Rheological properties of phase transitions in polydisperse and monodisperse colloidal rod systems

Rheological modifiers are added to formulations to tune rheology, enable function and drive phase changes requiring an understanding of material structure and properties. We characterize two colloidal rod systems during phase transitions using multiple particle tracking microrheology, which measures the Brownian motion of probes embedded in a sample. These systems include a colloid (monodisperse polyamide or polydisperse hydrogenated castor oil), surfactant (linear alkylbenzene sulfonate [LAS]), and nonabsorbing polymer (polyethylene oxide [PEO]) which drives gelation by depletion interactions. Phase transitions are characterized at all concentrations using time-cure superposition. We determine that rheological evolution depends on LAS:colloid. The critical PEO concentration required to form a gel, c_c/c*, is independent of LAS:colloid, critical relaxation exponent, n, is dependent on LAS:colloid, and both are independent of colloid polydispersity. n indicates the material structure at the phase transition. At LAS:colloid > 16, the scaffold is a tightly associated network and at LAS:colloid = 16 a loosely associated network.