Abstract
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 probeâs motion is governed by a balance between the external force and the entropic âreactiveâ force of the dispersion resulting from the microstructural deformation. A âmicroviscosityâ 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=FaâkT)âthe ratio of âdrivenâ (F) to diffusive (kTâa) transportâas well as of the volume fraction of the force-free bath particles making up the colloidal dispersion. At low Peâin the passive microrheology regimeâthe microviscosity can be directly related to the long-time self-diffusivity of the probe. As Pe increases, the microviscosity âforce-thinsâ until another Newtonian plateau is reached at large Pe. 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).
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