Electroacoustics / Conductivity

The fundamental method behind the mean pore size measurement is called the seismoelectric effect. An applied ultrasonic pressure wave causes the so-called electrical "double-layer" at the interface between the surface and a suitable liquid (i.e. low conductivity, for example water, polar or non polar organic) to shear, resulting in an oscillating current. When double layers overlap in pores, the seismoelectric effect depends on pore width.

Overlapping double layers produce a pore size relative response.

"Seismoelectric effect: A non-isochoric streaming current. 1.
Experiment." A.S. Dukhin, P.J. Goetz and M. Thommes (2010) J. Coll. Interface Sci., 345, 547-553.

The same seisomoelectric effect is also employed to reveal the zeta potential of the surface inside the pore structure. Propagation of ultrasound through a porous body creates motion of the given liquid, in this case one of high conductivity (to create isolated double-layers) inside of the pores relative to the solid matrix. This, in turn, causes relative motion of charges that are located in the diffuse layer and on the pores' surfaces. This is expressed as an oscillating electric current called the Streaming Vibration Current.

The measurement of percent porosity uses very high frequency conductivity measurements. And unlike direct current conductivity measurements, the high frequency oscillating current reveals the porosity of all pores - including dead-ended (blind) pores - not just those that form a connected pathway on a macroscopic scale.

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