Probe microrheology without particle tracking by differential dynamic microscopy

We present a new method for performing passive probe microrheology. Using a simple theoretical framework, we show how probes' mean-squared displacements can be extracted by analyzing intensity fluctuations in optical microscopy videos via differential dynamic microscopy (DDM). Applying the method to optically dilute probes in Newtonian and viscoelastic fluids quantitatively reproduces mean-squared displacements extracted from multiple particle tracking (MPT), and exposes the relative strengths and weakness of DDM. Furthermore, DDM can be used to measure the mean-squared displcement in optically dense fluids where MPT fails, demonstrating that DDM can extend the range of microrheology experiments while circumventing many of the drawbacks of MPT.