Dynamical simulation of dipolar Janus colloids: Equilibrium structure and thermodynamics

The static microstructures and thermodynamics of a colloidal dispersion of dipolar Janus (DJ) particles-that is, dipolar spheres in which each hemisphere is specified by a different charge interaction-have been investigated through simulation. DJ particles are modeled at a high level of detail with pairwise potentials represented as a sum of a spherically symmetric soft repulsion and an orientation-dependent electrostatic component using continuous potentials. The latter is important because it allows for the use of conventional molecular dynamics simulations, and is in contrast to the patch model and dipolar hard sphere model, which are discontinuous and therefore do not. The electrostatics are represented through a rigorous pointwise (PW) covering of two different hemispheres filled by points of corresponding charge. An isotropic coarse-graining (CG) of the PW models serves as a limit of the structure wherein the orientations of the DJ particles can be pairwise averaged. Over the range of volume fractions and DJ charge densities studied-consistent with reversible structures absent of long-range correlations-the CG model agrees well with the PW model with respect to equilibrium structure (isotropic pair correlation) and ensemble free energy. Time-dependent relaxation simulations of the PW model suggest that chain structures are not expected in liquid phases in contrast to that which has been observed for point dipole models of simple polar fluids. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737432]