(535g) Effect of Shear Stress on Rheological Properties of Fibrous Colloidal Gels during Gel-Sol Transitions Induced By Osmotic Pressure Gradients

The manufacturing of creams and colloidal suspensions requires a variety of processing steps including dilutions and mixing that can cause permanent changes to the microstructure of the product. This work investigates the effects of shear, such as from mixing, on the material properties of a colloidal gel after repeated phase transitions. Hydrogenated castor oil (HCO) is a fibrous colloid that is a stable suspension in the presence of a surfactant, and has large aspect ratios ranging from 50-2500. A gel-sol phase transition is induced by an osmotic pressure gradient. Material properties are measured using both bulk rheology and μ²rheology, the combination of microfluidics and multiple particle tracking microrheology (MPT). Experiments are performed with two starting materials, a non-sheared 4 wt% HCO gel and a sheared 0.125 wt% HCO sol. Bulk rheology measures the degradation of the 4 w% gel and determines that osmotic forces are not large enough to completely degrade the gel into a sol. This is evidenced in the data because there is no crossover point between the storage and loss moduli. Gelation of the 0.125 wt% sol does show a crossover point during a change of phase, but the final material properties are a weaker gel than the equilibrated 4wt% sample. In MPT, fluorescent 0.5 μm probe particles are embedded into a sample and video microscopy records their Brownian motion. Particle trajectories are then related to material properties using the Generalized Stokes-Einstein Relation. The microfluidic device used in μ²rheology enables repeated phase transitions on a single sample while minimizing the amount of shear during fluid exchange. μ²rheology results conclude that the material can undergo repeated phase changes without changes to microstructure, but again determine that the material properties are dependent on the shear history of the sample. This work shows the importance of sample handing during processing, and the ways that different processing steps can permanently alter material properties.