Size dependence of microprobe dynamics during gelation of a discotic colloidal clay

oft materials, such as gels and colloidal glasses, often exhibit different rheological properties at ulk and microscopic scales as a result of their complex microstructure. This phenomenon has ecently been demonstrated for a gel-forming aqueous dispersion of Laponite clay Oppong et al. hys. Rev. E 78, 021405 2008 . For this material, microrheology reveals a significantly weaker el and a longer gelation time than bulk measurements. By performing multiple particle tracking icrorheology experiments with different probe sizes, we show that length-scale–dependent heology is a general feature of Laponite gels. Small changes in probe size are accompanied by rder of magnitude differences in the observed rheological properties and gelation time. The probe ynamics also exhibit size-dependent spatial heterogeneities that help to elucidate a microstructural ength scale in the system. Through analytical theory and Brownian dynamics simulations, we find hat the correlations described by previous authors between successive displacements of individual robes are more directly a result of material elasticity than of microstructural confinement. The pparent gelation times of dispersions with different Laponite concentrations exhibit a self-similar ependence on probe size, suggesting a superposition of Laponite concentration and probe size. rom these observations, we propose an accordant description of the microstructural evolution of he gel. © 2011 The Society of Rheology. DOI: 10.1122/1.3532979