Ultramicroscope technique

Grimley (G10, Gil) used an ultramicroscope technique to determine the velocities of colloidal particles suspended in a falling film of tap water. It was assumed that the particles moved with the local liquid velocity, so that, by observing the velocities of particles at different distances from the wall, a complete velocity profile could be obtained. These results indicated that the velocity did not follow the semiparabolic pattern predicted by Eq. (11) instead, the maximum velocity occurred a short distance below the free surface, while nearer the wall the experimental results were lower than those given by Eq. (11). It was found, however, that the velocity profile approached the theoretical shape when surface-active material was added and the waves were damped out, and, in the light of later results, it seems probable that the discrepancies in the presence of wavy flow are due to the inclusion of the fluctuating wavy velocities near the free surface. 

The electroosmotic flow rate can then be calculated hy Q = Mave c- Although it is possible to directly measure the electroosmotic mobility using the ultramicroscope technique [1],... 

The electro-osmotic flow rate can then be calculated by Q = Mave c- Although it is possible to directly measure the electro-osmotic mobility using the ultramicroscope technique [1], however, it is common to measure it by means of measuring the average electro-osmotic velocity. Once the average velocity is obtained, the electro-osmotic mobility can be easily determined by /jLqq =... 

Microelectrophoresis is the most common technique for electrokinetic measurements in colloidal systems. Here individual particles can be observed, in their normal environment, under the microscope. Very dilute dispersions can be studied and very small particles, down to about 0.1 pm diameter, can be observed using the dark-field microscope (ultramicroscope). High magnifications allow minimization of observation times, and in polydisperse systems a given size range of particles can be studied to the exclusion of others. 

Similar correlations with d-band character have been found by means of Auger (238) and EELS (239). Where the need arises to pinpoint the oxidation state of a particular transition element in multicomponent or heterogeneous systems (as is often the case in the study of heterogeneous catalysts), a technique such as EELS, which can focus on ultramicroscopic quantities and still yield the number of d-electron states, adds further power to the X-ray absorption method. 

An alternative technique when the particles are too small to be readily seen by an ultramicroscope is that of moving boundary electrophoresis, which has its parallel in hindered settling. Here, the motion of the interface formed between a zone of the suspension and the solvent or dispersion medium is measured under the influence of an electric field. This technique is particularly useful for separating and identifying dissolved macromolecules such as proteins. 

The invention of the ultramicroscope finished a long debate of the nineteenth century on the true composition of colloidal solutions as it eventually disclosed their heterogeneous, disperse nature. For this reason, this technique attracted... 

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