An Introduction to DLS Microrheology

Microrheology techniques involve tracking the motion of dispersed probe (or tracer) particles in a complex fluid, to extract local and bulk rheological properties of the matrix. Analogous to mechanical rheometry techniques, a stress is applied to the system by motion of the probe particle, and the deformation (or strain) is measured through changes in the probe particle position.

Dynamic Light Scattering (DLS) Microrheology is classified as a passive technique, whereby the colloidal probe particles undergo thermal fluctuations in a system at thermodynamic equilibrium. The Mean Square Displacement (MSD) of the probe particles with time is followed by DLS, to enable linear viscoelastic parameters for the complex fluid matrix to be extracted.

DLS Microrheology offers significant measurement advantages for low viscosity, weakly-structured complex fluids since it offers a much wider frequency range than conventional mechanical rheometry (fundamentally limited by inertia), and can access the very high frequencies required to measure the critical (short timescale) dynamics of such low viscosity materials. DLS Microrheology also requires very small sample volumes - microliter-scale volumes are possible - and enables rheological characterization of material types not available in larger volumes e.g. protein-based formulations.

This paper introduces microrheology techniques for the rheological characterization of soft complex fluids, with subsequent emphasis on DLS Microrheology and the underlying theory. The paper goes on to review the applicability of DLS Microrheology measurements, and some important practical aspects of method development to ensure robust data. Finally some example measurements are shown - including the high frequency viscoelastic characterization of a polymer solution, and the use of the technique for protein solutions - both as a method for making viscosity measurements over an industrially-relevant concentration range, and for evaluating the onset of aggregation and structure development in denaturing systems.

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