Nomenclature for diffusion coefficients

The time dependence of the field correlation function is therefore determined by the time correlation function of a spatial Fourier component of the concentration, namely 

In a solution containing a single macromolecule species that dominates the scattering, two physically distinct diffusion coefficients characterize translational diffusion. The two-particle or mutual diffusion coefficient Dm describes, via Tick s law. 

When is a probe species dilute The careful experimenter makes an adequate control study of the effect of probe concentration Cp. The ideal Dp is the extrapolation Cp 0 of the measured probe diffusion coefficient. In many systems Dp for small Cp is substantially independent of Cp, and no extrapolation is needed. In other systems Dp depends significantly on Cp, and an extrapolation Cp O must be performed. Several authors have measured the initial slope kp of the dependence of Dp on probe concentration, and the effect of matrix concentration on kp. 

A variety of physical techniques has been used to measure probe diffusion in polymer solutions. In the fluorescence recovery after photobleaching (FRAP) technique, a small fluorescent label is attached to the probes. An intense pulse of light (the pump laser beam) then destroys ( bleaches ) the fluorescent labels in some volume of the solution. Much weaker illumination (the probe beam) is used to monitor recovery of the fluorescence intensity as unbleached fluorescent molecules diffuse back into the regions in which the bleaching occurred. 

Physically distinct from FRAP and FRS is fluorescence correlation spectroscopy (FCS), in which optical methods are used to measure fluctuations in the number of fluorescently-tagged molecules in a small volume of space. In nondilute solution 

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