Electroacoustic techniques

Particle mobility and zeta potential can now be measured by more sophisticated techniques. With photoelectrophoresis, particle mobility is measured as a function of pH under the influence of ultraviolet radiation. At pH < 8, the electrophoretic mobility of irradiated hematite particles (A = 520 nm) was markedly different from that measured in the absence of UV irradiation. This was attributed to the development of a positive surface charge induced by photo-oxidation of the surface Fe-OH° sites to (Fe-OH) sites (Zhang et al., 1993). The electroacoustic technique involves generation of sound waves by the particles in the colloidal dispersion and from this data. 

Use of Ultrasonic Vibration Potential To Monitor Coalescence. The complex chemical nature of crude oils makes it difficult to relate the dispersion behavior to the physicochemical properties at the crude-oil-water interface. In addition, the nonpolar and nontransparent nature of the oleic phase provides significant obstacles for studies of the interactions of the suspended water droplets in real systems. Recent development (28, 29) of electroacoustical techniques has shown considerable promise for electrokinetic measurements of colloidal systems and the direct monitoring of the rate and extent of coagulation (flocculation and coalescence) of water droplets in nontransparent water-in-oil media. The electroacoustic measurement for colloidal systems in nonpolar media is based on the ultrasound vibration potential (UVP) mode, which involves the applica-... 

In this paper the use of electroacoustic techniques involving the application of a sonic field and the detection of an electric field, for monitoring coalescence of water droplets in non-polar media will be discussed. This technique was used to evaluate the rate and extent of dewatering in oil continuous emulsions when surface active chemicals were added. The results showed that a combination of an oil soluble demulsifier and water soluble surfactant was substantially more effective in causing droplet coalesence than the individual components. An explanation for these findings were based on studies of time-dependent interfacial tensions at the oil/water interface and electrokinetic properties. The results indicated that a direct relationship exists between the adsorption behavior at the oil/water interface (apparent rate of spreading) and emulsion stability. 

The zeta-potential, which is traditionally measured with electrokinetic and electroacoustic techniques, is an important parameter for the computation of particle-particle interactions and the corresponding electroviscous effects. However, it does not reveal the complete structure of the electric double layer, in particular its immobile part (Stern layer). New arising techniques that are sensitive to the real interface without affecting the double layer equilibrium may facilitate a complementary characterisation. 

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] ... 

Surface conductance also has pronounced effects on the usual electrophoretic mobUity it can lead to a large drop in the mobUity and therefore to a large error in the reported zeta potential obtained using standard electrophoresis formulae. To determine the true zeta potential in these systems it is necessary to determine the surface conductance. The electroacoustic technique provides the most convenient method for doing this. 

Rowell and co-workers [62-64] have developed an electrophoretic fingerprint to uniquely characterize the properties of charged colloidal particles. They present contour diagrams of the electrophoretic mobility as a function of the suspension pH and specific conductance, pX. These fingerprints illustrate anomalies and specific characteristics of the charged colloidal surface. A more sophisticated electroacoustic measurement provides the particle size distribution and potential in a polydisperse suspension. Not limited to dilute suspensions, in this experiment, one characterizes the sonic waves generated by the motion of particles in an alternating electric field. O Brien and co-workers have an excellent review of this technique [65]. 

Recent advances have been made in the theory and application of acoustic and electroacoustic spectroscopies for measuring the particle size distribution (PSD) and -potential of colloidal suspensions, respectively.67-69 To date, the use of acoustics has been confined mainly to industrial applications, despite the clear potential for the technique to characterize colloids with environmental or agricultural significance. 

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... 

Chemical and physical processing techniques for ferroelectric thin films have undergone explosive advancement in the past few years (see Ref. 1, for example). The use of PZT (PbZri- cTi c03) family ferroelectrics in the nonvolatile and dynamic random access memory applications present potentially large markets [2]. Other thin-film devices based on a wide variety of ferroelectrics have also been explored. These include multilayer thin-film capacitors [3], piezoelectric or electroacoustic transducer and piezoelectric actuators [4-6], piezoelectric ultrasonic micromotors [7], high-frequency surface acoustic devices [8,9], pyroelectric intrared (IR) detectors [10-12], ferroelectric/photoconduc-tive displays [13], electrooptic waveguide devices or optical modulators [14], and ferroelectric gate and metal/insulator/semiconductor transistor (MIST) devices [15,16]. 

Acoustics has a related field that is usually referred to as electroacoustics (8). Electroacoustics can provide particle size distribution as well as zeta potential. This relatively new technique is more complex than acoustics because an additional electric field is involved. As a result, both hardware and theory become more complicated. There are even two different versions of electroacoustics depending on what field is used as a driving force. Electrokinetic sonic amplitude (ESA) involves the generation of sound energy caused by the driving force of an applied electric field. Colloid vibration current (CVC) is the phenomenon where sound energy is applied to a system and a resultant eleetrie field or eurrent is created by the vibration of the colloid electric double layers. 

We are optimistic about the future of acoustics in colloid science. It is amazing what this technique can do especially in combination with electroacoustics for characterizing electric surface properties. We hope that this review will allow you to taste the power and opportunities related to these sound-based techniques. 

The DT-1200 has two separate sensors for measuring acoustic and electroacoustic signals separately. Both sensors use the pulse technique. The acoustic sensor has two piezo crystal transducers. The gap between the transmitter and receiver is variable in steps. In default mode, the gap changes from 0.15 mm up to 20 mm in 21 steps. The basic frequency of the pulse changes in steps as well. In default mode, the frequency changes from 3 to 100 MHz in 18 steps. The number of pulses collected for each gap and frequency is automatically adjustable in order to reach the target signal-to-noise ratio. 

The combination of acoustic and electroacoustic spectroscopy provides a much more reliable and complete characterization of the disperse system than either one of those techniques separately. Electroacoustic phenomena are more complicated to interpret than acoustic phenomena because an additional field (electric) is involved. This problem becomes even more pronounced for a concentrated system. It makes acoustics favorable for characterizing particle size, whereas electroacoustics yields electric surface properties. 

Electroacoustics provide a unique opportunity to estimate both the size of emulsion droplets and the state of the surface (kinetic) charge in a single measurement. The next two chapters, by Hunter and by A. Dukhin, Wines, Goetz and Somasundaran describe in detail the advantages of these techniques and their current development. The latter chapter also describes the application of acoustic techniques to microemulsion systems, revealing interesting structural details. 

Hunter, R. J., The electroacoustic characterization of colloidal suspensions, in Handbook on Ultrasonic and Dielectric Characterization Techniques for Suspended Particulates, Hackley, V. A. and Texter, J. (Eds), American Ceramic Society, Westerville, OH, 1998, pp. 25-46. 

Electroacoustic phenomena. They are electrokinetic phenomena that have recently gained interest, both experimentally and theoretically. In the ESA (electrokinetic sonic amplitude) technique, an alternating electric field is applied to the suspension and the sound wave produced in the system is detected and analyzed. The colloid vibration potential (CVP) or colloid vibration current (CVI) is the reciprocal of the former a mechanical (ultrasonic) wave is forced to propagate through the system, and the resulting alternating potential difference (or current) is measured. 

In this paper we examine the role of mixed surfactants in the demulsification of water-in-Leduc oil emulsion by application of the spreading rate method which is then correlated with the electroacoustic results and centrifugation. Microelectrophoresis using the reverse emulsion was also used to investigate the adsorption process. The results show both a very good correspondence between the various techniques and provide insight on the synergistic adsorption behavior of the hydrophobic and hydrophilic surfactants. 

An electroacoustic method based on the application of a sonic field and detection of the electric signal proved to be a reliable technique for measuring the kinetics of coagulation/coalesence process of colloidal systems in non-polar and non-transparent media like water-in-crude oil emulsion. 

In the last decade the electroacoustic waves in colloids [37] attracted considerable attention. Electrically induced concentration fluctuations were detected by means of different experimental techniques. The Bragg diffraction technique was used to analyze the concentration gradients induced in colloidal crystals by dc electric pulses [38], Density fluctuations in ac electric fields were observed with time-resolved reflection spectroscopy and transmitted light spectroscopy [39,40]. The conventional light scattering was also applied to detect the induced translation... 

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