Laser Doppler scattering

Laser Doppler Scattering for the Determination of Ionic Velocity Distributions in Channels and Membranes... 

Although most univalent ions are permeable in the gramicidin channel, the channel has some conductance properties that suggest that the permeation process may be more complicated than ion motion down a narrow tube . Channel block by divalent cations is mechanistically complicated (14). Also, when Tl+ ion is present as the sole permeant ion, it is an excellent permeant ion that retains linear behavior to very large transmembrane potentials (15). However, when this ion is present as the minority cation in variable mole fraction solutions of Tl+ and Na+, Tl+ ion severely limits set channel currents (16). Such anomalous behavior can be elucidated if the local velocity of a specific ion (e.g., Tl+ ion) can be determined experimentally. In fact, the laser Doppler scattering technique produces a detectable scatter only with the Tl+ so that ion motions within the gramicidin channels can be differentiated. 

Laser Doppler Velocimeters. Laser Doppler flow meters have been developed to measure Hquid or gas velocities in both open and closed conduits. Velocity is measured by detecting the frequency shift in the light scattered by natural or added contaminant particles in the flow. Operation is conceptually analogous to the Doppler ultrasonic meters. Laser Doppler meters can be appHed to very low flows and have the advantage of sensing at a distance, without mechanical contact or interaction. The technique has greatest appHcation in open-flow studies such as the deterrnination of engine exhaust velocities and ship wake characteristics. 

The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path. A laser is commonly used as the source of incident illumination. The measurements are essentially independent of local temperature and pressure. This technique can be used in many different flow systems with transparent fluids containing particles whose velocity is actually measured. For a brief review or the laser-Doppler technique see Goldstein, Appl. Mech. Rev., 27, 753-760 (1974). For additional details see Durst, MeUing, and Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976. 

Bickel, W. S., J. F. Davidson, D. R. Huffman, and R. Kilkson, 1976. Application of polarization effects in light scattering a new biophysical tool, Proc. Natl. Acad. Sci. USA, 73, 486-490. Bilbro, J. W., 1980. Atmospheric laser Doppler velocimetry an overview, Opt. Eng., 19, 533-542. Blanchard, D. C., 1967. From Raindrops to Volcanoes Adventure with Sea Surface Meteorology, Doubleday, New York. 

As shown in Fig. 12 fluid flow can be determined by measuring the doppler shift in laser radiation scattered from particles in the moving fluid stream. No sensor is required in the moving stream. The laser radiation focal point can be moved across the flow tube to measure velocity profiles. Fluid linear flows from 0.01 to 5000 inches (0.03 centimeter to 127 mctersi per second hate been measured. Contaminants, such as smoke, may have to be added to gases to provide scattering centers for the laser beam. 

The methods described in this book are primarily concerned with the measurement of the microstructure of complex fluids subject to the application of external, orienting fields. In the case of flow, it is also of interest to measure the kinematics of the fluid motion. This chapter describes two experimental techniques that can be used for this purpose laser Doppler velocimetry for the measurement of fluid velocities, and dynamic light scattering (or photon correlation spectroscopy) for the determination of velocity gradients. 

Laser Doppler Velocimetry and Dynamic Light Scattering... 

The current status of prediction and modelling in the area of fuel spray combustion requires, among other parameters, the measurement of droplet or solid particle size distribution and the relative velocity between the fuel and the surrounding gas. Many optical techniques, based on laser light scattering, have been investigated to this purpose (Refs.1,2,2,]+,, 6 and j), but the only system able to simultaneously determine the size and the velocity is the dual-beam laser Doppler velocimeter shown in Figure 1. 

Dynamic light scattering (DLS), also called photon correlation spectroscopy (PCS) or laser light scattering (LLS) is a technique based on the principle that moving objects cause a frequency shift due to the Doppler effect. If a solution of macromolecules with random Brownian motion is illuminated with monochromatic laser light, the scattered light should contain a distribution of frequencies about the incident frequency the spectral line is virtually broadened. The width of the distribution is related to the MMD. 

FIGURE 11.2 (a) Schematic representation of dynamic light scattering, and (b) laser Doppler velocimetry. (Part (a) modified from Buffle, J. and Leppard, G.G., Environ. Sci. Technol., 29, 2176, 1995. With permission.)... 

Dynamic light scattering (photon correlation spectroscopy, PCS) Laser doppler velocimetry... 

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