What is NMR ?
NMR is a powerful technique that has had a huge impact and is widely used in a range of scientific disciplines. When an atomic nucleus is placed in a magnetic field there is an interaction between the nuclear magnetic moment of the nucleus and the field. Because the magnetic moment of the nucleus is quantised the nucleus reacts to a radiofrequency field of a specific frequency which corresponds to the energy of the transition between the quantum states of the nucleus. It is the dependence of the frequency on the magnetic field that gives NMR such a direct insight into the molecular state of matter.
The nucleus can experience different magnetic fields because of, for instance, the magnetic field of other atoms in the molecules, which means a molecule can have a spectrum of NMR frequencies leading to the field of NMR spectroscopy.
Alternatively, the nucleus can experience a different magnetic field due to the application of a magnetic field gradient, the resulting frequency spectrum then provides information about the position of the nucleus in the sample and leads in particular to the field of NMR Imaging and the now well know non-invasive technique of MRI used in many modern hospitals. As the resolution of NMR Imaging can be at the micron level it is also known as NMR Microscopy when high spatial resolutions are obtained. NMR is also sensitive not only to molecular position, but also molecular motion leading to techniques that can measure molecular diffusion and flow velocity fields.
Because NMR provides information about the nuclear environment it is able to provide a unique insight into the state and organisation of matter and can simultaneously measure both macroscopic and molecular properties of the sample in question. It is this combination of techniques that is so useful in measuring the rheological properties of complex fluids.