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

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