DLS Microrheology is an example of a passive technique. In other words, no external forces are exerted on the probe particles. Passive or thermal diffusion microrheology measures the linear rheological properties of colloidal particles undergoing thermal fluctuations in a system at thermodynamic equilibrium. Microrheology techniques can involve the application of passive or active measurement protocols. Alternative approaches to microrheology measurements Microrheology techniques, which can move beyond the limitations of mechanical rheometry and extend rheological characterization to a wider range of material types, are therefore of growing interest to academics and to those working at the forefront of commercial product development. The limits of mechanical rheometry also become evident when studying behavior at very high frequencies (over very short timescales), where system inertia rapidly dominates measurement response, and/or when the amount of sample available is minimal, i.e., down to the microliter scale, as may be the case with high-value protein-based formulations.Ĭomplex fluids often have a range of rich dynamics (over multiple length scales and timescales) that stimulate academic interest and inspire application in the formulation of everyday products such as foodstuffs, personal care and household items, and industrial chemicals. The characterization of such systems therefore demands application of very low stresses-typically a tiny fraction of the lowest torque range available from a standard laboratory rotational rheometer (which is typically optimized for measurements over multiple decades of stress). These microstructures convey viscoelasticity, imparting rheological behavior that is somewhere between that of a liquid (viscous) and solid (elastic), but in some systems they are extremely weak and highly strain sensitive. Complex fluids have a liquid base, but also encompass supra-molecular structures formed by the constituent macromolecules or surfactant phase. However, for some complex fluids, as exemplified by synthetic and biopolymer or protein solutions, and surfactant systems, there may be measurement limitations. Properties such as viscosity and viscoelasticity play an important role in controlling the performance and in-process behavior of many products, ranging from paint, polymers, and adhesives to pharmaceuticals and foods.Ĭonventional rheological techniques use a mechanical system to impose a controlled force or displacement upon the sample. Rheological testing yields parameters that directly quantify aspects of product performance and, as a result, it is applied across a number of industries. Rheology is the study of the flow and deformation of materials under an applied force. Following an introduction to microrheology, this article discusses how the use of DLS Microrheology complements conventional rotational rheometry by extending rheological characterization into new areas, and presents data to demonstrate how this can be achieved. While microrheology methodology is currently being pursued in academic and industrial research laboratories, there is growing general interest in the technique and in its potential. An added benefit of DLS Microrheology is the ability to work with much smaller sample volumes than are typically needed for a mechanical rotational rheometer. The technique has significant potential for measuring low-viscosity, weakly structured samples-for example, polymer and protein solutions-because it allows access to the very high frequencies needed to measure critical short timescale dynamics of these systems. In DLS Microrheology, the average movement of an ensemble of dispersed probe particles is tracked using dynamic light scattering (DLS). The motion of dispersed probe, or tracer, particles can be followed using either light scattering techniques or particle-tracking video microscopy. The term “microrheology” is used to describe a range of techniques that enable the determination of local and bulk viscoelastic properties for a soft material by tracking how dispersed probe particles move within it. Microrheology is a relatively new and developing analytical methodology of growing interest to those working at the forefront of rheological characterization.
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