Electrokinetic Effects with Small Particles

Electrokinetic phenomena is a generic term applied to effects associated with the movement of ionic solutions near charged interfaces. Although a respectable antiquity can be claimed for· the subject, our understanding of the fluid mechanics begins with Smoluchowski's studies in the early part of the twentieth century. Much of the current interest stems from the use of electrokinetic measurements in bio­ chemistry and biophysics. Although electrokinetic phenomena play important roles in many diverse natural and technological processes no attempt is made to deal with their ubiquitous nature here. Instead our understanding of two archetypal problems is reviewed. Other reviews may be consulted for insight into the diversity of the phenomena (see Abramson 1934; Abramson, Moyer Overbeek 1950; Overbeek Dukhin & Deryagun 1974). Another reason for current interest in these phenomena derives from their roles in the bulk behavior of suspensions. A focus of the work on suspensions is the use of micro scale phenomena, e.g. flow around a single particle, to explain bulk behavior. Considerable attention has been paid to suspensions of uncharged particles but not to charged particles in ionic solutions. Here attention focuses on the description of the two prototypical problems that serve as the basis for an understanding of the bulk behavior. These problems are the motion of a small charged particle in response to an externally applied field and the behavior of a similar particle exposed to some sort of flow without any externally applied field. In both cases the essential complicating feature is the presence of diffuse space charge arising from the response of the ions in solution to the presence of the charged interface. The basis for the description of the phenomena is reviewed first with emphasis on the ways in which electrokinetic effects alter the conservation equations. The current understanding of the two archetypal problems is reviewed in subsequent sections.