Electrophoretic light scattering, electrically

Beside of the progress in the theory of a particle movement in the zetameter measurement cell, there was progress in particle measurement techniques. New models of zetameters enable automatic measurement of electrophoretic mobility on the basis of the shift of light wave scattered on the particle that moves in the electric field [82]. This technique is called photon correlation spectroscopy (PCS). To increase the sensitivity of the measurement, it is supported by multiangle electrophoretic light scattering (ELS). This combination, allows one also to measure the particle size distribution of the dispersed phase [83]. 

Electrophoretic light scattering (ELS) is commonly used to measure v. The electrophoretic mobility /r can be calculated from v and the known value of E according to Eq. (I). Theoretical models [ I.7-I0] that describe colloidal electrostatics and hydrodynamics can then be used to relate the measured values of n to particle electrical characteristics including surface charge density and surface electric potential. Because /r depends on the surface electrostatic properties but not particle bulk properties, ELS can characterize surface electrostatic properties exclusively for a wide range of colloidal materials. 

Electrophoretic light scattering (ELS) is a subset of DLS (Section II.C.3) that is commonly used to measure the electrophoretic mobility /a of colloidal particles (Section I.A.2). ELS can be used to probe particle surface electrostatic properties via theoretical models of colloidal electrostatics and hydrodynamics that relate H to particle electrical characteristics. 

Electrophoretic light scattering measurements. If an electric held of strength E acts on an isolated charged particle of charge Q, it causes a motion, which is at a velocity u balanced by the friction force. For a sphere one obtains the simple expression... 

The electrical charge of emulsion droplets was determined via zeta-potential measurement by electrophoretic light scattering. The oil droplet size of the emulsions was analysed by static light scattering after dilution of the emulsion to the required optical density or, in the case of spray-dried particles, after dissolution of an aliquot of the microcapsules. Microencapsulation efficiency was calculated from total oil content in the formulation and the gravimetric determination of the oil extracted from the microcapsules with petrol ether [62]. 

Provided that fluid motion is uniform in the illuminated region of the suspension, then similar information may also be extracted by analysis of laser light scattering from particles undergoing electrophoretic motion, that is migratory motion in an electric field, superimposed on that motion. 

The best prospect for this field appears to be the electrophoretic technique. Variants of the light-scattering method such as fluorescence and Raman and infrared intensity fluctuations are also promising, although only the fluorescence technique has been used so far to measure reaction rate constants. Feher (1973) has introduced a technique which directly measures fluctuations of electrical conductivity of dilute electrolyte solutions. These fluctuations may be related to diffusion coefficients and reaction rate constants by methods similar to those described in this chapter. 

Recent development of laser-based instrumentation for electrophoretic mobility determination has made it possible to determine zeta potential of particles suspended in liquid media for systems that were difficult or impossible to study by classical techniques. These instruments measure electrophoretic mobilities by making direct velocity measurements of particles moving in an applied electric field by analyzing the Doppler shift of laser light scattered from the moving particles. Electrophoretic mobilities can then be converted into zeta potentials by use of standard equations 48-50),... 

The electrophoretic motion is either measured microscopically or by light scattering. The former way is called microelectrophoresis and usually employs ultramicroscopes when dealing with colloidal particle systems. The optical instrumentation can be identical to that of DUM, while the software has to be modified because only the displacement in the direction of the electric field is relevant. The method yields a number weighted distribution of zeta-potentials. Similar to DUM, a sufficiently large number of trajectories has to be evaluated in order to keep the statistical uncertainty within an acceptable level. Moreover, the method may be insensitive to weak scatterers within a polydisperse colloidal suspension. 

K. Ito and R. Hayakawa. Quasi-elastic light scattering with the sinusoidal electric field New measurement methods and frequency dispersion of the electrophoretic mobility and diffusion constant of polyions. Colloids and Surfaces A, 148 (1999), 135-148. 

The electronic ink is supplied by E ink corporation (Comiskey 1998). The film consists of electrophoretic microcapsules in a polymer binder, coated on to a 25 pm polyester/indium tin oxide sheet (Fig. 14.10). Optical contrast is achieved by moving black and white sub-micron particles with opposite charge in a transparent fluid within a microcapsule. Depending on which sub-micron particles are closest to the viewer, light is scattered back (white state) or absorbed (black state). The electrophoretic effect is multi-stable - without any electric field the microcapsules keep their switching state. This greatly reduces the power consumption of the display (Ritter 2001). 

The application of laser Doppler velocimetry (LDV) to measure the electrophoretic mobility n of charged colloidal particles is known as laser Doppler electrophoresis (LDE). In a typical LDE experiment, an applied electric field drives the collective motion of charged colloidal particles. The particles pass through an interference pattern created by a dual-beam experimental setup (Section III.A.2). The collective electrophoretic velocity of the particles is then determined via intensity- or spectrum-based analysis of the scattered light, and the electrophoretic mobility n is calculated by dividing the velocity by the applied electric field strength. 

By applying an electric field to a solution of macromolefl les, the molecules are accelerated to a terminal velocity determined by their electrophoretic mobilities.22 The spectrum of scattered light is thereby Doppler-shifted to a frequency determined by Eq. (5.8.15). From the Doppler shift the mobility can be measured, whereas from the width of the Doppler shifted line the diffusion coefficient can be measured [cf. Eq. (5.8.10)]. In this way Ware and Flygare (1971) have measured the electrophoretic... 

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