The mechanical properties of the cytoplasm play a crucial role in the transport and interaction of intracellular components as well as the cell's response to its physical environment. Like other complex fluids, cytoplasmic mechanics can be probed with passive microrheology, by tracking the thermal motion of individual particles over time. In cells that are actively moving or deforming, however, large-scale persistent flow of the cell contents tends to obscure the stochastic motion from which material properties of the cytoplasm can be derived.
We present a novel technique for analyzing single particle trajectories confounded by slowly-varying flow, in order to characterize the underlying stochastic motion. Our method leverages the separation of time-scales to rigorously subtract out the persistent component of the motion so as to enable robust extraction of the diffusion coefficient and subdiffusive scaling exponent. This technique allows the characterization of a flowing complex fluid as a viscous or viscoelastic material.
We apply this method to analyze the motion of lysosomes within motile neutrophils crawling in a two-dimensional environment, demonstrating that the cytoplasm of these cells behaves as a viscous fluid. The technique is widely applicable to passive microrheology studies in the presence of spatially heterogeneous slowly-varying drift, such as cytoplasmic microrheology in moving cells or particle motion in bulk flow of a complex medium.