(317b) Defining the Structure and Properties of Colloidal Rod Systems during Dynamic Phase Transitions

Rheological modifiers are added to formulations to tune rheology, enable function and drive phase changes requiring quantitative characterization of material structure and properties. Of particular interest is how these modifiers change the rheology during phase transitions. We characterize the dynamic evolution of a colloidal rod systems using multiple particle tracking microrheology (MPT). MPT measures the Brownian motion of embedded probes to extract rheological properties. In this work, we use an extension of MPT, bi-disperse MPT, which embeds two different particles sizes into the material to simultaneously measure length-scale dependent rheology. In this work, we use microfibrillated cellulose (MFC), a renewable material, which is surface-oxidized (OMFC) making it a negatively charged colloidal rod. We measure rheological properties and structure of OMFC during gelation, which is induced by incubation in either an anionic or cationic surfactant. We measure that gelation evolution is dependent on the charge of the surfactant used to induce the phase transition. In anionic surfactant, OMFC undergoes gradual gelation. In cationic surfactant, we measure rapid gelation followed by length-scale dependent rearrangement. The initial OMFC concentration also dictates the structure at the phase transition, with higher initial concentrations resulting in more tightly associated networks. We also characterize these materials using bulk rheology and measure that OMFC forms stiffer networks but yields at lower strains in cationic surfactant than in anionic surfactant. This characterization can be used in future materials design where OMFC is the rheological modifier, enabling the structure and the rheology at the phase transition to be precisely designed into the scaffold minimizing trial-and-error experiments.