One- and two-point micro-rheology of viscoelastic media

Abstract

In recent years optical tracer techniques have been developed to determine themicro-rheology of soft viscoelastic materials. Recent theoretical arguments (LevineA J and Lubensky T C 2001 Phys. Rev. E 65 011501) suggest that the correlatedfluctuations of a pair of widely separated probe particles should reflect the bulkrheology of the medium that they are embedded in more accurately than themotion of a single particle. We present a experimental test of these arguments.Using optical tweezers techniques (Henderson S, Mitchell S and Bartlett P 2002Phys. Rev. Lett. 88 088302), we measure at high spatial and temporal resolutionthe thermal motion of a pair of colloidal particles suspended in a semi-diluteviscoelastic solution of non-adsorbing polystyrene in decalin. From themeasured particle trajectories we determine both the one-and two-particlecorrelations and extract the local and bulk rheology. A comparison of thetwo measurements shows significant differences which are interpreted interms of the depletion of polymer molecules from the particle surface.