Colloidal dynamics in polymer solutions: Optical two-point microrheology measurements

TL;DR

This paper extends optical two-point microrheology to high frequencies, enabling detailed analysis of colloidal dynamics in polymer solutions through correlated probe particle measurements.

Contribution

The authors develop a high-frequency extension of two-point microrheology using optical tweezers and interferometry, with a theoretical model for response functions.

Findings

Validated method with polystyrene in decalin

Observed anomalous response ratio indicating slip boundary conditions

Evidence of polymer depletion at particle surfaces

Abstract

We present an extension of the two-point optical microrheology technique introduced by Crocker \textit{et al.} [Phys. Rev. Lett. \textbf{85}, 888 (2000)] to high frequencies. The correlated fluctuations of two probe spheres held by a pair of optical tweezers within a viscoelastic medium are determined using optical interferometry. A theoretical model is developed to yield the frequency-dependent one- and two-particle response functions from the correlated motion. We demonstrate the validity of this method by determining the one- and two-particle correlations in a semi-dilute solution of polystyrene in decalin. We find that the ratio of the one- and two-particle response functions is anomalous which we interpret as evidence for a slip boundary condition caused by depletion of polymer from the surface of the particle.