Light scattering Doppler shift

Figure 2. Light of incident angular frequency o>0 is scattered from two particles undergoing random Brownian motion. Because the scattered light is Doppler shifted by the two particles respectively, the scattered signal contains two discrete frequencies.

Since the velocities of the dissolved macromolecules are far less than the velocity of light, the Doppler shift will be extremely small and thus the frequency shifts are very small. The high intensity of the laser source makes it possible, however, to measure weakly scattered light and to observe very small frequency shifts. Like conventional light scattering, DLS can be used successfully to determine the molecular weight, size and shape of macromolecules. Moreover, DLS is such a powerful tool that also the diffusion coefficient can be measured, and that in a more elegant manner than many other classical tools (Sun, 1994,2004). 

The scattering techniques, dynamic light scattering or photon correlation spectroscopy involve measurement of the fluctuations in light intensity due to density fluctuations in the sample, in this case from the capillary wave motion. The light scattered from thermal capillary waves contains two observables. The Doppler-shifted peak propagates at a rate such that its frequency follows Eq. IV-28 and... 

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 capillary wave frequency is detected by an optical heterodyne technique. The laser beam, quasi-elastically scattered by the capillary wave at the liquid-liquid interface, is accompanied by a Doppler shift. The scattered beam is optically mixed with the diffracted beam from the diffraction grating to generate an optical beat in the mixed light. The beat frequency obtained here is the same as the Doppler shift, i.e., the capillary wave frequency. By selecting the order of the mixed diffracted beam, we can change the wavelength of the observed capillary wave according to Eq. (11). 

When a spherical particle enters the crossing volume of two laser beams, a Doppler effect occurs not only in frequency shift but also in phase shift of the scattered light. The frequency shift yields the velocity of the sphere, whereas the phase shift gives the particle size. The phase Doppler principle has been employed to measure the size and size distributions of spheres in addition to the particle velocity. The phase Doppler principle was first reported by Durst and Zare (1975) and became a viable measurement tool one decade later [Bachalo and Houser, 1984]. 

This technique involves the creation of a two-dimensional image of skin perfusion. It operates by emitting laser light on to the skin tissue, which upon partial absorption and diffuse scattering, is then reflected with doppler shifted frequencies from blood cells and with unshifted frequencies from stationary tissue. It is a popular method, as it is easy to use and non-invasive, however subjects must refrain from smoking for 4 h prior to measurements and no caffeine intake is permitted 1 h prior to measurements.26... 

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. 

Electrophoretic instruments for analysis of colloidal suspensions can be divided into two basic classes optical instruments for which the operator observes the migration of particles in a field using a microscope and laser-based instruments that measure the Doppler shift in the frequency of the scattered light from particles... 

A third possibility is to follow the dynamics of droplets. With dynamic light scattering, the Doppler shift relative to the frequency of the incident light in the scattered light is measured. This Doppler shift comes from the Brownian motion of the droplets. Since larger droplets show slower Brownian motion, one can obtain information on the droplet size distribution. 

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