On the Relationship of Dielectrophoresis and Electrowetting

Electrostatic fields have two observable effects upon the hydrostatics of liquids. These are (i) a net force exerted on the liquid if the electric field is nonuniform and (ii) a change in the contact angle. For the case of electrodes coated with thin layers of dielectric material, both of these effects are most certainly electromechanical in nature. Though often seen to occur together, the bulk electromechanical and electrowetting effects are distinct and can be expected to occur independently under proper conditions. A derivation using the methods of lumped parameter electromechanics demonstrates that the net force on a liquid mass situated between two parallel, coated electrodes can be determined with no reference to the actual shape of the meniscus. By the very nature of its derivation, this electrostatic force cannot be localized as to where it acts on the liquid. The conclusion is that the term "electrowetting" should be restricted in its use to denote the effect of the electric field upon the contact angle. On the other hand, translational forces harnessed in certain microfluidic applications are better referred to as examples of the net electromechanical force resulting from electric field nonuniformity. The high-frequency limit of the electromechanical response, equivalent to the case of an insulative liquid, is recognized as liquid dielectrophoresis.