Direct visualization of flow-induced microstructure in dense colloidal gels by confocal laser scanning microscopy

Unconstrained uniaxial compression (or squeeze flow) of high volume fraction gels of fluorescent silica particles of diameter 832 nm results in the formation of voids (at phi = 0.26) and cracks (at phi = 0.40) that are of scale 10-100 mum. This evidence of inhomogeneous material deformation was obtained by direction visualization of three-dimensional structure by confocal laser scanning microscopy. Flow-induced void and crack microstructures are quantified by locating all particle centroids with quantitative image processing, by performing Voronoi volume tessellation with computational geometry and by analyzing the particle number density fluctuations as a function of averaging volume. Average short-range real space structural measures, such as the pair correlation function, are little changed by the flow. However, the probability distribution of excess normalized Voronoi polyhedra volume is profoundly extended by squeeze flow, particularly at large polyhedra volumes. Comparison of the Voronoi polyhedra volume distributions and particle bond distributions. indicates that: (1) in the low 0 gel, large flow-induced voids are formed by the reorganization of the existing quiescent voids without significant effect on the local structure; and (2) in the high 0 gel, cracks are formed by reorganization of the local structure itself. Analysis of the number density. fluctuations shows that the gels respond to applied squeeze flow deformation with structural distortion on the length scale of 5-10 particle diameters. (C) 2003 The Society of Rheology.