viscoelastic properties of the solution. In other studies, probe diffusion has been viewed as being of interest because it measures solution properties that are not the same as the viscoelastic properties of the solution.

A valuable complement to quasielastic scattering studies of probe diffusion is provided by measurements on probes subject to external driving forces. The overwhelming bulk of the literature on driven probe motion is provided by studies of capillary electrophoresis. The electrophoretic literature primarily emphasizes improving separations of different charged species. However, electrophoretic separations often use solutions of neutral polymers as the support medium. At the same time as these experiments are performing separations, these experiments are also giving information on the dynamics of the neutral polymers[@phillies2011a; @phillies2012e]. A substantial literature exists on buoyancy-driven probe motion in the ultracentrifuge[@laurent1963a; @ye1998a]. In a few experiments, magnetic[@hough1999a; @schmidt2000a] or optical[@amblard1996a] tweezers were used to examine oscillatory driven movements of probes. Tweezer experiments are particularly interesting because the experimenter can separately control two of the three: drive force, drive frequency, and particle displacement. An alternative complement to probe diffusion is provided by tracking probes in complex fluids in which the fluid itself is performing driven motion, e.g., shear[@tapadia2006a].

Quasielastic scattering of 