Dielectrophoresis positive

Figure 3. Diagram showing the principle of dielectrophoresis (DEP), which only occurs in a non-homogeneous electric field, (a) Particle more polarizable than the medium positive dielectrophoresis (pDEP) (b) particle less polarizable than the medium negative dielectrophoresis (nDEP).

DEP force. The frequency dependence and the direction of the DEP force are governed by the real part of the Clausius-Mossotti factor. If the particle is more polarisable than the medium, (Re[/cm] > 0). the particle is attracted to high intensity electric field regions. This is termed as positive dielectrophoresis (pDEP). Conversely, if the particle is less polarisable than the medium, (Re[/cm] < 0), the particle is repelled from high intensity field regions and negative dielectrophoresis (nDEP) occurs. Therefore the real part of the Clausius-Mossotti factor characterizes the frequency dependence of the DEP force, as demonstrated in Fig. 1. 

For a positive value of Re[ T( o)], the particle is attracted to high electric field gradients with positive dielectrophoresis (pDEP). With dielectrophoresis it is possible to manipulate particles using a variety of techniques and applications. 

When an AC field is applied, the cells introduced into the channel can be guided to the areas with the highest electric field with positive dielectrophoresis. This facilitates self-assembly of the cells at the micro orifice and results in cell contact and pearl-chain formation (Fig. 2b). When the diameter of the micro orifice is smaller than that of the cells, one-to-one cell contact is guaranteed as there is space for only one cell to fit in the orifice. 

This dielectric force pushes particles toward regirms of high field density or low field density depending on whether the Clausius-Mossotti factor is positive or negative, respectively. In other words, if Op < ct then negative dielectrophoretic motion away from sharp points in electrodes or insulator obstacles is observed the converse is true for positive dielectrophoresis, which is rarely observed in DC-DEP due to other electrokinetic forces. For a truly insulating particle, Op = 0, the Clausius-Mossotti factor is simply 1/2, and motion away from high field... 

Fig. 4 Plot of the Clausius-Mossotti factor against frequency for two different solid particles. In the shaded area, one particle experiences positive dielectrophoresis and the other negative dielectrophoresis, enabling separation in this frequency window...

These surface vortices provide an efficient means for microfiuidic mixing, as shown in Fig. 10, where a dye is rapidly mixed within several seconds. The mixing can be enhanced by inducing the vortex instabilities wherein turbulent-like mixing efficiencies are observed [18]. In addition, particles dispersed in the flow are also observed to be drawn into the vortices due to positive dielectrophoresis toward a point on the interface closest to the needle where the field is most intense. Once a sufficient particle concentration is achieved within the vortex, shear-induced migration leads to crossstreamline transport such that the interior of the vortex is populated [18], as shown in Fig. 11a. 

Fig. 1.13 SEM images of (a) vertically aligned CNT mat and (b) CNTs trapped in castellated microelectrode gaps of sensors by positive dielectrophoresis (DEP). DEP is the electrokinetic motion of dieiectricediy polarized materials in nonuniform electric fields and has been used to manipulate CNTs for separation, orientation, emd positioning of CNTs ((a) Reprinted with permission from Huang et al. 2005, Copyright 2005 Elsevier, (b) Reprinted with permission from Suehiro et al. 2007, Copyright 2007 Elsevier)...

FiGURE 3.1 Schematic representation of the principle of detection of S. typhimurium using positive dielectrophoresis-driven online enrichment with fluorescent NP label. (Reprinted with permission from He, X. et al. 2013. Biosens. Bioelectron. 42 460-466.)... 

He et al. (2013) described a method for the rapid and sensitive detection of Salmonella typhimurium, a foodborne infectious pathogen, in microfluidic channels using positive dielectrophoresis-driven online enrichment and fluorescent NP label. The bacteria were labeled with antibody-conjugated Rubpy-doped fluorescent NPs and enriched by positive dielectrophoresis, then being detected continuously in a microfluidic chip (Figure 3.1) by fluorescence microscopy. The assay of the bacteria at low concentration levels was achieved in less than 2 h with a LOD of 110 cfu mL" in artificially contaminated mineral water samples. 

Ele, X., C. Hu, Q. Guo, K. Wan, Y. Li, and J. Shangguan. 2013. Rapid and ultrasensitive Salmonella typhimurium quantification using positive dielectrophoresis driven on-line enrichment and fluorescent nanoparticles label. Biosens. Bioelectron. 42 460-466. 

Note that the dielectric force F is directed along the gradient of the electric field intensity V . For the metallic particle, the force direction is always toward the direction of the largest field. On the other hand, the force on a nonmetallic particle will be toward the direction of the largest field only if Ep > e (positive dielectrophoresis) it will be toward the lowest field if e > p (negative dielectrophoresis). If Ep E or Ea Ep, the magnitude of the force is not influenced, but the direction is. Obviously if there are two particles with Ep [Pg.81]

The sign of the quantity cp-Ed) is important If Cp is greater than Ed, then the direction of the force is toward the region of high electrical field strength it is called positive dielectrophoresis. On the other hand, if Ep is smaller than Ed, the direction of the force is reversed, and one has what is called negative dielectrophoresis the particle is forced in the direction of lower field strength (Pohl and Kaler, 1979). (Of course, the particle may experience zero force as weU.) Note further that the dielectrophoretic force... 

Cells undergoing positive dielectrophoresis move in the radial direction of the merging of the curved rear electrode and the flat front electrode, whereas cells experiencing negative dielectrophoresis move in the opposite direction. Meanwhile, the bulk carrier flow perpendicular to this electrical force carries the cells forward toward the carrier exit. The cells or particles unaffected by the nonuniform electrical field continue to move along the line of original introduction. Thus different cells trace out different trajectories and therefore can be withdrawn at different locations from the stream near the liquid outlet. 

Note (1) The direction of the dielectric force depends on the sign of the real part of the Clausius-Mossoti factor, that is, if R6(/ cm) > 0, the particle is attracted to the region where the field is maximum (positive dielectrophoresis). In the other case, that is, negative dielectrophoresis, the particle is repelled. 

The same electrochemical detection scheme was used in a study where the HeLa cells had been individually seeded using positive dielectrophoresis (pDEP, Figure 12.21a through c). This patterning technique involves flowing the cells in a microchannel sandwiched between two ITO electrodes. A patterned microwell array is mounted on the bottom ITO electrode. The application of a pDEP pulse of 3 V pp and 1 MHz between the two ITO electrodes generates an electric field across the microchannel that favors cell seeding and adhesion. " ... 

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