[6] The electrical double layer can be regarded as behaving like a parallel plate capacitor with a compressible dielectric filling.
When sound waves induce a local pressure variation, the spacing of the plates varies at the frequency of the excitation, generating an AC displacement current normal to the interface.
When a sound wave 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.
[11] With regard to the theory of CVI and ESA, there was an important observation made by O'Brien,[12] who linked these measured parameters with dynamic electrophoretic mobility μd.
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 contribution of the surface conductivity.
This assumes a small Dukhin number: Restriction of the thin double layer limits applicability of this Smoluchowski type theory only to aqueous systems with sufficiently large particles and not very low ionic strength.
This allows introduction of so-called "quasi-homogeneous" approach, when overlapped diffuse layers of particles cover the complete interparticle space.
[13] Their derivation predicts that surface charge density σ is a better parameter than ζ-potential for characterizing electroacoustic phenomena in such systems.