Flow-induced microstructural evolution in colloidal particulate gels

This research investigated the effect of applied flow on the microstructure of colloidal gels, formed by short-range attractive interactions, by means of light scattering and confocal laser scanning microscopy. The quiescent microstructure of dilute gels (0.01 < &phis; < 0.10) was consistent with that of fractal clusters, and the size of these clusters was dependent on volume fraction. Flow light scattering experiments revealed a profound effect of applied shear on the long-range structure. Upon application of shear, cluster densification, expansion and anisotropy in the flow-vorticity plane were observed.; The microstructure of dense colloidal gels (0.25 < &phis; < 0.50) was directly visualized and quantified by means of confocal laser scanning microscopy. The short-range gel structure was similar to that of dense amorphous hard sphere systems. Quantification of the volume fraction dependence of the gel isothermal compressibility and Voronoi volume distribution established that long-range structural heterogeneity, characterized by voids and density correlations, is a distinguishing feature of the static structure of dense colloidal gels formed by short-range attractions. The application of unconstrained uniaxial compression or squeeze flow was found to result in the growth of voids and cracks in dense gels. While average structural measures, such as the pair correlation function were unaffected, distribution of structural measures such as Voronoi polyhedra volume distributions were profoundly affected by flow. In particular, the squeeze flow resulted in an extension of the Voronoi polyhedra volume distribution to significantly greater magnitudes, consistent with flow-induced microstructural heterogeneity that is manifested as the observed voids and cracks. Based on these observations, it is evident that the long range structural features such as clusters and voids play a predominant role in determining the rheological response of colloidal gels.