Active and Nonlinear Microrheolog

Professor John F. Brady
Department of Chemical Engineering
California Institute of Technology

Date: Thursday, February 24, 2005
Time: 4:00 p.m.
Place: Engineering II, Room 3361

ABSTRACT

Over the last decade a set of experimental techniques collectively known as ‘microrheology' has emerged as an alternative to traditional ‘macrorheology', with the ability to probe the viscoelastic properties of soft heterogeneous materials (e.g. polymer solutions, colloidal dispersions, biomaterials, etc.) at the micrometer (and smaller) scale. In microrheology, elastic and viscous moduli are obtained from measurements of the fluctuating thermal motion of embedded colloidal probes. In such experiments, the probe motion is passive and reflects the near-equilibrium (linear response) properties of the surrounding medium. By actively pulling the probe through the material, further information about nonlinear material properties can be obtained, analogous to large-amplitude measurements in macrorheology. We consider a simple model of such systems: a colloidal probe pulled through a suspension of neutrally buoyant bath colloids. The non-equilibrium spatio-temporal configuration or microstructure of particles induced by the motion of the probe is calculated analytically and via Brownian Dynamics simulations and used to infer the dispersion's ‘effective microviscosity,' The computed effective viscosities compare well with analogous macrorheology studies of sheared colloidal dispersions, suggesting that active tracking microrheology can be a valuable tool with which to explore the rich nonlinear behavior of complex materials.