Impact of shear on the structure and rheological properties of a hydrogenated castor oil colloidal gel during dynamic phase transitions

Rheological modifiers are subject to processing steps, including mixing and dilution, that can have permanent structural effects. This work investigates rheological changes of a fibrous colloid, hydrogenated castor oil (HCO), when sheared during sample preparation. HCO is a polydisperse system that undergoes phase transitions in response to osmotic pressure gradients. Two HCO materials are characterized during phase transitions, a nonsheared 4 wt. % gel and a presheared 0.125 wt. % solution. Material properties are measured using multiple particle tracking (MPT) microrheology, μ²rheology, the combination of microfluidics and MPT, and bulk rheology. MPT quantitatively determines the critical relaxation exponent, n, which is constant for a material. MPT determines n is dependent on the starting HCO material, indicating that preshear has changed the structure. μ²rheology identifies consistent equilibrium states during consecutive phase transitions. Bulk rheology determines that the nonsheared gel does not completely degrade into a sol, indicated by no G′ and G″ crossover. The presheared material has a crossover indicating a sol-gel transition. The phases of HCO are identified by comparison of rheological data to previous work by Wilkins et al. [Langmuir 25, 8951–8959 (2009)], who determined the structure of a similar colloidal fiber system using confocal microscopy. The equilibrium moduli at the completion of both experiments are similar and indicate that the scaffold is in a transitional phase. These three techniques give consistent measurements of the rheological properties, and indicate the structure of the scaffold by comparison to previous works. During degradation, nonsheared HCO gels change from entangled networks to a transitional state with fiber entanglement. During gelation, presheared HCO solutions transition from bundles in solution to an associated network of bundles with few entanglements. These measurements confirm that shear history can permanently change rheological properties, affecting the scaffolds applications.