Transport mechanisms of biological colloids.

An effort was made to distinguish among various mechanisms by which colloidal particles can be transported through liquid solutions. Figures 1-3 provide a visual (Figure: see text) summary of these differences. The various "phoretic" motions (electrophoresis, diffusiophoresis, osmophoresis) differ from sedimentation in that the driving forces operate within a thin interfacial layer at the surface of a particle rather than on the bulk of the article. Because of these surface forces, the velocity field of solvent dragged by a particle undergoing phoretic motion is much weaker (O(r-3)) than in the case of sedimentation (O(r-1)). As a consequence, particle-particle and particle-pore interactions are weak for the phoretic motions; Figures 4 and 5 demonstrate this point. It should be emphasized that the results presented in this paper are strictly valid only for very thin interfacial layers (kappa-1 much less than a, L and K much less than a). The Einstein relation (Eq. 1), which applies to sedimentation and diffusion with Equation 2 used for f, has been incorrect for the phoretic transport. As shown in Table 2, there is no unique value of f that fits all the phenomena, and in fact the magnitude of f for phoretic motion greatly exceeds the Stokes' law coefficient. Unfortunately, Equation 1 is well ingrained in the literature; one objective here was to bring its inadequacies to light. The phoretic transport processes offer important advantages in separations because of the selectivity that may be achieved through surface chemistry. For example, experiments are underway that demonstrate transport of particles across porous membranes by diffusiophoresis. It may prove economically attractive to develop both analytical and preparative separation methods that rely more on the surface properties of biological colloids rather than just their size.