Microrheology and Structural Reconfiguration of Artificial Biofluids Composed of Xanthan Gum in Salt Solutions

We use a particle tracking method to investigate the microrheology and the timescales of structural rearrangement of solutions of xanthan gum that are being formulated for use as synthetic biofluids. By suspending Brownian fluorescent particles inside the medium and tracking changes of their mean-squared displacement (MSD), we can identify timescales when the particles are sub-diffusive, purely diffusive, or super-diffusive in behavior.

Solutions of various concentrations of xanthan gum and salt are investigated. At high concentrations of xanthan gum, we observe that the particles are inhibited from free diffusion, indicative of a purely elastic response from the material, which is consistent with the bulk rheology. However, for lower concentrations, we observed that the particles showed a super-diffusive behavior on the order of hours to days. We hypothesize that this behavior is a result of the rearrangement of the structures of the solutions over this timescale, and we use the transient changes in the MSD to provide structural information of this reconfiguration process. After equilibrium is reached, consistent short-time sub-diffusive behavior and long-time diffusive behavior is observed. These results indicate that the particles are able to eventually escape the cages of xanthan gum, and the intersection of the two regions can be used to estimate the cage size and the relaxation time of the solutions.