Sedimentation of Colloidal Particles through a Polymer Solution

We report sedimentation measurements of small colloidal particles through a nonadsorbing polymer solution. The experiment reveals that the particles ``feel'' the single-chain viscosity rather than the solvent viscosity when their radius Rc is smaller than the correlation length ξ of the polymer solution. The particles experience the macroscopic viscosity of the polymer solution when Rcξ. In the transition region, the particle's friction coefficient does not have the predicted scaling form. Instead, a new switch function is found to be of universal form independent of the polymer molecular weight.