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Electrokinetic separation techniques have been widely used for the analysis and characterization of charged materials of biological origin.3,4 Under terrestrial conditions preparative methods based on zone electrophoresis, isotachophoresis, and isoelectric focusing are prominent in the purification of charged macromolecules and small particles but have only been of limited usefulness in the separation of biological cells and larger particles. The major difficulties in the latter applications arise primarily from In the Apollo 16 experiment, a mixture of polystyrene latices wasprocessed in an improved version of the apparatus used on Apollo14. Whereas with such a device it is not possible to conduct theprocessing under terrestrial conditions without the appearance ofextensive thermal convection which destroys the separation bound-aries, under microgravity conditions flight photographs showed noboundary deterioration, but did indicate that electroosmosis was amajor factor in producing overlapping elongated sample zones in thecylindrical tube. Consequently, more subtle perturbations ofsample zones were not distinguishable.

From the contact angle θ determined with one liquid (of known surface tension) only, it is possible to obtain the values of the surface tensions γSV and γSL, via the equation of state (1): [image omitted]  where the subscripts S and L stand for the solid S and the liquid L, whilst the subscript V stands for vapor. In conjunction with Young's equation (2): [image omitted]  relation [l] yields: [image omitted]  With the help of a computer program values for γsv (1) and for γSl (3) can be derived for different values of the contact angle 8 obtained with a liquid of given γLV

Electrophoresis is the transport of dissolved molecules or suspended particles in a homogeneous polar liquid (such as water) under the influence of an electric field. Most molecules or particles acquire a surface electric charge when dissolved or suspended in buffered water (or other polar liquids), owing to ionization or adsorption of ions present in the water. The sign of the surface charge of molecules or particles determines whether they will migrate towards the positive or the negative electrode of the applied electric field, and the velocity of migration depends on the surface potential of the molecules or particles, as well as on the potential of the electric field.

It has long been surmised, as a theoretical curiosity, that conditions could arise under which the sign of the van der Waals interaction between two different uncharged bodies, surrounded by a liquid, might be negative, i.e. that such bodies would repel each other. This possibility is implicit in Hamaker's classical paper on van der Waals-London interactions, although, remarkably enough, Hamaker somehow became persuaded that this view was erroneous, and wrote his paper with the apparent purpose to gainsay it: "If two particles are embedded in a fluid and the London-van der Walls force between particles and fluid is greater than between the particles themselves, it might be thought that the resultant action will be a repulsion rather than an attraction. As has been pointed out to the author by Dr. J.H. de Boer, this conclusion does not hold. Owing to a peculiar property of the London-van der Waals forces, the resultant force is generally attractive even when the particles are surrounded by fluid. This is a matter of considerable interest which warrants a detailed discussion."1 However, In a more recent review on Hamaker constants, Visser stated explicitly: "When two materials are immersed in a liquid medium, and the interactions of each of these materials with that of the liquid medium is larger than the interaction between these materials themselves, spontaneous separation can occur due to dispersion forces only."2 Fowkes demonstrated such a repulsive interaction with poly (tetrafluoroethylene)-glycol-iron oxide.3

As a general rule the electrophoretic mobility of molecules and particles is solely governed by the potential at their surface of shear (ζ-potential) and does not depend on their size or shape. There are hawever two sets of exceptional conditions under which the size or shape of molecules and particles may influence their electrophoretic mobility. These are:1) Conditions under which the ratio between the dimensions of the molecules or particles and the thickness of their diffuse ionic double layer is such that the distortion of the electric field by the particles is neither at its minimum nor at its maximum.2) Conditions under which the molecules or particles are migrating under the influence of an electric field inside a porous medium, the size of the pores of which is such that they impede the larger molecules or particles more than the smaller ones.