Dielectric force

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. 

Fig. 11 Non-destructive cell sorting system using dielectric force [36]...

Static is dissipated toward air when the field force, due to the build-up of diai es on the fiber surface, exceeds the dielectric force of air. This depends on pres-mre, humidity, and the state of ionization of the surroundit air. 

In AC and DC dielectrophoresis, the dielectrophoretic force acting on the polarized particle causes it to move either up or down the induced electric field gradient, which is created by (a) nonuniform electrode geometry in AC fields or by (b) a nonuniform insulator geometry of obstacles in an otherwise uniform DC field or DC-offset AC field. This spatially nonuniform field or gradient dotted with the polarized particle dipole yields a net dielectric force ... 

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... 

As a result of the complexity of this dielectric force phenomenon, many spatially nonuniform geometries and electric field operating conditions are possible. Specifically in DC fields, the nonuniform insulator geometry of obstacles results in a dielectrophoretic force acting on the polarized particle, which causes the particle to move either toward or away firom electric field intensity without having the cells in direct contact with the electrode. 

For instance. Hunt et al. [16] used dielectric forces of the DEP tweezers to hold a single yeast cell at the end of a micromanipulator. In one report, the dielectrophoresis-based microfluidic device was applied to manipulate single T cells and pair them up with the individual anti-CD3/ anti-CD28 presenting microbeads to study cell activation based on the activation marker molecule CD69. 

This equation gives a physical explanation for the threshold for director rotation. The threshold is determined by field strength rather than voltage, and the director starts to rotate when the dielectric force overcomes the strength of the memory of the initial director at cross-linking. 

We previously described an LC droplet-based microlens developed by Cheng et al. [7]. The focal length was tuned using the dielectric force described in Section 5.2.3. Cheng further extended the design and introduced new liquids to a packaged liquid lens actuated by a dielectric force [12]. 

Figure 5.16a depicts the configuration of the new type of liquid lens actuated by dielectric force. The liquid lens consisted of a 15 pL liquid droplet with... 

Liquid-crystal electro-optic phenomena can be divided into two categories—those caused only by dielectric forces and those induced by the combination of dielectric and conduction forces. The two conduction-induced phenomena discussed later are dynamic scattering and the storage effect. Four of the dielectric phenomena, or field effects as they are sometimes known, are discussed first (1) induced birefringence, (2) twisted nematic effect, (3) guest-host interaction, and (4) cholesteric-nematic transition. 

A third phenomenon depending solely upon dielectric forces is the guest-host or electronic color-switching interaction in which guest pleochroic dyes are incorporated within nematic host materials. The dyes have different absorption coefficients parallel and perpendicular to their optical axes. As illustrated in Fig. 6, the dye molecules can be oriented by the liquid crystal. With zero field, the liquid crystal is in the uniform parallel orientation and the dye mole-... 

The different electro-optic phenomena have been classified into those that involve only dielectric forces and those that depend upon the interaction of conduction and dielectric torques. The field-effect phenomena possess several common properties. The resistivity of the materials may be as high as chemically practical, i.e., p 10 ohm-cm. For the induced birefringence, twisted nematic, and guest-host color switching effects, the threshold voltages are less than 3 or 4... 

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]

Baker mentions two other effects that are of lesser importance but may be involved the surface adsorption of the sample on the matrix powder, and dielectric forces. Only the latter effect will be discussed here. 

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