travels through a colloidal suspension of particles whose density differs from that of the surrounding medium, inertial forces induced by the vibration of the suspension give rise to a motion of the charged particles relative to the liquid, causing an alternating electromotive force. The manifestations of this electromotive force may be measured, depending on the relation between the impedance of the suspension and that of the measuring instrument, either as colloid vibration potential or as colloid vibration current. Colloid vibration potential and current was first reported by Hermans and then independently by Rutgers in 1938. It is widely used for characterizing the ζ-potential of various dispersions and emulsions. The effect, theory, experimental verification and multiple applications are discussed in the book by Dukhin and Goetz. Electric sonic amplitude Electric sonic amplitude was experimentally discovered by Cannon with co-authors in early 1980s. It is also widely used for characterizing ζ-potential in dispersions and emulsions. There is review of this effect theory, experimental verification and multiple applications published by Hunter. Theory of CVI and ESA With regard to the theory of CVI and ESA, there was an important observation made by O'Brien, who linked these measured parameters with dynamic electrophoretic mobility μd. where A is calibration constant, depending on frequency, but not particles properties; ρp is particle density, ρm density of the fluid, φ is volume fraction of dispersed phase, Dynamic electrophoretic mobility is similar to electrophoretic mobility that appears in electrophoresis theory. They are identical at low frequencies and/or for sufficiently small particles. There are several theories of the dynamic electrophoretic mobility. Their overview is given in the Ref.5. Two of them are the most important. The first one corresponds to the Smoluchowski limit. It yields following simple expression for CVI for sufficiently small particles with negligible CVI frequency dependence: where: ε0 is vacuum dielectric permittivity, εm is fluid dielectric permittivity, ζ is electrokinetic potential η is dynamic viscosity of the fluid, Ks is conductivity of the system, Km is conductivity of the fluid, ρs is density of the system. This remarkably simple equation has same wide range of applicability as Smoluchowski equation for electrophoresis. It is independent on shape of the particles, their concentration. Validity of this equation is restricted with the following two requirements. First, it is valid only for a thin double layer, when the Debye length is much smaller than particle's radius a: Secondly, it neglects the