(519e) Microrheological Characterization of the Dynamic Transition of Hydrogenated Castor Oil Colloidal Gels Due to Osmotic Pressure Gradients

Authors: 
Schultz, K., Lehigh University
Lindberg, S., Procter & Gamble
Wehrman, M., Lehigh University
Nature provides colloidal materials with wide ranges of structure and function leading to materials with diverse rheology and applicability. Although these materials are used in industrial applications, their potential is not fully realized due to a lack of in depth understanding of the structure-function relationship on the microscale as they undergo dynamic processes. Uncovering the heterogeneous, dynamic structure during critical transitions of colloidal gels will enable these materials to be used for more advanced applications, including rheological modification of composite materials and dynamically adaptable materials that tolerate harsh. We characterize the dynamic transitions (sol-gel and gel-sol) of hydrogenated castor oil (HCO), which transitions due to an osmotic pressure gradient. This scaffold consists of HCO colloids, a surfactant (linear alkylbenzene sulfonate) and water. Multiple particle tracking microrheology (MPT) measures these phase transitions. In MPT, we embed fluorescent probe particles into the sample and measure their Brownian motion, which is related to rheological properties using the Generalized Stokes-Einstein Relation. Probe movement is tracked during expansion and contraction of the gel until a steady state has been reached. MPT data are analyzed using time-cure superposition identifying critical transition times and critical relaxation exponents that determine the amount of energy stored by the material. HCO gels have critical relaxation exponents of nexp = 0.77±0.08, for expansion, and ncon = 0.90±0.15, for contraction. During expansion and contraction HCO gels evolve heterogeneously, this heterogeneity is measured spatially and rheologically. van Hove correlation functions, characterize the one-dimensional probability distribution function of particles. Heterogeneity of the gel during the phase change is quantified by comparing variances of single particle van Hove correlation functions using an F-test with a 95% confidence interval determining if individual particles are probing statistically different microenvironments within the gel. These phase changes have rheological heterogeneous microenvironments that are homogeneously distributed throughout the field of view. Although HCO gels do evolve heterogeneously, this work determines that these heterogeneities do not significantly change traditional MPT measurements but the analysis techniques developed provide additional information on the unique heterogeneous scaffold microenvironments. This creates a toolbox that can be widely applied to other scaffolds during dynamic transitions.
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