Mesoscopic heterogeneity in soft materials by local rheological measurements

Soft materials including polymers, micelles, gels and liquid crystals possess hierarchical structure with various length scales ranging from nanometer to micrometer. Thus, to give a better understanding of dynamics, it is necessary to examine the structure and physical properties at various length scales, and clarify the correlation between them. Recently, measurements of local rheological properties in soft materials have become possible with the advent of various techniques, called microrheology. In these techniques, rheological information can be accessed on the basis of the movement of probe particles dispersed in a sample to be measured. We have made an effort for time- and spatialresolved rheological measurements for soft materials, by using optical tweezers and particle tracking that fall under active and passive methods in microrheology, respectively. Such studies can provide information on a concentration fluctuation and a spatial heterogeneity in the systems. Notably, a worm-like micelle solution, a supramolecular hydrogel and lyotropic liquid crystals were spatially heterogeneous on the comparable or at a smaller length scale less than the characteristic length of network and interfacial structures, and on the time scale shorter than the relaxation time.