Tunable effective interactions between dendritic macromolecules.

We employ extensive Monte Carlo and molecular-dynamics simulations to investigate the effective interactions between the centers of mass of dendritic macromolecules of variable flexibility and generation number. Two different models for the connectivity and steric interactions between the monomers are employed, the first one being purely entropic in nature and the second explicitly involving energetic interactions. We find that the effective potentials have a generic Gaussian shape, whose range and strength can be tuned via modifications in the generation number and flexibility of the spacers. We supplement our simulation analysis by a density-functional approach in which the connectivity between the monomers is approximated by an external confining potential that holds the monomer beads together. Using a simple density functional for the interactions between the monomers, we find semiquantitative agreement between theory and simulation. The implications of our findings for the interpretation of scattering data from concentrated dendrimer solutions are also discussed.

Azimuthal frustration and bundling in columnar DNA aggregates.

The interaction between two stiff parallel DNA molecules is discussed using linear Debye-Hückel screening theory with and without inclusion of the dielectric discontinuity at the DNA surface, taking into account the helical symmetry of DNA. The pair potential furthermore includes the amount and distribution of counterions adsorbed on the DNA surface. The interaction does not only depend on the interaxial separation of two DNA molecules, but also on their azimuthal orientation. The optimal mutual azimuthal angle is a function of the DNA-DNA interaxial separation, which leads to azimuthal frustrations in an aggregate. On the basis of the pair potential, the positional and orientational order in columnar B-DNA assemblies in solution is investigated. Phase diagrams are calculated using lattice sums supplemented with the entropic contributions of the counterions in solution. A variety of positionally and azimuthally ordered phases and bundling transitions is predicted, which strongly depend on the counterion adsorption patterns.

Phase behavior of columnar DNA assemblies.

The interaction between two stiff parallel DNA molecules depends not only on the distance between their axes but also on their azimuthal orientation. The positional and orientational order in columnar B-DNA assemblies in solution is investigated, on the basis of the electrostatic pair potential that takes into account DNA helical symmetry and the amount and distribution of adsorbed counterions. A phase diagram obtained by lattice sums predicts a variety of positionally and azimuthally ordered phases and bundling transitions strongly depending on the counterion adsorption patterns.

Decoration lattices of colloids adsorbed on stripe-patterned substrates.

The equilibrium structure of decoration lattices composed of colloidal particles adsorbed on periodic stripe-patterned substrates is calculated as a function of the stripe width and separation and for different interparticle interactions. Due to a competition of length scales, a wealth of different stable decoration lattices occurs such as triangular, quadratic, rhombic, kitelike, and sheared honeycomb lattices, triangular slices as well as triangle superlattices. This is of relevance for constructing templates that enforce crystal growth of unusual solid structures.

Sedimentation profiles of systems with reentrant melting behavior.

We examine sedimentation density profiles of star polymer solutions as an example of colloidal systems in sedimentation equilibrium that exhibit reentrant melting in their bulk phase diagram. Phase transitions between a fluid and a fluid with an intercalated solid are observed below a critical gravitational strength alpha*. Characteristics of the two fluid-solid interfaces in the density profiles occurring in Monte Carlo simulations for alpha