-star polymers are a simple, robust, and tunable system, which hierarchically self-assembles into soft-patchy particles and mechanically stabilizes selected, open crystalline structures. The self-aggregating patchy behavior can be fully controlled by the number of arms per star and by the fraction of attractive monomeric units at the free ends of the arms. Such self-assembled soft-patchy particles while forming, upon augmenting density, gel-like percolating networks, preserve properties as particle size, number, and arrangement of patches per particle. In particular, we demonstrate that the flexibility inherent in the soft-patchy particles brings forward a novel mechanism that leads to the mechanical stability of diamond and simple cubic crystals over a wide range of densities, and for molecular sizes ranging from about 10 nm up to the micrometer scale.

Lattice dependence of reaction-diffusion in lattice Boltzmann modeling

We consider the influence of the lattice symmetry and size of the lattice Boltzmann method on the behavior of a pure reaction diffusion system. We show that the effect of the dispersion relation in the diffusion coefficient can be minimized, by tuning the fraction of rest particles and the relaxation parameter. For the reaction, we focus on the Selkov model and study the dynamics of pattern formation due to the Turing Instability. For the chosen reaction parameters, however, no clear influence of the lattice symmetry is found.

Charge-induced conformational changes of dendrimers

We study the effect of chargeable monomers on the conformation of dendrimers of low generation by computer simulations, employing bare Coulomb interactions. The presence of the latter leads to an increase in size of the dendrimer due to a combined effect of electrostatic repulsion and the presence of counterions within the dendrimer and also enhances a shell-like structure for the monomers of different generations. In the resulting structures the bond length between monomers, especially near the center, will increase to facilitate a more effective usage of space in the outer regions of the dendrimer.

Do cylinders exhibit a cubatic phase

We investigate the possibility that freely rotating cylinders with an aspect ratio L/D=0.9 exhibit a cubatic phase similar to the one found for a system of cut spheres. We present theoretical arguments why a cubatic phase might occur in this particular system. Monte Carlo simulations do not confirm the existence of a cubatic phase for cylinders. However, they do reveal an unexpected phase behavior between the isotropic and crystalline phase.

Dynamical arrest in low density dipolar colloidal gels

We report the results of extensive molecular dynamics simulations of a simple, but experimentally achievable model of dipolar colloids. It is shown that a modest elongation of the particles and dipoles to make dipolar dumbbells favors branching of the dipolar strings that are routinely observed for point dipolar spheres (e.g., ferrofluids). This branching triggers the formation of a percolating transient network when the effective temperature is lowered along low packing fraction isochores (phi<0.1). Well below the percolation temperature the evolution of various dynamical correlation functions becomes arrested over a rapidly increasing period of time, indicating that a gel has formed. The onset of arrest is closely linked to ongoing structural and topological changes, which we monitor using a variety of diagnostics, including the Euler characteristic. The present system, dominated by long-range interactions between particles, shows similarities to, but also some significant differences from the behavior of previously studied model systems involving short-range attractive interactions between colloids. In particular, we discuss the relation of gel formation to fluid-fluid phase separation and spinodal decomposition in the light of current knowledge of dipolar fluid phase diagrams.

The effects of pH, salt and bond stiffness on charged dendrimers

We have performed molecular dynamics simulations of charged dendrimers with various charge distributions, and including both rigid and soft bonds between the monomers. Whereas the rigid bonds result in a shell-like structure, the soft bonds lead to a larger dendrimer size and a more homogeneous monomer distribution. The measured density profiles of counter-ions and co-ions are compared with those stemming from Poisson-Boltzmann theory. The latter is in very good agreement with simulations for the soft-bond model, whereas for rigid bonds, significant discrepancies arise caused by the fact that Poisson-Boltzmann theory neglects finite-size ion effects. The addition of monovalent salt has no significant influence on the behavior of the dendrimers.

Computer simulations of polyelectrolyte stars and brushes

We briefly review results pertaining to the conformations, interactions and phase behavior of two related soft matter systems: star-branched polyelectrolytes and spherical polyelectrolyte brushes. Moreover, we present new results on the complexation of stars with oppositely charged, spherical, hard colloids, demonstrating the versatility of these systems to form novel complexes that result in a variety of patchy colloids, whose morphology can be affected by small amounts of added salt. Finally, we demonstrate that spherical polyelectrolyte brushes with low grafting density have distinct characteristics from dense brushes as regards the condensation of counterions. Here, condensation of counterions takes place along the rods and the brush conformation seems to be much more robust to added salt than that of star polyelectrolytes. (Some figures in this article are in colour only in the electronic version)

Reversible gelation and dynamical arrest of dipolar colloids

We use molecular dynamics simulations of a simple model to show that dispersions of slightly elongated colloidal particles with long-range dipolar interactions, like ferrofluids, can form a physical (reversible) gel at low volume fractions. On cooling, the particles first self-assemble into a transient percolating network of cross-linked chains, which, at much lower temperatures, then undergoes a kinetic transition to a dynamically arrested state with broken ergodicity. This transition from a transient to a frozen gel is characterised by dynamical signatures reminiscent of jamming in much denser dispersions.

Microscopically Resolved Simulations Prove the Existence of Soft Cluster Crystals

We perform extensive monomer-resolved computer simulations of suitably designed amphiphilic dendritic macromolecules over a broad range of densities, proving the existence and stability of cluster crystals formed in these systems, as predicted previously on the basis of effective pair potentials [B. M. Mladek et al., Phys. Rev. Lett. 96, 045701 (2006)]. Key properties of these crystals, such as the adjustment of their site occupancy with density and the possibility to heal defects by dendrimer migration, are confirmed on the monomer-resolved picture. At the same time, important differences from the predictions of the pair potential picture, stemming from steric crowding, arise as well, and they place an upper limit in the density for which such crystals can exist.