Challenges in determining anomalous diffusion in crowded fluids

Abstract

Anomalous diffusion in crowded fluids, e.g. in the cytoplasm of living cells, is a frequentphenomenon. Despite manifold observations of anomalous diffusion with severalexperimental techniques, a thorough understanding of the underlying microscopic causesis still lacking. Here, we have quantitatively compared two popular techniqueswith which anomalous diffusion is typically assessed. Using extensive computersimulations of two prototypical random walks with stationary increments, i.e. fractionalBrownian motion and obstructed diffusion, we find that single particle tracking (SPT)yields results for the diffusion anomaly that are equivalent to those obtainedby fluorescence correlation spectroscopy (FCS). We also show that positionaluncertainties, inherent to SPT experiments, lead to a systematic underestimation of thediffusion anomaly, regardless of the underlying random walk and measurementtechnique. This effect becomes particularly relevant when the position uncertainty islarger than the average positional displacement between two successive frames.