Over-time measurement of solution viscosity change using quantification of Brownian motion through Particle Diffusometry

Real-time viscosity measurement techniques have been used to analyze the transition of hydrogels from a liquid state to a gel state. Measuring real-time changes in viscosity can be done through passive rheometry with the addition of tracer particles. Particle Diffusometry quantifies the Brownian motion of sub-micron-sized fluorescent particles by computing diffusion coefficients via statistical averaging. Herein, we demonstrate a method to study viscosity change over time using Particle Diffusometry on polyacrylamide hydrogel formation for a temporally and spatially resolved rheometry technique. We used synthetic images of particles undergoing a decreasing sigmoidal trend for the simulation of the viscosity change of the solution. For the experiment, polyacrylamide hydrogel was initiated using the point light source from the microscope. Using a diffusion gradient plane through PD, we confirmed a radial gel formation from the center of the imaging window. Also, by varying the initiator concentration, we observed the time-varying onset of hydrogel formation. This work establishes the groundwork for quantifying over time changes in Brownian motion.