(518c) Dynamic Microstructural Evolution of a Hydrogenated Castor Oil Colloidal Gel Measured Using Passive Microrheology

Colloidal gels have wide applicability ranging from commercial products to biomaterials due to their unique rheological characteristics. The structure and properties of colloidal gels is dramatically changed by environmental conditions and these changes can cause decrease in the shelf life and performance of the products. Understanding the conditions that cause a colloidal gel to undergo the gel-sol transition will lead to design of stable products. Multiple particle tracking (MPT) is used to determine the material properties and structure of a colloidal gel, hydrogenated castor oil (HCO), in water-surfactant environments. MPT measures the Brownian motion of embedded probe particles to extract rheological properties. HCO is crystallized to form rod-like particles, and is dehydrated with 20 wt% water, a similar composition to that used in home care products. The osmotic gradient between solution and gel drives the expansion and contraction of the material. Probe particle movement is tracked during expansion and contraction of the colloidal gel over six hours in a series of surfactant solutions with varying water content (0-100% water). Complete expansion of the material outside of the measurement apparatus occurs over six hours, while contraction occurs over one hour. Heterogeneity of the material during dynamic expansion and contraction is quantified by comparing particle trajectories using a F-test with a 95% confidence interval to determine if the movement of individual particles is statistically distinct. During expansion, the colloidal gel contains heterogeneous microenvironments that are homogeneously distributed throughout the field of view. Finally, MPT quantifies and measures cage jumping. Cage jumping occurs when the colloidal gel structure is heterogeneous with microscopic islands of gel and sol. A particle can become stuck within the colloidal gel structure and is able to ``jump'' between two islands of gel. The frequency of cage jumping in this colloidal gel was measured and increases as the material expands and decreases as the material contracts. The bulk viscosity of this material is linearly related to the frequency of cage jumping.