microrheology

Microrheology is a set of experimental techniques used to characterize the viscoelastic properties of soft materials at microscopic length scales by observing the motion of embedded tracer particles or by applying forces to such particles. It is particularly useful for heterogeneous, delicate, or small-volume samples where conventional bulk rheology is impractical.

There are two main branches: passive microrheology and active microrheology. In passive microrheology, the thermal fluctuations of tracer particles are tracked, and their Brownian motion is analyzed to extract the material’s complex shear modulus G*(ω). The approach relies on the generalized Stokes-Einstein relation, which connects the mean-squared displacement of particles to G*(ω) under appropriate conditions. Active microrheology employs external forces, such as optical or magnetic tweezers, to drive a particle and measure its response, enabling measurements over a wider range of stresses and frequencies and often in more nonuniform or nonlinear samples.

Common implementations include particle-tracking microrheology (PTM), dynamic light scattering microrheology (DLS-MR), optical-tweezer microrheology, and magnetic-tweezer microrheology. These methods can probe high-frequency behavior and local mechanical properties, making them valuable for studying complex fluids, polymer networks, gels, and biological systems.

Advantages of microrheology include requiring only small sample volumes, enabling high spatial and temporal resolution, and allowing investigation of local heterogeneity and high-frequency response. Limitations include dependence on accurate knowledge of tracer size and surface chemistry, potential probe-matrix interactions, boundary effects, and assumptions of linear, homogeneous, isotropic media. The approach was popularized in the 1990s by Mason and Weitz and has since become a widely used tool in soft matter and biophysics.