Electrokinetic sonic amplitude effect

Different from acoustic attenuation spectroscopy, in electroacoustic spectral analysis, sound waves are generated by an applied high frequency electric field across a colloidal suspension and subsequently detected. This is called the electrokinetic sonic amplitude effect (ESA) [38]. These sound waves arise because the alternating electric field pushes the suspended particle forwards and backwards. By measuring the magnitude and phase angle of the sound waves at multiple frequencies (typically from 1-10 MHz), the particle dynamic mobility, Pd, can be determined, provided the concentration and the density of the... 

Electroacoustics — Ultrasound passing through a colloidal dispersion forces the colloidal particles to move back and forth, which leads to a displacement of the double layer around the particles with respect to their centers, and thus induces small electric dipoles. The sum of these dipoles creates a macroscopic AC voltage with the frequency of the sound waves. The latter is called the Colloid Vibration Potential (CVP) [i]. The reverse effect is called Electrokinetic Sonic Amplitude (ESA) effect [ii]. See also Debye effect. 

The colloid vibration potential (difference) E or CVP is the a.c. potential difference measured between two Identical relaxed electrodes, placed in the dispersion if the latter Is subjected to an (ultra)sonlc field. CVP Is a particular case of the more general phenomenon, ultrasonic vibration potential (UVP), applying to any system, whether or not colloids are present. This field sets the particles into a vibrating motion, as a result of which the centres of particle charge and countercharge are periodically displaced with respect to each other. This phenomenon is the a.c. equivalent of that observed in the Dorn effect. Counterpart to this is the electrokinetic sonic amplitude, ESA, the amplitude of the (ultra)sonlc field created by an a.c, electric field in a dispersion. 

When an alternating voltage is applied to a colloid, the particles move back and forth with a velocity that depends on their size and zeta potential and on the frequency of the applied field. As they move, the particles generate sound waves. This phenomenon is called the electroacoustic effect, which can be measured and what was named electrokinetic sonic amplitude (ESA) [5],... 

Because electrophoresis uses optical detection, this technique is limited to the analysis of dilute systems however, the recent development of electroacoustic methods has extended analysis to concentrated slurries containing up to 50% vol/vol solids [73], The electroacoustic effect is the response of charged particles to an applied alternating electrical or acoustical field [74], in contrast to the static field employed in electrophoresis. The acoustical response results from relative vibratory motion between particle and medium if the two phases differ in density. If an alternating electrical field is applied, charged particles vibrate in a back-and-forth motion in phase with the applied field, producing a sound wave whose pressure amplitude is proportional to the particle mobility and This technique is termed electrokinetic sonic amplitude (ESA). Alternatively, if an ultrasonic wave is applied, the particles vibrate at the sound... 

Another method for the determination of electrophoretic mobility which has emerged in recent years is that of the measurement of the electrokinetic sonic amplitude (ESA) for a particle subjected to an alternating current (8). This electroacoustic effect is a result of the oscillation of the particles near the electrodes where a sound wave is produced that can be picked up by a pressure transducer located behind the electrode. The ESA pressure signal is simultaneously proportional to the dynamic mobility of the particle, the particle volume fraction and the density difference between particle and solvent. Thus, the electroacoustic effect is appropriate for concentrated dispersions where conventional electrophoretic methods are inappropriate. However, one disadvantage of the method is that it is not appropriate to systems having low density differences between the particles and suspending liquid. 

Similarly to LFDD, there is a set of electrokinetic techniques that involves ac fields and that can be applied to suspensions of arbitrary particle concentration, as they do not rely on optical techniques of evaluation. These are the so-called electroacoustic techniques, which enable the determination of the dynamic or ac mobility, u, of colloidal particles (the ac counterpart of the dc or classical electrophoretic mobility) as a function of frequency. There are basically two such techniques. One is based on the determination of the electric potential difference induced by the passage of a sound wave through the system it is called colloid vibration potential (CVP) or colloid vibration current (CVI), depending on the quantity measured. In the second technique, reciprocal of CVP or CVI, the basic process is the generation of a pressure wave when an ac electric field is applied to the suspension the amplitude of the sound wave, A sa is known as electrokinetic sonic amplitude, and so we speak of the ESA effect. After the very early works in the subject, O Brien [27,28] was the first author to perform a rigorous investigation on the physical foundations of electroacoustic techniques, and he found that Me is in fact proportional to [28] ... 

The Electrokinetic Sonic Amplitude (ESA) effect in this context refers to the generation of ultrasound by the application of an alternating electric field to a colloid. Previous reviews on the ESA have mainly focused on the determination of particle size and zeta potential from the ESA. While this is certainly a very important application of the ESA phenomenon, there is more information in the ESA spectmm than just particle size and zeta. It can be used, for instance, to determine the thickness of adsorbed polymer layers or the surface conductance under the shear plane. It is these other applications that will be our main interest here. To begin we will give an alternative explanation for the ESA phenomenon, one that allows a deeper understanding of the underlying physics. 

In classical electrokinetic phenomena, the forces and fluxes are independent of time. Electroacoustic effects are analogs of electrophoresis and sedimentation potential in which the forces and fluxes are variable in time. Alternating forces induce alternating fluxes of the same frequency, with a time delay. The phenomenological coefficients between the force and coupled flux can be used to calculate the potential. The phase shift is a source of additional information about the system. The electric sonic amplitude (ESA) is the amplitude of the ultrasonic field... 

---