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Induced-Charge Electrokinetics Phenomena

Consider a neutral electrically conducting particle with arbitrary geometry, immersed in an aqueous solution. Once an electric field is applied, the electric field passes through the conducting particle (Figure 1a) and drags the charges inside the particle to its conducting surface. Shortly after, the negative charges migrate to that side of the particle which is closer to the higher voltage; while the positive charges move to the opposite side (Figure 1b). Consequently an internal electric field is generated around the particle which acts like a shield and resists the external electric field from passing the conducting particle. Meanwhile, the induced surface charges of the conducting particle attract the counter-ions of the aqueous solution and EDL forms around it. At this stage, the particle behaves like an insulator and the electric field around the particle reaches steady-state condition (Figure 1c). The established configuration of the charges and EDL on the conducting particle is equivalent to the non-flux electrostatic boundary condition. Because the induced charges under the surface of the conducting particle have an opposite sign to that of the ions attracted on the surface of the particle (from the aqueous electrolyte solution). The electric double layers on these two sides have opposite electrical polarity; consequently the induced EDL on these two sides also opposite polarity [1]. The interaction between the applied electric field and the net charges in the dipolar EDL will generate opposite electro-osmotic flow and hence vortexes near the conducting object. The existence of these vortices is predicted theoretically. However, very few experimental evidences are reported, especially for conducting objects that do not used as working electrodes under DC field.

A key characteristic in the theory of the induced charge electrokinetic flow is the vortices generated by the interaction of the applied electric field with the induced dipolar electric double layers around an electrically conducting surface. The vortices can significantly influence the electrokinetic motion of a particle with an electrically conducting surface.

For the first time, Daghighi et. al. [2] experimentally provided the visual evidence of the vortices in the induced charge electrokinetic flow in a DC field around a metal surface that is not an active electrode. In their study, a spherical carbon steel particle of 1.2mm in diameter was placed in the middle of an open chamber PDMS plate. The chamber was filled with DI water. A DC electric field of 40V/cm from left to right was applied via two electrodes placed at the ends of the chamber to induce electric charge on the surface of the conducting particle. Figure 2 shows four vortices around the conducting particle. The dashed line represents the particle’s surface. Since particle size and the surrounding area with the vortices are large, the particular microscope (TE2000-E Nikon) has a limited field of view, the whole picture in Figure 2 was obtained in two steps.

Their experimental study proved the existence of vortices close to a non-electrode conducting surface submerged in an electrolyte solution under DC electric field. They confirmed a key prediction of the theory of induced charge electrokinetics flows while DC electric field is involved by showing four vortices formed around a spherical carbon-steel particle. The size and the strength of these vortices is a function of the external DC electric field. Furthermore, the induced charge electrokinetic motion of heterogeneous particles, polymer particles with nickel film coated on one hemisphere, in a microchannel was investigated experimentally. The velocity of the heterogeneous particles is significantly higher than the polymer particles of the same size under the same applied electric field. The quantitative results verified the theoretical predication that the vortices on the conducting surface side push the particle moving to downstream. The direction of heterogeneous particle's motion when DC electric field is involved is totally different from using AC electric field in the same system. The heterogeneous particle moves in the same direction as the DC electric field axis is. Proven vortices can be used for mixing [3], fluid manipulation and control [4] in micro-devices of biochemical, medicine, and many other useful and critical aspects.