In passive microrheology, the viscoelastic function can be obtained from the trajectory of Brownian motion of the probe particle. Various methods can be used to measure particle position. In this review, we describe typical methods using video tracking and dynamic light scattering and present the results of our recent studies. We also present our recently developed method for determining the viscoelasticity of a matrix from the orientation correlation function of the probe particles using dielectric relaxation measurements. This method allows the use of molecular-sized probes and is applicable to materials with high elastic moduli such as glass. This method allows us to extend our interest from microrheology to nanorheology.

Passive microrheology is a relatively modern rheology technique for measuring linear visco-elasticity of soft material such as emulsion, polymer solution and micellar solution by observing Brownian motion of micron-sized probe particles embedded in the medium. The generalized Stoles-Einstein relation (GSER) is used to derive the dynamic modulus of viscoelastic material from the mean square displacement (MSD) of probe particles undergoing Brownian motion in the material. In this review article, we briefly explain fundamental concept of passive microrheology and derivation of the GSER. Among several methods for observing mean square displacement of probe particles, we focus on the light scattering technique called diffusing-wave spectroscopy (DWS) that is performed in multiple scattering regime. A merit of using DWS-based microrheology is its ability to probe higher frequency regime than the other techniques such as more conventional dynamic light scattering method made in the single-scattering regime and particle-tracking method. Some examples of DWS-based micro-rheology are presented.

Microrheology (MR) using optical trapping and laser interferometry is particularly useful for biological samples whose rheology remarkably depends on time and length scales. However, it has been challenging to measure samples like living cells that exhibit large fluctuations (flow) and can be damaged by laser light. Recently, with the feedback control of the optical-trapping force and the probe position, it has become possible to conduct precise, minimally invasive MR measurements of soft matter and living samples. In this paper, after explaining these latest techniques, we will present examples of measurements: the linear viscoelasticity within living cells, and the force-induced nonlinear fluctuations in the in vitro cytoskeleton.