Microsystems use several techniques to actuate particles beyond the electrophoresis discussed in Chapter 13. Two physical phenomena are described in this chapter – dielectrophoresis and magnetophoresis – which are commonly used in microdevices to manipulate particles or droplets in suspension. This chapter also discusses digital microfluidics, which is not a physical phenomenon, but rather a system concept for manipulating fluid droplets using AC electric fields.

DIELECTROPHORESIS

Dielectrophoresis (DEP) is often used in microsystems as a mechanism for manipulating particles. It is appealing because the dielectrophoretic force on a particle scales with the characteristic length scale of the system to the –3 power, and dielectrophoretic forces are quite large when small devices are used. Further, particle response varies based on the frequency and phase of the applied field. Because the user can change particle response by changing a setting on a function generator, DEP measurements afford great flexibility to the user. Because of this, DEP has been used for many applications, with one example shown in Fig. 17.1.

The term dielectrophoresis refers to the Coulomb response of an electrically polarized object in a nonuniform electric field. In contrast to linear electrophoresis, it (a) does not require that the object have a net charge and (b) has a nonzero time-averaged effect even if AC electric fields are used.

Consider, as an example, a spherical, uncharged, uniform, ideal dielectric particle with a finite polarizability, expressed using its electrical permittivity εp, suspended in empty space.