Rheology of polymer solutions using colloidal-probe atomic force microscopy

We use colloidal-probe atomic force microscope (AFM) to study the rheological behavior of polymer solutions confined between two surfaces: the surface of a sphere and a flat surface on which the fluid is deposited. Measurements of the hydrodynamic force exerted on the sphere by the flowing liquid allowed retrieving the viscosity of the solution for different distances between the sphere and the flat surface. This method has been experimentally tested for Newtonian fluids for which the viscosity does not vary versus the gap dimensions. On the other hand, for non-Newtonian fluids, such as the large molecular weight polymer solutions used here, the measured viscosity depends on the gap height D between the flat surface and the sphere. The decrease of the viscosity versus gap height is similar to previously observed variations in colloidal suspensions. Depletion of polymers in the gap region due to the high shear rates involved is a possible cause for such a variation.