Colloid vibration current is an electroacoustic phenomenon

Electroacoustic phenomena

Electroacoustic phenomena arise when ultrasound propagates through a fluid containing ions. The associated particle motion generates electric signals because ions have electric charge. This coupling between ultrasound and electric field is called electroacoustic phenomena. Fluid might be a simple...

 that arises when ultrasound

Ultrasound

Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 propagates through a fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

 that contains ions and either solid

Solid

Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...

 particles or emulsion

Emulsion

An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...

 droplets .

The pressure gradient

Pressure gradient

In atmospheric sciences , the pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pressure per unit length...

 in an ultrasonic wave moves particles relative to the fluid. This motion disturbs the double layer

Double layer

Double layer may refer to:* Double layer , a structure in a plasma and consists of two parallel layers with opposite electrical charge* Double layer , a structure that appears on the surface of an object when it is placed into a liquid...

 that exists at the particle-fluid interface

Interface (chemistry)

An interface is a surface forming a common boundary among two different phases, such as an insoluble solid and a liquid, two immiscible liquids or a liquid and an insoluble gas. The importance of the interface depends on which type of system is being treated: the bigger the quotient area/volume,...

. The picture illustrates the mechanism of this distortion. Practically all particles in fluids carry a surface charge

Surface charge

Surface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field...

. This surface charge is screened with an equally-charged diffuse layer; this structure is called the double layer. Ions of the diffuse layer are located in the fluid and can move with the fluid. Fluid motion relative to the particle drags these diffuse ions in the direction of one or the other of the particle's poles. The picture shows ions dragged towards the left hand pole. As a result of this drag, there is an excess of negative ions in the vicinity of the left hand pole and an excess of positive surface charge at the right hand pole. As a result of this charge excess, particles gain a dipole moment

Electric dipole moment

In physics, the electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of charges, that is, a measure of the charge system's overall polarity with SI units of Coulomb-meter...

. These dipole moments generate an electric field that in turn generates measurable electric current. This phenomenon is widely used for measuring zeta potential

Zeta potential

Zeta potential is a scientific term for electrokinetic potential in colloidal systems. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential...

 in concentrated colloids.