Ultramicroscopic surfaces

The velocity of particle migration, v, across the field is a function of the surface charge or zeta potential and is observed visually by means of an ultramicroscope equipped with a calibrated eyepiece and a scale. The movement is measured by timing the individual particles over a certain distance, and the results of approximately 10-15 timing measurements are then averaged. From the measured particle velocity, the electrophoretic mobility (defined as v/E, where E is the potential gradient) can be calculated. 

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. 

Zocher and Stiebel (1930) showed how dark ground ultramicroscopic examination of a surface film yields invaluable information as to the extent to which the material is spread on the surface ( 8). 

Size. Stratum corneum surfaces that impede diflFusion fall into four sizes. The largest structures, such as hair and the cornifled surface, are visible. Cell surfaces, their interfaces, and their elaborations are microscopic. Keratinized elements are ultramicroscopic. Finally, interactions between solvents, solutes, and barrier surfaces occur on the molecular level. The functions of these structures are integrated into an overall barrier capability. 

One must, however, be cautious in translating optical properties of gross fibers into properties of ultramicroscopic fibrils or subfibrillar parts. Theoretical and experimental difficulties result from the marked anisotropy of surface distribution in fibers. Also, the models are derived to account for interband structure primarily, and the considerable band portions of the fibrils may involve distorted other arrangements. 

It is however also conceivable that the flocculation produced consists of cohering ultramicroscopic colloid crystals. In this case the contact surface between colloid-rich and colloid-poor phase could decrease by recrystallisation and thermodynamic equilibrium would be reached when all the very small crystalline individuals had made way for a large crystalline individual. 

The development of colloid chemistry was divided by Wolfgang Ostwald into three main periods (i) Graham (1851-64), (2) Bams and Schneider (1891) and Picton and Linder (1892), who recognised that many colloids contain finely-divided particles of substances in their ordinary state, (3) the invention of the ultramicroscope (Siedentopf and Zsigmondy, 1902) to the present day. Ostwald emphasised the importance of surface energy which results from the fine state of division of one phase in a colloidal system. 

Dehydration Along the Line OOi. — Butschli has observed, and it has been corroborated by work with the ultramicroscope, that the dehydration does not go on from the outside toward the interior, but that holes are formed in the interior free of liquid. It may be assumed that at point 0 the water on the surface capillaries has a radius of curvature corresponding to the vapor tension, as shown in Fig. 20. According to 3, page 145, this would cause the evolution of gas in the interior of the mass much like the effervescence under reduced pressure of a... 

Colloidal State n Particular state in which any substance may exist under the proper conditions, determined by fineness of particle subdivision. The colloidal state is defined by a more or less well-marked ultramicroscopic zone in the scale of subdivision, the lower extreme of the zone approaching molecular dimensions, and the upper end gradually passing over into molecular aggregates (suspensions) visible under the ordinary microscope (Becher P (1989) Dictionary of colloid and surface science. Marcel Dekker, New York). 

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