The nature of anisotropic particles with short - range interactions

Anisotropic particles are used widely to control the thermodynamic and rheological properties of pharmaceuticals, cosmetics, food products, composite materials, and cements. In particular, rod-like particle suspensions exhibit unique equilibrium and non-equilibrium states at low particle loadings due to large excluded volumes and orientation-dependent attractions. However, the coupled effects of particle shape and interactions are not quantitatively understood with regard to dynamically arrested states, such as colloidal gels and glasses. To better quantify these coupled effects, a tunable model system of adhesive hard rods (AHR) was synthesized to independently control the particle aspect ratio and the temperature-dependent, short-range attractions. AHR suspensions were composed of octadecyl-coated silica rods suspended in tetradecane, which displayed thermoreversible dynamic arrest transitions. The dynamic moduli and particle dynamics were quantified systematically using macroscopic rheological methods and microscopic quasi-elastic light scattering methods, providing a quantitative map of the gel and glass boundaries as a function of aspect ratio (L/D 3 to 7), particle volume fraction (φ 0.1 to 0.5), and absolute critical temperatures (Tgel 25 °C to 30 °C). Small-angle neutron and X-ray scattering methods probed the arrested AHR microstructure to extract an effective interaction strength between rods on a reduced temperature scale, as defined by a dimensionless sticky parameter, τeff.