A nuclear-magnetic-resonance method is explored, which effectively attenuates the dipolar interaction in solids. The experimental technique corresponds to the observation of a free-induction decay in a frame of reference rotating with the frequency of an applied rf field. When the amplitude Hi of this field is much greater than the local field in the solid, and when its frequency is appropriately chosen, the secular part of the dipolar interaction is removed. As a result the rotary saturation line is extremely narrowed. At smaller values of Hi, nonsecular terms in the dipolar interaction come into play and contribute to line broadening. These nonsecular effects are investigated both theoretically and experimentally. All the measurements were made in single crystals of calcium fluoride. The calculation of the nonsecular contribution to the line width utilizes the unitary transformation method of Jordhal and Pryce. Theory and experiment are in good agreement.

A review of recent experiments in two-dimensional turbulence is presented. Work on flowing soap films and on thin layers of fluid driven electromagnetically is covered. Theoretical notions of turbulence in two and three dimensions are introduced.

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Introduction:ynamic Light Scattering is also known as Photon Correlation Spectroscopy.This technique is one of the most popular methods used to determine the size of particles. Shining a monochromatic light beam, such as a laser, onto a solution with spherical particles in Brownian motion causes a Doppler Shift when the light hits the moving particle, changing the wavelength of the incoming light.This change is related to the size of the particle. It is possible to compute the sphere size distribution and give a description of the particle's motion in the medium, measuring the diffusion coefficient of the particle and using the autocorrelation function.This method has several advantages: first of all the experiment duration is short and it is almost all automatized so that for routine measurements an extensive experience is not required. Moreover, this method has modest development costs.Commercial "particle sizing" systems mostly operate at only one angle (90°) and use red light (675 nm). Usually in these systems the dependence on concentration is neglected. Using more sophisticated experimental equipment (projector, shortwavelength light source), the methods can be not only considerably extended, but also more complicated and expensive.Although dynamic scattering is, in principle, capable of distinguishing whether a protein is a monomer or dimer, it is much less accurate for distinguishing small oligomers than is classical light scattering or sedimentation velocity. The advantage of using dynamic scattering is the possibility to analyze samples containing broad distributions of species of widely differing molecular masses (e.g. a native protein and various sizes of aggregates), and to detect very small amounts of the higher mass species (<0.01% in many cases). Furthermore, one does not have to worry that protein aggregates are being lost within a D -3 -chromatographic column (a common problem in using SEC to characterize aggregates), because there is no chromatographic separation involved.Moreover, with this technique it is also possible to obtain absolute measurements of several parameters of interest, like molecular weight, radius of gyration, Translational diffusion constant and so on. However, the analysis might be difficult for non-rigid macromolecules. Another limit is that above the zero degree Kelvin molecules fluctuate (i.e. molecules deviate from their average position). The light scattering:ccording to the semi-classical light scattering theory [Berne and Pecora, "Dynamic Light scattering" John Wiley, 1975], when light impinges on matter, The theory behind:he experiment's theory is based essentially on two assumptions. The first condition is that the particles are in Brownian motion (also called 'random walk'); in this situation we know the probability density function, given by the formula: The second assumption is that the beads used in the experiment, are spherical particles with a diameter small compared to the molecular dimensions. If it is so, then it is possible to apply the Stoke-Einstein ...

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