(414a) Using ?2rheology to Characterize Consecutive Phase Transitions in a Hydrogenated Castor Oil Colloidal Gel

The gelation and degradation of colloidal particles is of importance in the design of these materials for use in commercial products, such as consumer and home care products. Of concern in the design is whether the gradient that induces phase change can overcome any processing history, particularly due to shear stress. In this work, we characterize a hydrogenated castor oil (HCO) colloidal scaffold using μ²rheology, multiple particle tracking microrheology (MPT) in a microfluidic device. HCO is a colloidal fiber that gels and degrades in response to osmotic pressure gradients. When an HCO gel is contacted with water it will degrade and when the surrounding environment is changed to a gelling solution, composed of glycerine and surfactant, the material will gel. Using μ²rheology, we determine the scaffold properties and structure during consecutive phase changes on a single sample. The microfluidic device design is a two-level device with two chambers: sample and suction chambers. Channels that deliver solvent from the second level into the sample chamber are spaced 60° around the chamber. This creates equal pressure around the HCO sample locking it in place when solvent is exchanged. MPT measures gelation and degradation of the HCO scaffold in the microfluidic device. In MPT, the Brownian motion of probe particles embedded in the material is measured and related to rheological properties using the Generalized Stokes-Einstein Relation. μ²rheology measurements of consecutive phase changes of HCO are taken starting with both a solution of HCO (0.125 wt%) and a gel (4 wt%). The HCO gel has minimal shear added prior to measurement. The HCO solution is created by diluting HCO in water and adding large amounts of shear. Single samples are successfully measured through 4 – 9 phase transitions. MPT measurements suggest that the structure of the HCO gel is dependent on the starting materials shear history. When gelled the sheared HCO solution cannot form a tightly associated, entangled network structure. During degradation, the samples that begin as an HCO gel cannot completely degrade into single colloids in solution. From these results, we conclude that the osmotic gradient is not a strong enough force to break or reform entanglements. Therefore, equilibrium structures depend on the shear history of the starting material, which can have important implications in end use products made with colloidal gel scaffolds.