We study the motion of a Brownian probe particle subjected to a small amplitude oscillatory
external force and immersed in a colloidal dispersion, as a theoretical benchmark for particle-
tracking microrheology experiments. The application of an external force on the probe drives
the microstructure of the dispersion out of equilibrium; opposing this is the Brownian
diffusion of the probe and colloidal “bath” particles. The degree to which the microstructure is
driven away from equilibrium is given by the nondimensional external force on the probe, or …