Electrohydrodynamic Conduction Pump with Asymmetrical Electrode Structures in the Microchannels

Abstract

In this study, we have developed a microfluidic pump that utilizes the electrohydrodynamic (EHD) conduction of a working fluid. Ring-shaped pumps have been used in previous studies on EHD conduction, but these require a three-dimensional arrangement of electrode pairs, which makes it difficult to downsize the apparatus. Here, we propose a mechanism to achieve one-way fluid flow in the microchannels by arranging non-parallel electrodes in a plane to generate an asymmetric electric field. One advantage of this design is that it can be easily and precisely fabricated using microelectromechanical system (MEMS) processing techniques: this has allowed us to integrate a micropump and a microchannel into a single device. Moreover, the pressure generated by the pump is induced solely by electrochemical reactions; since mechanical components such as gears are not required, this helps reduce the noise generated by the device. The demand for small actuators is increasing due to the development of highly integrated electronics and the miniaturization of devices.1,2 The use of electrohydrodynamics (EHD), where a voltage is applied to induce the flow of a working fluid, may help reduce the size of actuators.3 In EHD, three forces are known to act directly on a fluid (unlike in electrophoresis where the forces act on dispersed particles in a fluid). These forces can be described by eq 1.Here, F is the electric body force, µ e is the charge density, E is the electric field magnitude, E is the electric field, ¾ is the permittivity, and µ is the mass density. The first term in the equation represents the Coulomb force acting on free charges in an electric field. The second and third terms represent the dielectrophoretic and electrostriction forces, respectively, which are polarization forces acting on polarized charges. The dielectrophoretic force acts on a permittivity gradient and does not exist for isothermal singlephase flow, and the electrostriction force is only applicable to compressible fluids. EHD phenomena can be divided into EHD conduction and EHD injection. It is known that EHD conduction occurs at less than 10 7 V m ¹1 . 4 EHD injection can produce large pressures, but can also lead to deterioration of the electrodes. Electrode deterioration is not a major issue for EHD conduction, and therefore this has attracted more research interest. Pearson and Seyed-Yagoobi have described the mechanism of EHD conduction as follows.5 When a high-voltage electric field is applied to a fluid, an equilibrium is achieved between the dissociation of some of the neutral species, AB, into ions (A + and B ¹ ), and recombination of the ions back into AB.When the voltage is increased above a certain threshold value, the rate of dissociation becomes faster than the rate of recombination, and the ions A + and B ¹ are attracted to the electrodes. As a result, the ions collide with the neutral AB species, and the fluid flows towards the electrodes.When EHD conduction is used for pumping, it is important to generate unidirectional flow by break...

Overview of electrohydrodynamic conduction pumping

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