Rheo-NMR

6 RHEO-NMR. Velocities near a wall, and indeed throughout the fluid, can be measured without tracer particles, and in opaque samples, by using nuclear magnetic 

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P. T. Callaghan 1999, (Rheo-NMR nuclear magnetic resonance and the rheology of complex fluids), Rep. Prog. Phys. 62, 599-670. 

Many investigations are made using the commercially available standard NMR microscopy hardware and software, although in some cases this hardware is modified in order to fulfill specific requirements and to expand the number of possible applications. Such modifications and expansions then become part of new commercially available hardware and software if they are useful for a larger number of users, as was the case recently with the development of the Rheo-NMR [17]. 

J. Gotz, K. Zick 2005, (Rheo NMR with applications on food), in Handbook of Modem Magnetic Resonance, ed. G. Webb, Kluwer Academic Publishers, London. 

A quite different approach to rheo-NMR was taken by Xia and Callaghan [12], in an NMR microscopy measurement of the velocity profile of a high molecular weight polymer solution flowing through a capillary. In this study anomalous polymer diffusion was found at a radius within the pipe at which the local shear rate exceeded... 

While it is possible to perform rheo-NMR in large scale MRI systems, the use of microscopic geometries with sub-millilitre volumes is highly desirable, especially where expensive speciality materials are to be examined. Such sample volumes... 

This is most easily achieved by rotating the inner cylinder and keeping the outer fixed in the laboratory frame. Note, however, that this geometry leads to the formation of Taylor vortex motion if inertial effects become important (Reynolds number Re 1). Most rheo-NMR experiments are actually performed at low Re. In the cylindrical Couette, the natural coordinates are cylindrical polar (q, <(>, z) so the shear stress is denoted and is radially dependent as q 2. The strain rate across the gap is given by [2]... 

In order to illustrate the potential applications of rheo-NMR five examples have been chosen. The first example deals with wormlike micelles [22] in which NMR velocim-etry is used to profile anomalous deformational flow and deuterium NMR spectroscopy is used to determine micellar ordering in the flow. The second example concerns flow in a soft glassy material comprising a solution of intermittently jammed star polymers [23], a system in which flow fluctuations are apparent. The third... 

A significant contribution of rheo-NMR has been to show that the uniform shear-rate assumption may be violated in the case of certain classes of fluids in which pathological flow properties are exhibited. Figure 2.8.10 shows shear-rate maps [26] obtained for the wormlike surfactant system, cetylpyridinium chloride-sodium salicylate in water. While the velocity gradients show no deviation from uniformity at very low shear rates, above a certain critical value yc a dramatic variation in the rate-of-strain across the 7° cone gap is found. In particular a very high shear-rate band is found to exist at the mid-gap. 

The same flow-aligning side-chain liquid crystalline polymer has been studied [43] in extensional flow using a rheo-NMR method in which selective excitation of... 

In summary, we have commented briefly on the microscopic applications of NMR velocity imaging in complex polymer flows in complex geometries, where these applications have been termed Rheo-NMR [23]. As some of these complex geometries can be easily established in small scales, NMR velocimetry and visc-ometry at microscopic resolution can provide an effective means to image the entire Eulerian velocity field experimentally and to measure extensional properties in elastic liquids non-invasively. 

Rheo-NMR [86] methods have been shown to be well-suited to emulsion rheology studies [28] and could be combined with any of the topics described above. The combination of structural and rheological measurements is a promising area for further research. 

Callaghan (2006) recently reviewed work on rheo-NMR spectroscopy that is useful in determining simultaneous molecular ordering and molecular dynamics in relation to complex fluid rheology. Even though at present systems are limited to polymer solutions and low-temperature melts, extension of this technique to reactive polymer systems would be interesting. 

An overview of rheological measurements coupled with magnetic resonance is provided by Callaghan [67], Rheo-NMR of emulsified systems has been studied, the systems including formulations with yield stress exhibiting wall slip [68], Comparisons are provided between conventional rheological techniques and Rheo-NMR characterization. 

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