Single particle motion in colloids: Microrheology and<br/><br/> microdiffusivity

Thursday, October 15, 2009 - 9:00am - 9:40am
EE/CS 3-180
John Brady (California Institute of Technology)
Keywords: Colloidal dispersions, Brownian motion, rheology

Abstract: The motion of a single individual particle in a complex
material is fundamental to understanding the dynamical
properties of the material. Monitoring such motion has given
rise to a suite of experimental techniques collectively known
as ‘microrheology,’ 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 one can gain information about the nonlinear response,
analogous to large-amplitude measurements in macrorheology.
But what exactly is measured in a microrheological experiment?
And how does the micro-rheological response compare with
conventional macrorheology? To answer these questions, we
consider a simple model – a colloidal probe pulled through a
suspension of neutrally buoyant bath colloids – for which both
micro- and macro-results can be obtained exactly. The moving
probe distorts the dispersion’s microstructure resulting in a
reactive entropic or osmotic force that resists the probe’s
motion, which can be calculated analytically and via Brownian
Dynamics simulations and used to infer the dispersion's
'effective microviscosity.' By studying the fluctuations in
the probe’s motion we can also determine the force-induced
'micro-diffusivity.' Connections between micro and macro
behavior will be explored.
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