Image correlation spectroscopy as a tool for microrheology of soft materials

Image correlation spectroscopy (ICS) has been mostly used to quantify spatiotemporal events in biological experiments. Here, we develop and demonstrate the use of ICS for microrheology (mu R) of complex, viscoelastic materials. A special technique for extracting mean-squared displacements (MSD) of probe particles in the samples from image correlation data is developed. The method (ICS-mu R) is tested on Newtonian (glycerol solutions in water) and complex fluids (poly(ethylene oxide) (PEO) solutions). Accurate measurements of the viscosities of the Newtonian fluids (equivalently, the diffusion coefficients of the probe particles) as well as the storage and loss moduli of the PEO solutions could be obtained. The possibility to combine spatiotemporal assessment and time- and length-scale dependent microrheological measurements from images of fluorescent molecules makes ICS-mu R a prospective tool in many biophysical applications. Further, the method developed for extracting MSD from image (or intensity) correlation measurements can be applied to other microrheological techniques to provide improved accuracy of rheological measurements.