Dielectrophoretic Force

Dielectrophoresis is the translational motion of neutral matter owing to polarization effects in a non-uniform electric field. Depending on matter or electric parameters, different particle populations can exhibit different behavior, e.g. following attractive or repulsive forces. DEP can be used for mixing of charged or polarizable particles by electrokinetic forces [48], In particular, dielectric particles are mixed by dielectrophoretic forces induced by AC electric fields, which are periodically switched on and off. 

Dielectrophoretic forces depend on the polarizibility of species, rather than on movement of charges [99]. This allows the movement of any type of droplet being immersed by a dielectrically distinct immiscible carrier medium. Since dielectric forces are generated by spatially inhomogeneous fields, no mechanical actuation is required. In addition to this, dielectrophoretic droplet movement benefits from the general advantages given by droplet microfluidic, i.e. discrete, well-known very small volumes, no need for channels, avoidance of dead volumes and more. 

M 15] [P 15] Discrete aliquots of the liquid were injected onto a reaction surface by means of dielectrophoretic forces generated by a fluid processor [99], Micropipet injectors were immersed in the fluid reservoir and placed close to the electrodes. A vertical distance of 10-20 pm was maintained. By means of a micromanipulator, the lateral distance was adjusted. The pressure was controlled by a syringe pump. 

The droplet injection was initiated by energizing the electrode nearest the injector [99], By dielectrophoretic forces, fluid is drawn out of the pipet and forms a droplet, which then moves to the edge of the electrode. The dielectrophoretic forces have to be balanced properly with the interfacial tension forces to give uniformly sized droplets. 18 pm (3 pi) sized droplets for example, were made in that way. 

Positive DEP drives particles towards field maxima and negative DEP moves them to field minima. It is only possible to create field maxima at the surfaces of the electrodes, no maximum can exist elsewhere in an isotropic solution. Trapping or positioning of particles by positive DEP therefore requires the dielectrophoretic force to be balanced by some other force (e.g. sedimentation, buoyancy or flow) and to be subjected to feedback control. This was first done with a two electrode system by Jones and Kaler [33-37]. 

Fig. 10. Modifying the resonances in an electrode system can be useful when two cell types have only small differences in passive electrical properties and in dielectrophoretic force spectra. The best frequency to achieve a separation lies near the crossover frequencies when one cell is showing positive DEP and the other negative. But at such a frequency the forces are small. By suitable adjustment of capacitive and inductive elements at each electrode, it is possible to make a system resonate at the desired frequency, thereby increasing the drive voltage (and force) many fold without the need for expensive high voltage signal generators. The real dielectrophoretic force spectra (a) can be transformed into effective spectra (b)...

A more detailed derivation of the dielectrophoretic force in lossy dielectric media has been given by Sher (52) and is based in turn on a derivation of the potential electric energy of a lossy dielectric body given by Schwarz (53). 

Importantly, the induced dipole is also a function of frequency. Therefore, we see that the direction in which the particle moves is not only a function of the properties of the particle and the suspending medium but also the frequency of the applied field. The time-averaged dielectrophoretic force on the dipole is given by ... 

T. B. Jones, Mnltipole corrections to dielectrophoretic force, IEEE Trans. Ind. Appls., IA-21, 930-934 (1985). 

M. Washizu, Precise calculation of dielectrophoretic force in arbitrary field, J. of Electrostat., 29, 177-188 (1992). 

M. Washizu and T. B. Jones, Multipolar dielectrophoretic force calculation,... 

X. Wang, X-B Wang and P. R. C. Gascoyne, General expressions for dielectrophoretic force and electrorotational torque derived using the MaxweU stress tensor method, J. of Electrostat., 39, 277-295 (1997). 

T. Sun, H. Morgan and N. G. Green, Analytical solutions of ac electrokinetics in interdigitated electrode arrays electric field, dielectrophoretic and travelling-wave dielectrophoretic forces, Phys. Rev. E., 76, 046610 (2007). 

H. Hwang, D.-H. Lee, W. Choi and J.-K. Park, Enhanced discrimination of normal oocytes using optically induced pulling-up dielectrophoretic force, Biomicrofluidics, 3, 014103 (2009). 

It is appropriate to consider first the influences of long-range flocculation or collision mechanisms. Depending on the size and movement of dispersed droplets, different mechanisms will play different roles in the collision process. In a compact electrostatic coalescer (CEC), Brownian motion, sedimentation, laminar shear, turbulent shear, or turbulent inertia may play a role in droplet movement owing to hydrodynamic effects. Additionally, electrophoretic and dielectrophoretic forces, arising from the apphed electric field, may act on dispersed droplets. 

Neutral droplets can also be made to collide by inducing a dielectrophoretic force of interaction between neighboring droplets which arises from the polarization of the droplets in the applied electric field. The local electric field must be nonuniform, and the presence of the droplets will distort the field even if it is uniformly applied. The force, which is independent of field polarity, depends on the permittivity j, of the continuous phase and the volumes of the droplets. At larger separations, dielectrophoretic forces tend to be small in comparison with electrophoretic ones. However, at very close proximity, dielectrophoretic forces will dominate. The dielectrophoretic force acting on a droplet is given by ... 

ABSTRACT A brief history of the behavior of materials in nonuniform electrical fields is presented, followed by a theory of dielectrophoretic force and the derivation of the general force equation. Attention is paid to the several classes of polarization which lead to the experimental considerations of induced cellular dielectrophoresis. A distinction between batch and continuous methods is discussed, with a focus on a new microtechnique. While dielectrophoresis can induce aggregation of materials, i.e., cells, other orientational applications exist. Cell division, cellular spin resonance, and pulse-fusion of cells form topics appropriate to the realm of high-frequency electrical oscillations and are discussed in the context of living material. 

When living cells or their parts are in a nonuniform electric field, a force upon them usually arises. This can be used to help analyze, sort, or study such materials. This is not to say that such nonuniform field efEects are unique to living matter, but rather to call attention to the fact that such forces can be uniquely useful in the biological milieu even though they can be do occur in the case of inanimate matter. As we shall see, such nonuniform field forces dielectrophoretic forces) resemble (but are not the same as) those exerted by a magnet upon a piece of iron, or by the earth... 

One further point might be made for clarity. As we have seen, dielectrophoresis is the translational motion evoked by a nonuniform electric field. In the case of some solid materials and in certain semisolid ones (e.g., liquid crystals) there is seen still another mechanical response of a neutral body to an electric field, that of a distortion. This is electrostriction, and refers to the distortional response or strain resulting from an imposed electrical stress. Electrostrictive strains are used in sound transducers, for example. Historically speaking the two effects, translational (dielectrophoresis) and distortional (electrostriction), where both at times referred to as electrostriction with resultant confusion. Modem usage has tended to restrict the term electrostriction to the discussion of distortional strain that has been induced electrically. For the sake of brevity, we shall frequently use the abbreviation DEP response as that referred to properly as dielectrophoresis. One can, of course, couple a moment arm to the dielectrophoretic force (e.g., DEP force) producing a torsion, and possibly a realignment of the body in the field. 

The equating of (20) and (21) followed by the substitution e = e[ is then held to provide the effective moment, required in the equation for the dielectrophoretic force. 

Menachery, A., Pethig, R., 2005. Controlling cell destruction using dielectrophoretic forces. lEE Proc. Nanobiotechnol. 152 (4), 145—149. 

Dielectrophoresis is the translational motion of a neutral particle by induced polarization in a nonuniform electric field. The magnitude and direction of the induced dielectrophoretic force are dependent on the characteristics of the applied electric field as well as the dielectric properties of the surrounding medium and of the particle itself. 

Dielectrophoretic forces, though, can be induced by means other than an applied electric signal through electrodes. Optical tools can be implemented to modify an applied electric field, making these methods more susceptible for dynamic as opposed to static manipulation of electric fields with surface electrodes. Dielectrophoresis applications are not limited to particulate manipulation either. With properly configured surface-electrode geometry, it is possible to induce fluid motion and create nanoliter-sized droplets. Additionally, dielectrophoretic forces can be utilized to manipulate particles to buUd micro- and nanostructures such as wires. 

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