Diffusion-induced relaxation

A spin-A spin interaction. The fluctuation of the precession frequency is also induced by the microwave pulses used for the excitation and the refocussing the microwave pulses induce the transition betwen a and p spin states, so that the magnetic interaction of a particular A spin with other A spins is changed instantaneously during the second microwave pulse of the ESE measurements. The relaxation process due to the thus-created fluctuation is called instantaneous diffusion. The relaxation rate due to the instantaneous diffusion depends on the distance between the A spins and the number of the A spins. Because the concentration of the A spin depends on the intensity of the microwave pulse, the rate of the instantaneous diffusion also depends on the intensity of the microwave pulse this process can be eliminated in the ESE experiments by lowering the power of the microwave pulses. When the instantaneous diffusion is the dominant process in the phase relaxation, this provides us with a means of studying the local spatial distribution of radical species [13],... 

Endothelial-derived relaxing factor (EDRF) a small, diffusible molecule that is released by the vascular endothehal cell and diffuses to the overlying smooth muscle cell, where it induces relaxation and resulting vasodilation. EDRF is either NO alone or a bound form of -NO... 

Several solvent induced relaxation processes are involved in this process Diffusion, trapping, geminate recombination, and vibrational relaxation. In addition, the A X transition represents the important class of nonadiabatic reactions, here induced by the solute-solvent interaction. Furthermore, the interaction between the molecular species and the radiation field, used to initiate and to monitor the process, is modified by the solvent environment. Other important solvent induced processes Diffusion controlled reactions, charge (electron, proton)... 

Classic Case II transport behavior finds weight gain a linear function of time (18). A constant rate of absorption will be the result of a constant rate relax-ation process if diffusion of penetrant to the relaxing boundary is rapid when compared to penetrant induced relaxations. A relation describing penetrant uptake as a function of time has been given (26) ... 

Zheng et al derived a Redfield-type theory for magnetic field gradient-induced relaxation of spins undergoing restricted diffusion. The theory covered both transverse and longitudinal relaxation and the approach was valid in all diffusion regimes. The theory can be useful for MRI in structured media and was illustrated with experiments on gaseous, polarized He. 

Using sorption-desorption kinetics, the extent of non-Fickian behavior decreases as the thickness of the film increases. Presumably, as the ratio of the time scale for diffusion to the time scale for stress-induced relaxation I ID) becomes large, the time-dependent relaxation occurs so rapidly (relatively) that it has a small effect on the diffusion process. In other words, in the limit where molecular-scale responses to swelling stresses are rapid compared to the rate of diffusion, the process appears to be Fickian. This is the case for totally amorphous rubbery polymers that behave as high molecular weight liquids. 

We have presented here the first observation of transient molecular reorientation induced in a liquid crystal by a -switched laser pulse. The response time of molecular reorientation in the nematic phase is of the order of 10—100 psec. Although this is 10 —10 times longer than the duration of the laser pulse, transient molecular reorientation is still strong enough to yield an easily detectable phase shift in the probe beam. Residual al> sorption and subsequent very rapid radiationless conversion into heat can result in a temperature rise in the medium which decays via heat diffusion with relaxation times in the 10—200 msec range. The temperature rise also induces a refractive-index change in the medium and hence a phase shift in the probe beam. This thermal effect and the molecular reorientation are initiated simultaneously by the pulsed laser excitation. They are in general coupled... 

Since all materials are polarizable to some degree, the surface charge is generally not fixed. This leads to a broad class of nonlinear electrokinetic phenomena, where bulk electric fields interact with induced diffuse charge in solution to produce nonlinear electrophoretic motion, U =f(E). In electrolytes, such effects of induced-charge electrophoresis (ICEP) occur in addition to the purely electrostatic effect of dielectrophoresis (DEP) in low-frequency AC fields (< 100 kHz), where there is enough time for diffuse-charge relaxation around the particle within each period. ICEP is a complex phenomenon, which can lead not only to nonlinear mobility (in the field direction) but also to rotation and motion in arbitrary directions, even in uniform fields. 

The interaction of a (3-hairpin antimicrobial peptide, protegrin-1, with various lipid membranes has been investigated by solid-state P, and NMR. Mixed lipid bilayers containing anionic lipids and cholesterol were used to mimic the bacterial and mammalian cell membranes, respectively. P and spectra of macroscopically oriented samples showed that protegrin-1 induces the formation of an isotropic phase in anionic bilayers containing phosphatidylglycerol. 2D P exchange, H spin diffusion and relaxation time measurements were undertaken. In a related work the results of solid-state H, C, N and P NMR studies of the selective perturbation of lipid bilayers by the cyclic antimicrobial peptide have been presented. ... 

Translational self-diffusion. Whereas in bulk nematics, translational self-diffusion modulates only the dipolar interactions between protons on different molecules (i.e. intermolecular interactions), in microdroplets it also modulates intra-molecular dipolar interactions because the director field in a confined geometry is non-uniform. This new relaxation mechanism is called translational diffusion induced rotation (TDIR). Since intramolecular dipolar interactions are generally stronger than intermolecular interactions, translational selfdiffusion represents a much more effective relaxation mechanism in micro-droplets than in bulk nematics. TDIR is also effective at much lower frequencies than diffusion induced modulation of inter-molecular... 

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