(450h) Simulations of the Phases and Structures Formed from Field-Induced or Permanently Polarized Off-Center Dipolar Colloids

Colloidal particles with anisotropic charge distributions hold promise for serving as the building blocks for a broad range of new materials including photonic materials with novel symmetries and electrical materials for information storage. Of particular interest are systems of particles whose interactions can be induced and controlled through external magnetic and/or electric field(s). We modeled the assembly of systems of off-center extended dipolar colloidal spheres using quasi-2D Monte Carlo simulations in both the presence and absence of an external field. We find that these asymmetric colloids assemble into domains with a number of unusual symmetries whose phase behavior can be controlled by changing the temperature. The dipolar particles in our simulations are modeled as a hard spheres with two smaller, embedded spheres representing the positive and negative charges on an extended dipole. Since we use an extended dipole model (instead of the typical point dipole model) we are able to investigate how the angle between the charges and the center of the colloidal particle affects their aggregation behavior. We find that the particles typically aggregate into cyclical structures whose interior angle is equal to the angle between the charges on a single dipolar sphere. For example, systems of particles with an angle of 90° typically group together to form square aggregates composed of four particles. Cyclical structures of up to 8 particles have been formed while larger structures typically form chains. When placed in a strong external field these particles instead line up in a staggered chain configuration which has been found in experiments on magnetic Janus spheres in the presence of an external magnetic field. To quantify the differences between the systems in and outside of a field we have measured both the cluster size distribution and the percolation probability and found that systems within a field percolate at lower area fractions and at higher temperatures than their out-of-field counterparts. This difference could allow for precise control of the rheological properties of fluids containing such particles.