Ordering of sedimenting paramagnetic colloids in a monolayer.

Sedimentation enables self-assembly of colloidal particles into crystalline structures, as needed for catalysis or photonics applications. Here we combine experiments, theory, and simulations to investigate the equilibrium structure of a colloidal monolayer with tunable interparticle repulsion via an applied external magnetic field. Experimental observations of the equilibrium structure are in excellent agreement with density functional theory. Within a (zero-temperature) local density approximation, we derive a simple analytical expression that quantitatively captures the inhomogeneous ordering ranging from solid to liquidlike states. Monte Carlo simulations corroborate these findings and explore an even wider range of sedimentation conditions, thus providing a global view of the sedimentation-mediated ordering in colloidal monolayers with tunable long-ranged interparticle repulsions. Our findings shed further light on the classical sedimentation problem in colloidal science and related areas.

Dipolar Crystals: The Crucial Role of the Clinohexagonal Prism Phase.

We report a new phase called clinohexagonal prism (CHP) that accounts for all the ground states of dipolar hard spheres prepared at any density. This phase merely consists of an oblique prismatic lattice with a hexagonal base. Our calculations show that at intermediate densities, a special close packed body-centered orthorhombic phase coincides with the CHP phase in the ground state for a wide density window. In the high packing regime, i.e., in the vicinity of the density of the hexagonal close packed phase, it is a limiting case of the CHP phase with vanishing obliquity that emerges. These findings provide a unified and clarified view of the solid-solid transitions occurring at zero temperature in dipolar systems and should be relevant in other related molecular or soft matter systems governed by anisotropic (and possibly isotropic) soft potentials.

On the interaction of dipolar filaments.

The interactions of dipolar filaments such as magnetic needles and chains in strong homogeneous magnetic/electric field are investigated theoretically. Revisiting the case of uniformly magnetized/polarized parallel needles of finite size L and separated by a distance R , all the relevant regimes of attraction and/or repulsion are properly addressed and discussed. At short inter-needle separation ( R/L ≲ 0.2, the repuive pair potential of two facing needles is governed by R(-1) in strong contrast with R(-3) at long separations (R/L ≳ 2.5). This softening is attributed to an efficient long-range screening owing to the relatively long needle extension in this regime. This whole understanding of dipolar needles effective interaction is then used to grasp that of dipolar chains made up of spherical dipolar beads. When excluded-volume correlations are weak (i.e., the chains are a few beads apart), chains and needles possess virtually the same effective interaction. However, at short separation there is a remarkable hardening upon approaching two chains in registry in qualitative contrast to the needles case.