Microfabricated electrohydrodynamic pumps

Abstract

Pumping is often cited as a general application which motivates the development of microfabricated motors and other actuators. In that spirit, this paper studies microfabricated electrohydrodynamic fluid pumps. In electrohydrodynamic (EHD) pumping, fluid forces are generated by the interaction of electric fields and charges in the fluid. In contrast to forces generated by mechanical pumping using an impeller or bellows, EHD pumping requires no moving parts and consequently offers the possibility of simplified fabrication and high reliability. This paper discusses electrohydrodynamic pumping and issues concerning its use in micronsize scale systems. The fundamental operating principles of the EHD pump are outlined and examples of configurations which meet the requirement for inducing free electric charge are shown. The possible performance achievable in micron size-scale regimes is indicated. Issues concerning fluid conductivity, instability and surface tension are addressed. A microfabricated structure which demonstrates the EHD pumping of a highly insulating silicone oil is described. The structure consists of an array of 10 μm by 235 μm highly doped, LPCVD polycrystalline silicon electrodes patterned over silicon nitride. The electrode array is excited with a traveling wave of electric potential. Pumping results are qualitatively described. This paper describes a study of electrohydrodynamic pumping, including issues concerning its application to micron size-scale systems. The fundamental operating principles of EHD pumps are outlined and the possible performance achievable in micron size-scale regimes is indicated. Surface and bulk instabilities are addressed. Finally, the preliminary results of an EHD pumping experiment with a microfabricated structure are described. A practical requirement for EHD pumping is the induction of free electric charge in the volume of the fluid to be pumped or on its interface with another material. Charge accumulation on a material interface is readily achievable; however, if one material is a fixed rigid wall, such as the wall of a conduit, no pumping can take place. Consequently, the practical application of microfabricated EHD pumps may require the induction of free charge in the volume of the fluid, possibly by temperature-induced conductivity gradients. A further constraint on the usefulness of EHD pumping is its reduced effectiveness with conducting fluids. Hence its usefulness in many situations, including biological environments, may be limited. Another possible difficulty may be fluid instabilities; however, these instabilities may be useful for mixing and cooling purposes. Yet, it remains to be seen if they can compete with molecular diffusion in micron-size scale systems. Finally, it seems clear that surface tension will be a dominant force in virtually any micron-scale system with a liquid surface. In spite of these difficulties, electrohydrodynamic interactions may prove to be a reasonable way to achieve pumping without moving parts.