Light Scattering Brownian movement

Photon Correlation Spectroscopy. Photon correlation spectroscopy (pcs), also commonly referred to as quasi-elastic light scattering (qels) or dynamic light scattering (dls), is a technique in which the size of submicrometer particles dispersed in a Hquid medium is deduced from the random movement caused by Brownian diffusion motion. This technique has been used for a wide variety of materials (60—62). 

Colloidal dispersions may appear either translucent or cloudy, depending on the type of colloid and the degree of particle concentration and dispersion. The colloidal particles cannot be easily distinguished from water. They possess properties that are very different from other solid settable suspensions and from solutions. When the colloidal particles are < 5 pm, they have erratic aleatory movements known as Brownian movements, caused by collisions with molecules from the dispersion medium. When a light beam passes through a colloidal dispersion, this reflects and scatters light (Tyndall effect). 

The difference between elastic and "quasielastic" measurements is that in the latter, small changes in the frequency due to the translational ("Brownian") movement of the scattering particles are also measured. The broadness of the intensity distribution of the emitted light for frequencies around the primary monocluomatic beam frequency is directly related to the diffusion coefficient of the particles, which can then be related to the hydrodynamic radius if a model for the particle shape is available Dynamic light scattering can thus be used to follow the kinetics of particle coagulation by following the decrease in diffusion coefficient as the particle size increases. ... 

Micellar charge can be estimated by electrophoresis [22], electrochemically [49-51 ], or by analysis of the Brownian movement by quasi-elastic light scattering [46-48]. The use of ion-selective electrodes depends on the observation that the electrode... 

Dynamic light scattering involves the study of time-dependent fiuctuations in the intensity of scattered light which are the result of the Brownian motion of the particles. The random movement of the particles causes the distances between the particles to fiuctuate, causing constantly varying constructive and destructive interference patterns. This time-dependent fiuctuation in the intensity can be correlated with itself to obtain the diffusion coefficient of the particles. This method is also called photon correlation spectroscopy (PCS) and quasi-elastic light scattering (QELS) [21, 22]. 

Photon correlation spectroscopy (PCS) has been used extensively for the sizing of submicrometer particles and is now the accepted technique in most sizing determinations. PCS is based on the Brownian motion that colloidal particles undergo, where they are in constant, random motion due to the bombardment of solvent (or gas) molecules surrounding them. The time dependence of the fluctuations in intensity of scattered light from particles undergoing Brownian motion is a function of the size of the particles. Smaller particles move more rapidly than larger ones and the amount of movement is defined by the diffusion coefficient or translational diffusion coefficient, which can be related to size by the Stokes-Einstein equation, as described by... 

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