Abstract
In this work, we use structure and dynamics in sedimentation equilibrium, in the presence of gravity, to examine, via confocal microscopy, a Brownian colloidal system in the presence of an external electric field. The zero field equation of state (EOS) is hard sphere without any re-scaling of particle size, and the hydrodynamic corrections to the long-time self-diffusion coefficient are quantitatively consistent with the expected value for hard spheres. Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number Peg, and dipolar strength Λ are of order unity. In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength Λ ~ 1. The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes.
Highlights
Particle-solvent density mismatch that is typical in many microscopy experiments have Peg ~ O(1), and gravity can often not be neglected
Gelation and glassy behaviour have been observed in systems with a combination of short-range attractions in addition to repulsions, resulting in different rheological properties[19]
Our system is index matched and density matched for confocal microscopy experiments, and we show it to be an excellent candidate for true hard-sphere-plus-dipolar interactions
Summary
Particle-solvent density mismatch that is typical in many microscopy experiments have Peg ~ O(1), and gravity can often not be neglected. Stable cluster states have been identified in equilibrium colloidal systems with competing interactions on different lengthscales[4,20]. Understanding these cluster states remains a problem of fundamental interest. Our system is index matched and density matched for confocal microscopy experiments, and we show it to be an excellent candidate for true hard-sphere-plus-dipolar interactions. With colloidal spheres of diameter 2a = 0.8 and 1.0 μm-diameter in a nearly density-matched solvent mixture, our system, with Peg ~ O(1) is as strongly Brownian as possible while still allowing the particles to be just large enough for microscopy. We first demonstrate that our Brownian colloidal system has interactions that are hard-sphere-plus-dipolar. We examine structures and dynamics of clusters in sedimentation equilibrium, and characterize the dependence of cluster size and dynamics as a function of height, for two particle sizes
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