Spatially resolved microrheology using localized coherence volumes.

We propose an optical technique, based on dynamic light scattering, for investigating the local rheological response of a complex fluid over a frequency range larger than that provided by standard mechanical instrumentation. The low-coherence radiation used in a fiber optics configuration allows the measurements to be confined to a small volume around a tenth of a picoliter. The ability of the method to accurately measure both loss and storage moduli has been tested using both simple Newtonian liquids and viscoelastic, complex fluids. The possibility to monitor liquid-gel transitions in polymer solutions has also been demonstrated. The unique capability of the technique to localize the measurement volume suggests that this novel approach can be used for three-dimensional mapping of rheological properties in heterogeneous systems.