Rheological Study of Soft Matters: A Review of Microrheology and Microrheometers

Abstract

To overcome these shortcomings of macrorheology, a number of novel microrheological methods have emerged during the last few decades, [2] often involving a small tracking or probing particle (micrometer size) dispersed in a softmatter medium. By tracking the fluctuation due to thermal energy or applying an external force, usually in the form of magnetic force or optical trap, one can measure local mechanical responses. The local disturbance in a micrometer scale avoids reconstruction of local viscoelastic response, an inevitable problem in bulk rheology, especially when the probe particle size is smaller than the characteristic length of a soft matter. Depending on whether the probe is manipulated by an external force instead of a random walk due to thermal fluctuation, microrheometers can be generally characterized as active or passive ones. For an equilibrium system, the fluctuationdissipation theorem (FDT) guarantees that these two methods are equivalent. [3] In this review, we will detail these two classes of microrheometers, including their basic principles and typical setups as well as some recently developed hybrid microrheometers, [4][5][6] often used in characterizing polymer solutions and biological materials. Passive MicrorheologyIn a passive microrheometry method, the fluctuation of probes dispersed in a soft-matter medium is detected and analyzed by calculating the mean-square displacement (MSD) often from their video-based trajectories or scattered light intensities. Since the fluctuation of probes in space is driven by an extremely small thermal energy (k B T), the results from a passive method are always fallen inside the linear range of viscoelasticity. Generally, the elastic modulus is estimated within an upper limit of hundreds of Pa, which means that passive microrheometers are more suitable for measuring "soft" materials with a low viscosity and elasticity. Otherwise, the probes would be trapped without a measurable movement. Even newly developed super-resolution microscopes can provide a sub-nanometer resolution, [7] they are still not widely available methods and have a limited frequency range (up to 100 Hz). Mechanical properties of a soft matter can be obtained from MSD by using the generalized Stokes-Einstein relation (GSER). [8,9] In two extreme cases, (1) the probe is dispersed in a Newtonian viscous fluid and undergoes a Brownian motion with an MSD of 〈Δr 2 (t)〉. The viscosity η is related to the translational diffusion coefficient D as Microrheological TechniquesRheological properties of soft matter like polymer solutions/gels, colloidal dispersions, and biological materials have been extensively studied by macroscopic methods. Recently, a set of microrheometers has emerged as powerful tools to investigate the dynamics and structures of homogeneous or heterogeneous soft matter at the micro-or nanoscale. In this review, these microrheometers, including some novel hybrid microrheometers are summarized and compared.

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