Microrheology of colloidal dispersions by Brownian dynamics simulations

We investigate active particle-tracking microrheology in a colloidal dispersion by Brownian dynamics simulations. A probe particle is dragged through the dispersion with an externally imposed force in order to access the nonlinear viscoelastic response of the medium. The probe's motion is governed by a balance between the external force and the entropic "reactive" force of the dispersion resulting from the microstructural deformation. A "microviscosity" is defined by appealing to the Stokes drag on the probe and serves as a measure of the viscoelastic response. This microviscosity is a function of the Peclet number (Pe=FalkT)-the ratio of "driven" (F) to diffusive (kT/a) transport-as well as of the volume fraction of the force-free bath particles making up the colloidal dispersion. At low Pe-in the passive microrheology regime-the microviscosity can be directly related to the long-time self-diffusivity of the probe. As Pe increases, the microviscosity "force-thins" until another Newtonian plateau is reached at large Fe. Microviscosities for all Peclet numbers and volume fractions can be collapsed onto a single curve through a simple volume fraction scaling and equate well to predictions from dilute microrheology theory. The microviscosity is shown to compare well with traditional macrorheology results (theory and simulations). (c) 2005 The Society of Rheology.