Dielectrophoresis - Melissa Deschamps

Introduction

A dielectric particle is placed in an electric field where it becomes electrically polarized as a result in the partial charge separation which leads to an induced dipole moment. The dipole movement is the consequence of equal and opposite charges [4]. The magnitude of the dipole depends on the polarizability of the particle and the type of medium. If a particle polarizability is higher than the medium the particle will go towards regions of higher electric field. If the medium has a higher polarizability than the particle will be driven towards the low field strength region [5]. This is called positive and negative DEP, respectively.

Background and Theory

Figure 1. (A) Electrophoresis, charged and neutral particle (B) DEP, A neutral particle in a nonuniform electric field

A dielectric particle is placed in an electric field where it becomes electrically polarized as a result in the partial charge separation which leads to an induced dipole moment. The dipole movement is the consequence of equal and opposite charges [4]. The magnitude of the dipole depends on the polarizability of the particle and the type of medium. If a particle polarizability is higher than the medium the particle will go towards regions of higher electric field. If the medium has a higher polarizability than the particle will be driven towards the low field strength region [5]. This is called positive and negative DEP, respectively.

Figure 1. (A) Positive DEP (B) Negative DEP [5]

Dielectrophoretic Force

A particle in an electric field experience the lateral force described in equation one.

[math]\displaystyle{ \bar{F}_\mathrm{elec} =q\bar{E}+(\bar{m}\nabla\bar{E}+\frac{1}{6}\nabla(\overrightarrow{Q}:\nabla\bar{E})+... (1) }[/math]

Where [6]:

[math]\displaystyle{ \nabla }[/math] is the del operator

[math]\displaystyle{ q\bar{E} }[/math] is interaction between the net charge of particle [math]\displaystyle{ (q) }[/math] to the electric field [math]\displaystyle{ (\bar{E}) }[/math]

[math]\displaystyle{ \bar{m} }[/math] dipole force component

[math]\displaystyle{ \overrightarrow{Q} }[/math] is the Quadrupole Tensor

The time-averaged DEP force due to an imposed electrical field can be approximated in terms of dipole effects for a sphere using equation one [6].

[math]\displaystyle{ \langle F_\mathrm{DEP} \rangle = 2\pi r^3\varepsilon_m \textrm{Re}\left\{\frac{\varepsilon^*_p - \varepsilon^*_m}{\varepsilon^*_p + 2\varepsilon^*_m}\right\}\nabla \left|\vec{E}_{rms}\right|^2 (2) }[/math]

Where

[math]\displaystyle{ \nabla \left|\vec{E}_{rms}\right|^2 }[/math] is the gradient of the square of the electric field quantifying the non-uniformity of the electric field

[math]\displaystyle{ 2\pi r^3\varepsilon_m }[/math] is the surface area of square

[math]\displaystyle{ {Re} }[/math] Takes real component of the Clausius-Mossotti Factor

[math]\displaystyle{ \varepsilon }[/math] permitivity

The Clausius-Mossotti Factor, equation 3, is the effective polarizability of the particle and its medium [5]

[math]\displaystyle{ \frac{\varepsilon^*_p - \varepsilon^*_m}{\varepsilon^*_p + 2\varepsilon^*_m} (3) }[/math]

Where

[math]\displaystyle{ \varepsilon^*_p, \varepsilon^*_m }[/math] is the complex dielectric characteristics of the particle and the medium [math]\displaystyle{ \varepsilon^*=\varepsilon=-j\omega\sigma }[/math] [5]

[math]\displaystyle{ \sigma }[/math] conductivity

[math]\displaystyle{ \omega }[/math] angular frequency

The above equation has a few simplifications to get the final form. One the particles are homogeneous dielectric and show no conductive loses. The electric field is assumed to be nonuniform and to be only dipole movement and the equation does not take in account the presence of a boundary layer. However this deviation portrays key features of the process. The DEP force depends on particle volume; the greater the volume the greater the force. The induced moment is either a negative or positive force depending on the media [3].

Applications Need to Expand

Due to the properties of dielectrophoresis there has been much research in recent years for developing new technologies to study and capitalize on nanosized systems. The research is mainly concentrated in material science and biotechnology [3].

Filtration Systems

Comminution is an energy intensive process used in the mining process. Dielectrophoris is being reviewed as a better techniquie for the removal of course grains in order to lessen the amount of material going on to finer grinding processes [7].

Biological Separations

DEP depends on the structure and composition of a system which allows for more complicated separation than in electrophoresis because it focusing, translation, fractionation and characterization of analytes within a medium. [6]. There has been a push to intergrate this process to discriminate particles in microfluidic systems.Its due to enhanced microelectrode structures that allow for the sorting of different cells and microorganisms using AC electric fields [8].

-Angled Electrodes for bio material separation/ Lab on a chip

References

[1] Pohl, H. A. Journal of Applied Physics1951, 22 (7), 869–871. Doi: http://dx.doi.org.silk.library.umass.edu/10.1063/1.1700065

[2] Pohl, H. A.; Crane, J. S. Biophysical Journal1971, 11 (9), 711–727. Doi: http://dx.doi.org/10.1016/s0006-3495(71)86249-5

[3] Pethig, R. Journal of The Electrochemical Society 2016, 164 (5). Doi: http://dx.doi.org/10.1149/2.0071705jes

[4] Pethig, R. Biomicrofluidics2010, 4 (3), 039901. Doi: http://dx.doi.org/10.1063/1.3474458

[5] Hsu, T.-R. MEMS and microsystems: design, manufacture, and nanoscale engineering; John Wiley: Hoboken, NJ, 2008.

[6] Kadaksham, J.; Singh, P.; Aubry, N. Fluids Engineering 2003. Doi: http://dx.doi.org/10.1002/1522-2683(200207)23:13<1973::AID-ELPS1973>3.0.CO;2-1

[7]Ballantyne, G.; Holtham, P. Minerals Engineering 2010, 23 (4), 350–358. Doi: http://dx.doi.org/10.1016/j.mineng.2009.09.001

[8] Pethig, R.; Markx, G. Trends in Biotechnology 1997, 15 (10), 426–432. Doi: http://dx.doi.org/10.1016/s0167-7799(97)01096-2