Electrokinetic Motion of Polarizable Particles

Electrokinetic Motion of Polarizable Particles - Electroosmotic Flow (DC)... 

Nonlinear electrokinetic phenomena, such as the electrokinetic motion of polarizable particles, have only been studied for a few decades, and attention is just begiiming to be paid to the nonlinear motion of heterogeneous particles due to induced-charge electrophoresis [7-9]. Recent theoretical work has relaxed assumptions 1-3, but much remains to be done. Surprising new possibilities include particles that rotate continuously or translate perpendicular to a uniform AC field [9]. 

The underlying physical mechanisms for the electrokinetic motion of particles are described in other entries on Electroosmotic How (DC), Electrophoresis, Dielectrophoresis, Nonlinear Electrokinetic Phenomena, and Electrokinetic Motion of Polarizable Particles, along with various mathematical models. The effects of relaxing the assumptions above in these models, however, are often unexpected and have not yet been firUy explored, either theoretically or experimentally. Here, we simply give a few examples of how heterogeneous particles can move in electric fields. 

The canonical example is that of a Janus particle with one metallic and one insulating hemisphere [9], using the standard low-voltage model for electrokinetic motion of polarizable particles. In response to an applied electric field, the Janus particle rotates to align the interface between the two hemispheres with the field axis, due to both ICEP (electrohydrodynamics) and DEP (electrostatics). At the same time, for any orientation, the particle translates in the direction of its insulating end, propelled by ICEO flow on the metallic end, with a velocity... 

The electrokinetic motion of polarizable particles results from electroosmotic flow (induced-charge electrophoresis) of the first of second kind, in addition to electrostatic forces ( dielectrophoresis). 

Electrokinetic Motion of Polarizable Particles, Fig.1 (a) Induced-charge electroosmotic ICEO) flow around a symmetric, uncharged, ideally polarizable particle [3] (b) an example of ICEO flow and the resulting induced-charge electrophoretic ICEP) velocity for an asymmetric shape [4]... 

Electrokinetic Motion of Polarizable Particles, Fig. 2 (a) Mechanism for ICEP torque on a rodlike, polarizable particle in a uniform electric field, which... 

Electrokinetic Motion of Polarizable Particles, Fig. 3 Experiments on cylindrical silver particles (0.318 pm diameter, 6 pm length) sedimenting in deionized water by gravity alone (a) and in a 100 Hz, 100 V/cm AC field aligned with gravity (b). The experimental distribution of angles in different fields (c) agrees well with theoretical curves (solid) which take into account both ICEP rotation and electrostatic torque [14]... 

Other broken symmetries include irregular shapes (e.g., rods, polyhedra, etc.), nonuniform surface properties (e.g., partial dielectric or metallic coatings), and nonuniform background electric fields [10]. In each case, net pumping of the fluid by ICEO results if the object is held fixed, which requires a certain force and torque. Conversely, if the object is a colloidal particle, then broken symmetries cause it to translate and rotate ICEP, as described in companion articles on the electrokinetic motion of polarizable particles and heterogeneous particles. 

Electrokinetic Motion of Polarizable Particles Electrokinetic Sample Injection Electrokinetic Transport in Nanofluidic Sensing Devices... 

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