Active microrheology
Active microrheology uses external forces (most typically magnetic or optical) to force microrheological probes into motion. These techniques short-circuit the Einstein component of passive microrheology. Active microrheology provides an additional handle to probe material properties, and has been used both to extend the range of materials amenable to microrheological analysis, and to examine material properties that are inaccessible to passive microrheology. Three main topics are presented: the use of active microrheology to extend the range of passive microrheology, while maintaining many of the advantages (small sample size, wide frequency range, etc.); its use to complement passive microrheology in active systems, which convert chemical fuel to mechanical work, in order to elucidate the power provided by molecular motors, for instance; and its application (and potential limitations) to investigate the non-linear response properties of materials, including shear thinning and yielding.