Magnetic field-gradients stationary

It should be noted that the measurement of such small self-diffusion coefficients in the presence of a large local velocity shear requires the use of large magnetic field gradients (5Tm ) along with special pulse sequence which removes phase shifts due to coherent (i.e. stationary) flow but retains those due to stochastic motion. Such an effect is achieved by using a double echo variant of the PGSE pulse sequence [6, 8, 63]. 

The particles can also be oriented purely mechanically by a shear gradient, as well as by electric or magnetic fields. This type of flow with a linear shear can be produced when the liquid to be studied is placed in a narrow space between two concentric cylinders (Figure 7-2). One of the two cylinders turns (rotor), while the other remains stationary (stator). Owing to the partial orientation of the anisotropic molecules, the refractive indices n differ according to whether the long axis of the particles is parallel or at right... 

Diffusion and imaging experiments rely on the imposition of controlled field gradients to provide a coordinate system. Take, for example, the idealized one-dimensional system shown in Fig. 17, composed of two identical stationary spins. During a normal NMR experiment, the field is homogeneous across the sample, and the black and white spins precess at the same frequency, appearing at the same chemical shift (Fig. 17a). If, on the other hand (Fig. 17b), the field is varied linearly across the sample, the two spins do not experience the same magnetic field, and so resonate at different frequencies. They therefore appear as distinct peaks in the spectrum. 

However it differs from chromatography in one fundamental respect — separation is induced by an external field or gradient such as an electrical or magnetic field or a thermal gradient instead of interaction with a stationary phase. ... 

The second-order expansion given in eqn (6.96) recovers all of the physical quantities needed to describe a quantum system and determine its properties the charge density p and its gradient vector field Vp define atoms and determine many of their properties in a stationary state the current density determines the system s magnetic properties and the change in p in a time-dependent system and, finally, the stress tensor determines the local and average mechanical properties of the system. Thus, one does not need all the... 

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