Dielectrophoresis
Encyclopedia
Dielectrophoresis is a phenomenon in which a force
is exerted on a dielectric
particle when it is subjected to a non-uniform electric field
. This force does not require the particle to be charged
. All particles exhibit dielectrophoretic activity in the presence of electric fields. However, the strength of the force depends strongly on the medium and particles' electrical properties, on the particles' shape and size, as well as on the frequency of the electric field. Consequently, fields of a particular frequency can manipulate particles with great selectivity. This has allowed, for example, the separation of cells or the orientation and manipulation of nanoparticles and nanowires.
,nanoparticles and cell
s.
Pohl wrote in his book defining dielectrophoresis as the translational motion of neutral matter caused by polarization effects in a nonuniform electric field. The phenomenological bases are catalogued below:
Phenomena associated with dielectrophoresis are electrorotation
and traveling wave dielectrophoresis (TWDEP). Dielectrophoresis can be enhanced using multiple frequencies, in what is known as multiple-frequency dielectrophoresis (MFDEP).
and length 
(
) with complex dielectric constant 
in a medium with complex dielectric constant 
the time-dependent dielectrophoretic force is given by:
The complex dielectric constant is
, where 
is the dielectric constant
,
is the electrical conductivity, 
is the field frequency, and 
is the imaginary number
.
This equation is accurate for highly elongated ellipsoids when the electric field gradients are not very large (e.g., close to electrode edges). The equation only takes into account the dipole
formed and not higher order polarization
. When the electric field gradients are large, higher order terms become relevant, and result in higher forces.
To be precise, the time-dependent equation only applies to lossless particles, because loss creates a lag between the field and the induced dipole. When averaged, the effect cancels out and the equation holds true for lossy particles as well. An equivalent time-averaged equation can be easily obtained by replacing E with Erms, or, for sinusoidal voltages by dividing the right hand side by 2.
For a homogeneous sphere of radius
and complex permittivity 
in a medium with complex permittivity 
the (time-averaged) DEP force is:
The factor in curly brackets is known as the complex Clausius-Mossotti function
and contains all the frequency dependence of the DEP force.
.
Dielectrophoresis Field-Flow Fractionation, introduced by Davis and Giddings, is a family of chromatographic-like separation methods. In DEP-FFF, DEP forces are combined with drag flow to fractionate a sample of different types of particles. Particles are injected into a carrier flow that passes through the separation chamber, with an external separating force (a DEP force) being applied perpendicular to the flow. By means of different factors, such as diffusion and steric, hydrodynamic, dielectric and other effects, or a combination thereof, particles (<1 μm in diameter) with different dielectric or diffusive properties attain different positions away from the chamber wall, which, in turn, exhibit different characteristic concentration profile. Particles that move further away from the wall reach higher positions in the parabolic velocity profile of the liquid flowing through the chamber and will be eluted from the chamber at a faster rate.
, as the intensity of light is about 1000 times less.
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...
is exerted on a dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
particle when it is subjected to a non-uniform electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. This force does not require the particle to be charged
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
. All particles exhibit dielectrophoretic activity in the presence of electric fields. However, the strength of the force depends strongly on the medium and particles' electrical properties, on the particles' shape and size, as well as on the frequency of the electric field. Consequently, fields of a particular frequency can manipulate particles with great selectivity. This has allowed, for example, the separation of cells or the orientation and manipulation of nanoparticles and nanowires.
Background and Properties
Publications on dielectrophoresis go as far back as the 1950s, by Herbert Pohl. Recently, dielectrophoresis has been revived due to its potential in the manipulation of microparticlesMicroparticles
Microparticles are particles between 0.1 and 100 \mum in size. Commercially available microparticles are available a wide variety of materials, including those made of ceramics, glass, polymers, and metals. One encounters microparticles everyday in nature with items such as pollen, sand, and dust...
,nanoparticles and cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s.
Pohl wrote in his book defining dielectrophoresis as the translational motion of neutral matter caused by polarization effects in a nonuniform electric field. The phenomenological bases are catalogued below:
- The dielectrophoresis force can be seen only when particles are in nonuniform electric fields.
- Since the dielectrophoresis force does not depend on the polarity of the electric field, the phenomenon can be observed either with AC or DC excitation.
- Particles are attracted to regions of stronger electric field when their permittivityPermittivityIn electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
exceeds that of the suspension medium. - When the permittivity of the medium is greater than that of the particles, the motion of the particles is to lesser electric field.
- DEP is most readily observed for particles with diameters ranging from approximately 1 to 1000 μm. Above 1000 μm, gravity overwhelms DEP; below 1 μm, Brownian motionBrownian motionBrownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...
overwhelms the DEP forces.
Phenomena associated with dielectrophoresis are electrorotation
Electrorotation
Electrorotation is the circular movement of an electrically polarized particle. Similar to the slip of an electric motor, it can arise from a phase lag between an applied rotating electric field and the respective relaxation processes and may thus be used to investigate the processes or, if these...
and traveling wave dielectrophoresis (TWDEP). Dielectrophoresis can be enhanced using multiple frequencies, in what is known as multiple-frequency dielectrophoresis (MFDEP).
Dielectrophoretic force
For a field-aligned prolate ellipsoid of radius and length () with complex dielectric constant in a medium with complex dielectric constant the time-dependent dielectrophoretic force is given by:The complex dielectric constant is
, where is the dielectric constantDielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
,
is the electrical conductivity, is the field frequency, and is the imaginary numberImaginary number
An imaginary number is any number whose square is a real number less than zero. When any real number is squared, the result is never negative, but the square of an imaginary number is always negative...
.
This equation is accurate for highly elongated ellipsoids when the electric field gradients are not very large (e.g., close to electrode edges). The equation only takes into account the dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
formed and not higher order polarization
Multipole expansion
A multipole expansion is a mathematical series representing a function that depends on angles — usually the two angles on a sphere. These series are useful because they can often be truncated, meaning that only the first few terms need to be retained for a good approximation to the original...
. When the electric field gradients are large, higher order terms become relevant, and result in higher forces.
To be precise, the time-dependent equation only applies to lossless particles, because loss creates a lag between the field and the induced dipole. When averaged, the effect cancels out and the equation holds true for lossy particles as well. An equivalent time-averaged equation can be easily obtained by replacing E with Erms, or, for sinusoidal voltages by dividing the right hand side by 2.
For a homogeneous sphere of radius
and complex permittivity in a medium with complex permittivity the (time-averaged) DEP force is:The factor in curly brackets is known as the complex Clausius-Mossotti function
Clausius-Mossotti relation
The Clausius–Mossotti relation is named after the Italian physicist Ottaviano-Fabrizio Mossotti, whose 1850 book analyzed the relationship between the dielectric constants of two different media, and the German physicist Rudolf Clausius, who gave the formula explicitly in his 1879 book in the...
and contains all the frequency dependence of the DEP force.
Applications of Dielectrophoresis
Dielectrophoresis can be used to manipulate, transport, separate and sort different types of particles. Since biological cells have dielectric properties, dielectrophoresis has many medical applications. Prototypes that separate cancer cells from healthy cells have been made. Platelets have been separated from whole blood with a DEP-activated cell sorterFlow cytometry
Flow cytometry is a technique for counting and examining microscopic particles, such as cells and chromosomes, by suspending them in a stream of fluid and passing them by an electronic detection apparatus. It allows simultaneous multiparametric analysis of the physical and/or chemical...
.
Dielectrophoresis Field-Flow Fractionation (DEP-FFF)
The utilization of the difference between dielectrophoretic forces exerted on different particles in nonuniform electric fields is known as DEP separation. The exploitation of DEP forces has been classified into two groups: DEP migration and DEP retention. DEP migration uses DEP forces that exert opposite signs of force on different particle types to attract some of the particles and repel others. DEP retention uses the balance between DEP and fluid-flow forces. Particles experiencing repulsive and weak attractive DEP forces are eluted by fluid flow, whereas particles experiencing strong attractive DEP forces are trapped at electrode edges against flow drag.Dielectrophoresis Field-Flow Fractionation, introduced by Davis and Giddings, is a family of chromatographic-like separation methods. In DEP-FFF, DEP forces are combined with drag flow to fractionate a sample of different types of particles. Particles are injected into a carrier flow that passes through the separation chamber, with an external separating force (a DEP force) being applied perpendicular to the flow. By means of different factors, such as diffusion and steric, hydrodynamic, dielectric and other effects, or a combination thereof, particles (<1 μm in diameter) with different dielectric or diffusive properties attain different positions away from the chamber wall, which, in turn, exhibit different characteristic concentration profile. Particles that move further away from the wall reach higher positions in the parabolic velocity profile of the liquid flowing through the chamber and will be eluted from the chamber at a faster rate.
Optical Dielectrophoresis
The use of photoconductive materials (for example, in lab-on-chip devices) allows for localized inducement of dielectrophoretic forces through the application of light. In addition, one can project an image to induce forces in a patterned illumination area, allowing for some complex manipulations. When manipulating living cells, optical dielectrophoresis provides a non-damaging alternative to optical tweezersOptical tweezers
Optical tweezers are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force , depending on the refractive index mismatch to physically hold and move microscopic dielectric objects...
, as the intensity of light is about 1000 times less.
External links
- The American Electrophoresis Society: Dielectrophoresis
- Dielectrophoresis: a spherical shell model
- On the Relationship of Dielectrophoresis and Electrowetting
- Biological cell separation using dielectrophoresis in a microfluidic device
- Sandia’s dielectrophoresis device may revolutionize sample preparation
- Institute of Bioelectronic and Molecular Microsystems: Dielectrophoresis