Quantifying the dynamic transition of hydrogenated castor oil gels measured via multiple particle tracking microrheology

Abstract

Rheological modifiers are essential ingredients in commercial materials that exploit facile and repeatable phase transitions. Although rheological modifiers are used to change flow behavior or quiescent stability, the complex properties of particulate gels during dilution is not well studied. We characterize a dynamically evolving colloidal gel, hydrogenated castor oil (HCO), a naturally sourced material, used in consumer products. This HCO scaffold consists of fibrous colloids, a surfactant (linear alkylbenzene sulfonate) and water. The gel undergoes critical transitions, degradation and formation, in response to an osmotic pressure gradient. Multiple particle tracking microrheology (MPT) measures the evolving material properties. In MPT, fluorescent probe particles are embedded into the sample and Brownian motion is measured. MPT data are analyzed using time-cure superposition, identifying critical transition times and critical relaxation exponents for degradation and formation where tc,deg = 102.5 min, ndeg = 0.77 ± 0.09, tc,for = 31.9 min, and nfor = 0.94 ± 0.11, respectively. During degradation and formation HCO gels evolve heterogeneously, this heterogeneity is characterized spatially and temporally. Heterogeneity of the gel is quantified by comparing variances of single particle van Hove correlation functions using an F-test with a 95% confidence interval. HCO transitions 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.

Rheological properties and structure of step‐ and chain‐growth gels concentrated above the overlap concentration

“…n > 0.5 is a loosely cross‐linked porous network that dissipates energy. The structure at n = 0.5 is similar to the structure formed during percolation with equal ability to store and dissipate energy …”

“…Comparison of α to the critical relaxation exponent, n , determines the state of the material. n is determined from time‐cure superposition and will be discussed in depth below α<n is a gel, is a sol and is the gel point when the material transitions from a sol to a gel.…”

“…Time‐cure superposition (TCS) is used to analyze dynamic microrheological measurements during gelation to determine the critical relaxation exponent, n . TCS is the superposition of viscoelastic functions at different extents of reaction . Comparison of the logarithmic slope of the MSD to n determines the state of the material, as described previously.…”

“…Comparison of the logarithmic slope of the MSD to n determines the state of the material, as described previously. n also determines the structure of the scaffold and the amount of energy that is stored or dissipated in the scaffold . In this way, n can be thought of as a complex modulus.…”

“…An acidic buffer (pH 4.5, 70 mM acetic acid solution (Thermo Fisher Scientific) and 30mM sodium acetate solution (Thermo Fisher Scientific)) is used to slow the 3 wt % maleimide:thiol gelation to obtain more samples throughout gelation. The adjustment of the reaction time for this scaffold will not impact the final results of the scaffold structure, as these are only a function of the precursor molecule size and the polymeric interactions . The scaffold that undergoes the chain‐growth reaction (acrylate:thiol) is photopolymerized.…”