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Electroacoustics

Using Electroacoustic Phenomena for Pore size Analysis


There are various electroacoustic phenomena which can be generated when high frequency sound propagates through a fluid containing ions including electric sonic amplitude (ESA) typical in heterogeneous fluids and the ion vibration current (IVI) in homogeneous fluids. However, the electroacoustic response used to measure pore size is the streaming vibration current (SVI).

All of these effects are the result of charged particles in the sample moving under the influence of an applied ultrasonic signal. This movement generates a current or potential (depending on how it is measured) that is characteristic of the properties of the material. The electroacoustic effect can also work in the other direction, i.e. when a voltage is applied to a sample it produces an acoustic signal.

Seismoelectric Effect

Electroacoustics uses the seismoelectric effect to determine the mean pore size of a material. The seismoelectric effect is an electrokinetic phenomenon and is basically a type of a streaming current. Streaming currents are generated when a fluid passes over a solid and the electrical double layer at the phase boundary generates a thin layer of counterions. The movement of these counterions then generates an electric current.

When ultrasound is applied to a wetted porous material the resulting electroseismic current is dependent on the samples porosity – this is due to the electrical double layer. Unlike in a streaming current which is generated by its shearing, at higher frequencies the current generated is due to the compressibility of the double layer which acts like a parallel plate capacitor.

The seismoelectric effect is also influenced in a predictable manner by the overlapping of double layers in pores and can therefore be measured to determine the mean pore size.

History of Electroacoustics

1800 Nicholson and Carlisle shows that an electric current could decompose water into oxygen and hydrogen

1809 Reuss reports that applying an electric field to clay particles dispersed in water produces both particle motion and liquid motion (electrophoresis and electro-osmosis)

1853 Helmholtz develops model of the double layer while studying colloidal suspensions

1859 Quincke observes streaming potential

1879 Helmholtz publishes a mathematical expression for the streaming potential.

1903 Smoluchowski develops a theory to calculate zeta potential (electrokinetic potential in colloidal systems) from electrokinetic mobility

1932 Bikerman shows that each electric streaming potential causes an electric current in the contrary direction

1933 Debye predicts Ion Vibration Current (IVI) resulting from the difference in the effective mass or friction coefficient between anions and cations

1938 Hermans and Rutgers independently report Colloid Vibration Potential (CVP)

1944 Frenkel predicts that the electro-seismic electric field strength is proportional to porosity and independent of pore size

1948 Williams observes Streaming Vibration Current (SVI)

1983 Dukhin et al develop theoretical model for Streaming Vibration Current (SVI)


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