Probing the microstructure of yield-stress fluids using multiple particle tracking

Abstract

We have studied the microstructure of carbopol, a polymer, and laponite, a colloidal clay suspension, using multiple particle tracking and fluorescence video microscopy. From measurements of the positions of fluorescent spheres suspended in these fluids over a period of time, we obtain information about the microscopic properties of the fluids. -- For carbopol at low concentrations, the spheres diffuse freely in the fluid without any hindrance and the fluid environment is homogeneous. At intermediate concentrations, as the fluid gels, the environment becomes heterogeneous and the motion of the spheres becomes subdiffusive as the fluid structure starts to restrict their motion. At higher concentrations, stiffer gels are formed, the mean square displacement is low and the spheres are almost completely trapped by the fluid structure. From our data we estimate the length scales of the structures in the fluid to be on the order of 1 to 2 μm. -- The properties of laponite depend on both age and concentration. At small age and low concentrations, the motion of the spheres is diffusive and the fluid is homogeneous. At intermediate ages and concentrations, the motion is subdiffusive at short lag times but close to diffusive at longer lag times, which may be a signal of the sol-gel transition. At higher ages and concentrations the spheres are completely confined by the stiff gels formed and the fluid environment is heterogeneous. We were unable to determine a characteristic length scale for laponite, probably due to its fractal structure.