Size dependence of microprobe dynamics during gelation of a discotic
colloidal clay
By
Jason P. Rich, Gareth H. McKinley, Patrick S. Doyle
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 phenomena has recently been
demonstrated for a gel-forming aqueous dispersion of Laponite®
clay [Oppong et al. (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®
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® concentrations exhibit a self-similar
dependence on probe size, suggesting a superposition of Laponite® concentration
and probe size. From these observations, we propose an accordant description of
the microstructural evolution of the gel.