Direct observation of spatiotemporal dependence of anomalous diffusion in inhomogeneous fluid by...

The direct observation of a spatiotemporal behavior of anomalous diffusion in aqueous polymer [hyaluronan (HA)] solution was achieved by fluorescence correlation spectroscopy (FCS) using a modified instrument, enabling continuous change of the confocal volume of a microscope, namely, sampling-volume-controlled (SVC) FCS (SVC-FCS). Since HA chains form a mesh structure with a pore size of about 10-40 nm, the observed diffusion coefficient (Dobs) is markedly dependent on the diffusion distance (L). By SVC-FCS, the curve of the distance dependence of diffusion coefficient was directly obtained as a continuous profile in L = 245-600 nm showing evidence of anomalous diffusion. On plotting Dobs against either of the sampling time (tauobs) or the diffusion distance (L), Dobs turnover was observed near the anomalous diffusion area. The appearance of this turnover is attributed to the nonuniform mesh structure that can be observed only by a fast observation and that should be dynamically averaged by polymer motions with large tauobs. This behavior is similar to that revealed in glass, colloidal systems, and gel solutions using dynamic light scattering, neutron scattering, and other techniques.