Abstract
In Multiple-Particle-Tracking Microrheology (MPTM), rheological properties of fluids are determined from the Stokes-Einstein theory applied to Brownian motion of small suspended particles. As compared to conventional rheometers, this noncontact method does not have the problem of wall slip effects and requires a very small amount of a test fluid. MPTM measurements are typically performed in a quiescent fluid to ensure all particles are subject to random motion. Unfortunately, it is very difficult if possible to completely eliminate the deterministic motion of a test fluid during measurement because of thermal convection of the fluid, fluctuations and inclination of the experimental platform, and active transport of particles. In this work, we report our first results on development of MPTM that takes into account the deterministic velocity of a test fluid. In our approach, 0.1 or 0.9 um diameter Latex beads were suspended in a fluid located between a glass microscope slide and a glass coverslip. The movement of the particles was visualized through an inverted microscope with 40x and 60x objectives and recorded by a high-speed camera at 30 frames per second. The trajectories of particles were analyzed using a MATLAB code in which the viscosity of a test fluid was determined from the ensemble averaged MSD vs. lag time curves with the deterministic component of the fluid removed under the assumption that the ensemble-averaged velocity for randomly moving particles was equal to zero. We applied this approach to measure the viscosity of water and 5% dextran-water solution at different temperatures. With the elimination of the deterministic component, our results agree well with published viscosity data for these fluids.