Electrokinetic phenomena
Encyclopedia
Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids or in porous bodies filled with fluid. The term heterogeneous here means a fluid containing particles. Particles can be 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...

, liquid or gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

bubbles with sizes on the scale of a micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

or nanometer.

There is a common source of all these effects — the so-called interfacial 'double layer'
Double layer (interfacial)
A double layer is a structure that appears on the surface of an object when it is placed into a liquid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object...

of charges. Influence of an external force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

on the diffuse layer generates tangential motion of a fluid with respect to an adjacent charged surface. This force might be electric, 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...

, concentration gradient, gravity. In addition, the moving phase might be either continuous 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....

or dispersed phase.

Various combinations of the driving force and moving phase determine various electrokinetic effects. Following "Fundamentals of Interface and Colloid Science" by Lyklema (1995), the complete family of electrokinetic phenomena includes:
• electrophoresis
Electrophoresis
Electrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...

, as motion of particles under influence of electric field;
• electro-osmosis
Electro-osmosis
Electroosmotic flow is the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit...

, as motion of liquid in porous body under influence of electric field;
• diffusiphoresis
Diffusiphoresis
Diffusiophoresis is a motion of dispersed particles in a fluid induced by a diffusion gradient of molecular substances that are dissolved in the fluid. This gradient affects structure of the particles in an interfacial double layer and causes sliding motion of the fluid relative to the particle...

, as motion of particles under influence of a chemical potential
Chemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...

In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....

;
• capillary osmosis, as motion of liquid in porous body under influence of the chemical potential
Chemical potential
Chemical potential, symbolized by μ, is a measure first described by the American engineer, chemist and mathematical physicist Josiah Willard Gibbs. It is the potential that a substance has to produce in order to alter a system...

In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....

;
• sedimentation potential
Sedimentation potential
Sedimentation potential occurs due to dispersed particles motion relative to the fluid under influence of either gravity or centrifugation. This motion disrupts equilibrium symmetry of the particles double layer. Enveloping viscous flow around the particles drags ions of the diffuse layer away from...

, as electric field generated by sedimenting colloid
Colloid
A colloid is a substance microscopically dispersed evenly throughout another substance.A colloidal system consists of two separate phases: a dispersed phase and a continuous phase . A colloidal system may be solid, liquid, or gaseous.Many familiar substances are colloids, as shown in the chart below...

particles;
• streaming potential/current, as either electric potential or current generated by fluid moving through porous body, or relative to flat surface;
• colloid vibration current
Colloid vibration current
Colloid vibration current is an electroacoustic phenomenon that arises when ultrasound propagates through a fluid that contains ions and either solid particles or emulsion droplets ....

, as electric current generated by particles moving in fluid under influence of 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...

;
• electric sonic amplitude
Electric sonic amplitude
Electric sonic amplitude is an electroacoustic phenomenon that is the reverse to colloid vibration current. It occurs in colloids, emulsions and other heterogeneous fluids under the influence of an oscillating electric field...

, as ultrasound generated by colloidal particles in oscillating electric field.

There are detailed descriptions of electrokinetic phenomena in many books on colloid and interface science
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