(123b) Interfacial Colloidal Sedimentation Equilibrium Microstructures

To intelligently design, control, and optimize self-assembly of three-dimensional colloidal structures on substrates (e.g. photonic crystals), it is necessary to understand how interactions between colloids, surfaces, and external fields lead to various interfacial microstructures. In this talk, we present confocal microscopy measurements and statistical mechanical analyses of the sedimentation equilibrium of inhomogeneous colloidal fluids coexisting with crystals above macroscopic planar surfaces. In contrast to most previous sedimentation equilibrium studies, the present work focuses on relatively compact microstructures over 10-100 particle diameters due to the similarity of particle dimensions and the characteristic gravitational lengthscale (mgd/kT°Ö1). A new method is reported to obtain sedimentation equilibrium profiles of sub-micron fluorescent silica core-shell colloids by deconvoluting intensity profiles as density distributions without locating particle centers. This approach overcomes limitations on accurately locating very large numbers of 3D particle centers in the presence of significant Brownian motion, but still allows real-space visualization of equilibrium fluid and crystal phase microstructures. Inverse analyses of the resulting sedimentation equilibrium profiles using perturbation theory (based on the local density approximation), Monte Carlo, and a new density functional theory formulation are used to extract particle pair potentials in excellent agreement with independent measurements.