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Diffusion of spherical colloids in suspensions of rods.
Kyong Ok Kang Introduction
There are two different types of diffusion; self-diffusion that is related to the dynamics of a single Brownian particle in a homogeneous system and collective diffusion that is related to relaxation of inhomogeneities in concentration. We are interested in long-time self-diffusion of spherical “tracer” particles embedded in concentrated suspensions of rod-like colloids. Besides the concentration of rods, an important variable is the ratio of the size of the tracer sphere and the length of the rods.
Short-time self-diffusion: At small times, the tracer particle diffuses within local minima in the energy landscape set up by the rods, as depicted below. Diffusion on this short time scale is described by the so-called short-time self-diffusion coefficient.
Long-time self-diffusion: For later long times later, the tracer particle can “climb”
free energy barriers (as depicted below), and the energy landscape itself
changes due to the Brownian motion of the rods. The associated self-diffusion coefficient is referred to as the long-time self-diffusion coefficient.
The Experimental System
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Tracer particle: Silica particles with varying diameter. For Fluorescence Correlation Spectroscopy (FCS) measurements, these particles are fluorescently labeled with Rhodamine). Experiments are done for various salt concentrations.
Host particles: fd-virus particles – rod-like biological colloids with a contour length of 880 nm, diameter 6.6 nm, and persistence length 2200 nm.
Measurements
The self-diffusion coefficient of a tracer particle can be measured via Fluorescence Correlation Spectroscopy (FCS) and Dynamic Light Scattering (DLS). With DLS the full time dependence of the mean squared displacement can be obtained. This allows deciding whether with FCS the true long-time self-diffusion coefficient is measured. In addition, video microscopy measurements will be performed for the large tracer particles.
