Temporally and spatially resolved micro-rheometry of a transient viscous polymer formation

Real-time viscosity measurement techniques have been used to analyze the transition of hydrogels from a liquid state to a gel state. As viscosity is inversely proportional to diffusion coefficient, measuring real-time changes in viscosity can be done through passive rheometry with the addition of tracer particles. Particle diffusometry (PD) quantifies Brownian motion of sub-micron sized fluorescent particles by computing diffusion coefficients via statistical averaging. Herein, we demonstrate a method to study changes in diffusion coefficient as a function of time using PD for a temporally and spatially resolved rheometry measurement technique. We refined the PD algorithm using synthetic images of particles suspended in a liquid undergoing a sigmoidally decreasing diffusion trend to simulate the viscosity change of the solution during gelation. Then, the technique is applied to visualize the temporal and spatial gradients of diffusion coefficient during polyacrylamide hydrogel formation experiments. This work establishes the groundwork for quantifying over time changes in Brownian motion.