We present a neutron-scattering study of depletion interactions in a mixture of a hard-sphere-like colloid anda nonadsorbing polymer. By matching the scattering length density of the solvent with that of the polymer, wemeasured the partial structure factor Sc(Q) for the colloidal particles. It is found that the measured Sc(Q) fordifferent colloid and polymer concentrations can be well described by an effective interaction potential U(r)for the polymer-induced depletion attraction between the colloidal particles. The magnitude of the attraction isfound to increase linearly with the polymer concentration, but it levels off at higher polymer concentrations.This reduction in the depletion attraction presumably arises from the polymer-polymer interactions. The experimentdemonstrates the effectiveness of using a nonadsorbing polymer to control the magnitude as well asthe range of the interaction between the colloidal particles. @S1063-651X~96!10911-9#

We report a neutron scattering study of depletion interactions in a mixture of colloid andnonadsorbing polymer. It is found that the measured colloidal structure factor can be well describedby an effective interaction potential for the polymer-induced depletion attraction between the colloidalparticles. The amplitude of the attraction is found to increase linearly with the polymer concentrationbut it levels off at higher polymer concentrations.

(We report sedimentation measurements of small colloidal particles through a nonadsorbing polymersolution. The experiment reveals that the particles "feel" the single-chain viscosity rather than thesolvent viscosity when their radius Rc is smaller than the correlation length j of the polymer solution.The particles experience the macroscopic viscosity of the polymer solution when Rc À j. In thetransition 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.