Diffusion of microscopic tracer particles in a yield-stress fluid

The motion of micron-sized fluorescent tracer particles in a gel of Carbopol ETD 2050 is studied using fluorescence microscopy. For a Carbopol concentration at which the material shows yield-stress behavior on the bulk scale, the tracer particles display a range of behavior: Some of the particles show slightly subdiffusive behavior, while others are almost completely immobilized. This indicates that the material is inhomogeneous, with different particles sampling different microrheological environments. From our results we calculate the range of microrheological viscous and elastic moduli of the material, which we compare with bulk values of the moduli as determined by conventional shear rheometry.