Rheo-NMR in food science—Recent opportunities

For over 25 years, nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) techniques have been used to study materials under mechanical deformation. Collectively, these methods are referred to as Rheo-NMR. In many cases, it provides spatially and temporally resolved maps of NMR spectra, intrinsic NMR parameters (such as relaxation times), or motion (such as diffusion or flow). Therefore, Rheo-NMR is complementary to conventional rheological measurements. This review will briefly summarize current capabilities and limitations of Rheo-NMR in the context of material science and food science in particular. It will report on recent advances such as the incorporation of torque sensors or the implementation of large amplitude oscillatory shear and point out future opportunities for Rheo-NMR in food science.     

Anisotropic viscoelasticity and molecular diffusion in nematic wormlike micelles

Using rheo-NMR, pulsed-field-gradient NMR diffusometry and solution rheology, we observe temperature- and composition-dependent orientational order as well as director dynamics and molecular transport in a nematic wormlike micelle (WLM) system composed of cetyltrimethylammonium bromide in D₂O. We measure, for the first time over a comprehensive nematic range in WLMs, four of the five independent anisotropic viscosities by fitting nonlinear rheo-NMR director realignment data and comparing with traditional solution rheology measurements. Additionally, we extract self-diffusion coefficients in three orthogonal directions for the aligned WLM unimers as well as for the D₂O solvent molecules. Continued study and enhanced understanding of complex fluid dynamics in these anisotropic shear-modulated fluid systems can lead to advances in lubricants, coatings, oil extraction, drug delivery and tissue engineering.     

Measurement of multilamellar onion dimensions under shear using frequency domain pulsed gradient NMR

We present a simple method by which the dimensions of shear-induced multilamellar vesicles (MLVs), also known as onions, can be measured during the shearing process itself. This approach is based on the use of a closely spaced train of magnetic field gradient pulses applied during a CPMG echo sequence. The CPMG train compensates flow effects while the frequency-dependence of apparent diffusion can reveal the onion size. We present here a simple phenomenological model for restricted diffusion in multilamellar vesicles, which may be used to interpret the resulting diffusion spectrum. We demonstrate this approach with MLVs formed from the lamellar phase of sodium dodecyl sulfate (SDS) in water and octanol.