Dielectrophoretic Handling of Mesoscopic Objects Diego C. POLITANO, Paolo V. PRATI, Shao-Jü WOO, Otte J. HOMAN *) , Felix M. MOESNER and Andreas STEMMER Institute of Robotics, Nanotechnology Group *) Laboratory of Fieldtheory and Microwave Electronics Swiss Federal Institute of Technology Zurich (ETHZ) CH - 8092 Zurich, Switzerland ABSTRACT Efficient handling of minute objects in individual or col- lective mode is a delicate and most demanding task. Electric fields applied over a micro-patterned electrode array possess the capability to produce contactless driv- ing forces on small conducting or non-conducting parti- cles as presented in a former paper [1]. The transport and precise positioning of charged micro particles up to 400 μm in diameter has been demonstrated on the x-y plane, where an addressable electrode geometry allows flexible particle actuation trajectories. In this paper, the electrode pattern is further miniatur- ized allowing a gap width in the order of a 1 μm. The advantage of this downscaling is a clear reduction of Coulomb heating when small objects are transported in ionic solution. Additionally, high frequency electric fields are necessary to impart actuation forces onto the suspended objects in liquid environment as described in [2 - 4]. Since the electric field strength is a function of the gap between the electrodes and the applied voltage, a miniaturized pattern requires lower voltage amplitudes in order to attain a certain electric field strength. The electrodes are designed as a spiraled meander and fabricated by photolithographic processes within a monolayer. Under the application of six-phase voltages, a traveling electric field wave is created above the elec- trode array, immersed mesoscopic objects become polarized and negative dielectrophoretic effects cause object motion. It is shown that optimal object actuation occurs in a narrow, material-specific frequency spec- trum. Various simulations and their results are presented in this paper. I. INTRODUCTION A particle suspended in a medium becomes electrically polarized when subjected to an electric field. If the spatial distribution of phase and field magnitude are inhomoge- nous, a dielectrophoretic (DEP) force acts on the particle, as a result of the interactions between the induced polar- ization and the field. The DEP-induced particle motion is strongly dependent on the field frequency, the spatial con- figuration of the field, the dielectric properties of the sus- pending medium and, more importantly, of the particle itself. II. THEORETICAL BACKGROUND The term dielectrophoresis was coined by Pohl [5] to de- scribe the force exerted by a non-uniform electric field on a small polarized but uncharged particle. Most theoretical and experimental work in dielectrophoresis used the gen- erally accepted force expression of Pohl [5], namely (1) where is the non-uniform electric field and is the effective dipole moment to be discussed in more de- tail in this paper. The simplest derivation of the dielectro- phoretic force is accomplished by summing the forces on two equal but opposite electric charges (+q,-q) placed a vector distance apart in a non-uniform electric field (see Fig. 1). The net force on this small dipole is given as [6] . (2) If and only the lowest-order term of the Taylor series in is retained, we get . (3) F DEP p eff   L l   E o  = E 0 p eff d F DEP F DEP q E o r d + ( ) q E o  r  –   = d 0 fi d F DEP q d   ( ) E o   =