The influence of polymer on the diffusion of a spherical tracer

We analyze how the addition of a small number of polymer molecules influenc es the diffusion constant of a spherical tracer, whose radius is small comp ared to the size of the polymer. We show that the polymer chain can be rega rded as a two-dimensional object which is an impenetrable obstacle for the tracer. It is also shown that the diffusion constant of the tracer, in cont rast to the solution viscosity, is independent of chain length, depending o nly on the monomer concentration. (C) 1999 American Institute of Physics. [ S0021-9606(99)52437-9].