Interfacial colloidal sedimentation equilibrium. I. Intensity based confocal microscopy

Abstract

This paper reports confocal microscopy measurements of inhomogeneous colloidal sedimentation equilibrium profiles near planar wall surfaces for conditions when colloid dimensions are comparable to the characteristic gravitational length scale. The intensity based confocal method developed in this work enables real-space measurements of one-dimensional density profiles of Brownian colloids without identifying many single colloid centers in large imaging volumes. Measured sedimentation equilibrium profiles for single-phase interfacial fluids and for coexisting inhomogeneous fluid and solid phases are in agreement with a perturbation theory and Monte Carlo simulations within the local density approximation. Monte Carlo simulated colloid scale density profiles display some minor differences with confocal images in terms of microstructural transitions involving the onset of interfacial crystallization and the precise elevation of the fluid-solid interface. These discrepancies are attributed to polydispersity unaccounted for in the analyses, sensitivity of the perturbation theory to the effective hard sphere size, and the influence of ensemble, system size, and box shape in Monte Carlo simulations involving anisotropic/inhomogeneous solids. Successful demonstration of intensity based confocal microscopy provides a basis for future measurements of three-dimensional colloidal interactions, dynamics, and structure near surfaces.

Opals: Status and Prospects

“…The changes in the suspension (step 1 in Figure 11) clearly occur in field‐induced deposition methods, whereas they are much less important in the other deposition methods. The presence of gravity (or other external fields) results in a density profile in one direction, which can be measured by using light scattering or confocal microscopy 84. The magnitude of the gravitational field is usually described by the Péclet number that relates the gravitationally induced drift to the thermal diffusion and indicates which effect dominates 82.…”

A qualitative confocal microscopy study on a range of colloidal processes by simulating microgravity conditions through slow rotations

“…For this, the density profile should be measured as a function of height. [9][10][11][12] Recent work has shown that this can be obtained quite effectively from real-space microscopy techniques. 13 However, care has to be taken in the case of systems with long-range electrostatic interactions between the colloids, as the effect of the double layer complicates this scheme.…”

Experimental observation of the crystallization of hard-sphere colloidal particles by sedimentation onto flat and patterned surfaces

“…Questions remain not only about dynamics, but also about the equilibrium states. These questions have been addressed by a number of experiments and computer simulations: x-ray attenuation experiments, [28] particle-scale studies by confocal microscopy, [31,32] DFT calculations, [33] simulations of a fixed number of hard sphere particles in an increasing gravitational field, [29] and, most recently, a grand canonical Monte Carlo study that incorporated experimentally more accessible conditions (constant gravitation). Particularily interesting are the early stages of sedimentation and the crystallization of the very first layers.…”