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

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