(230c) Using ?2rheology to Measure Rheological Properties during Repeated Phase Transitions of Hydrogenated Castor Oil

Rheological modifiers are often used as ingredients in home care goods. These materials undergo phase changes during manufacturing and regular use. Rheological and material properties can be altered by repeated phase transitions, and it is important to know the extent to which microstructural changes affect the performance of the end use product. In this work, we investigate a hydrogenated castor oil (HCO) gel, a rod-like colloid (aspect ratio 50-2500) that forms a stable aqueous suspension with the addition of a surfactant and undergoes a phase transition due to an osmotic pressure gradient. μ²rheology, the combination of microfluidics and multiple particle tracking microrheology (MPT), is used to measure material properties during transition. A new microfluidic device design enables the exchange of fluids while minimizing shear stress on the sample. In MPT, fluorescent probe particles are embedded in the sample and their Brownian motion is recorded using video microscopy. The particle trajectories are then related to rheological properties using the Generalized Stokes-Einstein Relation. MPT data are compared to phases previously determined by Wilkins et. al., 2009. The phases of rod-like systems are network, where the particles have a sample spanning structure; a bundled phase where rods do not create a network; and a transitional phase where the colloidal particles partially lose scaffold structure and begin forming bundles. MPT results show that a 4wt% HCO gel undergoes a repeatable phase transition between the network and transitional phases due to the osmotic gradient, but never reaches the bundled phase. A 0.125wt% solution of colloidal rods that was prepared using a large amount of shear undergoes repeatable phase transitions but does not form the network structure measured in the 4wt% gel. This is due to the shear added during sample preparation permanently breaking the network structure. This work shows that sample preparation can have irreversible effects on the microstructure of the gels, which can cause changes in end use products made with rheological modifiers.