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Rheology is the study of the deformation and flow of materials, subject to an applied force. It is concerned with materials whose mechanical behaviors present features that can be partially described by both the classical theory of elasticity and by simple fluid mechanics.
Microrheology is a branch of rheology that includes active and passive optical measurement techniques, which are increasingly used for research into characterizations of materials. The microrheological measurements can be performed in situ, in an environment that cannot be reached by a bulk rheology experiment.
Furthermore, microrheology techniques are extremely sensitive even at low concentrations and have no inertial effect that can be obtained by mechanical rheometry. The small sample volume required at the micro-liter scale is particularly advantageous for biological systems.
Optical methods in rheology can provide essential means of establishing the connection between structures and flow properties for a wide range of materials including polymer solutions, emulsions, colloidal suspensions, liquid crystals, and surfactants. There are many optical methods used in the rheology that is discussed below.
Small Angle Light Scattering
Small Angle Light Scattering (SALS) is a well-established light scattering technique for structure analysis that bridges the gap between standard light scattering techniques such as dynamic or static light scattering (DLS and SLS) and microscopy.
The combination of rheology and SALS (Rheo-SALS) allows the formation of shear-induced structures during rheological measurements. However, the SALS technique offers much more than, both for static and dynamic light scattering.
Polarized Light Imaging
In polarized light imaging, no single element can be observed as it has no magnification option. Unlike microscopy, polarized light imaging is an integrative method, in which the entire sample can be seen.
The phase difference of light passing through various areas of the sample is detected and visualized as color mapping. Therefore, users can identify changes in the structure over the entire area of the sample.
By using polarized light imaging, a link between the macroscopic properties of a sample and the comparable microstructure can be made. Any changes in birefringent properties under shear can be monitored over the sheared sample, allowing visualization of the flow field. The method can be used not only to study the orientation phenomena in liquid crystals but also for the corresponding structure transitions induced by shearing.
Optical tweezers can be used to generate and measure fluid flow in two and three dimensions around microstructures. The measurements are highly reproducible and minimally invasive, opening the door for future in-depth studies of the rheological properties of microstructures and biological cells in lab-on-a-chip devices.
The optical tweezers are probably better suited than Micro-Electro-Mechanical Systems
(MEMS) or atomic force microscope (AFM) measurements because of a number of factors, including the ability to adjust the rigidity of the trap in real time during measurement; the extra degrees of freedom offered by the tweezers, which can be configured in all three dimensions, and the extreme sensitivity of the measurement.
Sources and Further Reading
- L. Pop, P. Heyer, J Laeuger. Optical methods in Rheology. 2008; 73760.
- Yao A, Tassieri M, Cooper J. Microrheology with optical tweezers. 2009.
- Robertson-Anderson RM. Optical Tweezers Microrheology: From the Basics to Advanced Techniques and Applications. ACS Macro Letters. 2018; 7:968–75.
- Loredana m. Völker-pop. Optical methods in rheology : polarized light imaging. 2014; 724:707–10.
- Mohan A. Mechanical and novel optical techniques for rheological characterization of cereal beta-glucan. 2014; 6(5):2732–8.
- Robertson-Anderson RM. Optical Tweezers Microrheology: From the Basics to Advanced Techniques and Applications. ACS Macro Letters. 2018;7: 968–75.