Control of particles using multi-frequency DEP S. Loire, Y.T. Zhang, F. Bottausci, I. Mezi´ c, N. C. MacDonald Mechanical & Environmental Engineering Department University of California, Santa Barbara, CA 93106, USA ABSTRACT We present numerical simulations and experiments on dielectrophoretic (DEP) separation and trapping per- formed in a titanium-based microchannel linear elec- trode array. The device is designed to allow for ef- fects driven by inhomogeneities in electric-field magni- tude driven (p-DEP and n-DEP) and inhomogeneities in electric-field phase-driven (travelling wave) DEP. It is also capable of inducing multi-frequency DEP, in con- trast with most of the previous, single-frequency, de- signs. The advantages of two-frequency DEP were shown by theoretical work (Chang et al. 2003) and permit pre- cise control of particles movements. We show that fluid flow effects are substantial and can affect the particle motion in a positive (enhanced trapping) and negative (trapping when separation is desired) way. To model the AC-electroosmosis, we use the theory developed by Gonzalez et al. Keywords: dielectrophoresis, multi-frequency, elec- troosmosis, bioparticles 1 INTRODUCTION The use of electric field and in particular dielectrophore- sis (DEP) have a great potential to help miniaturize and increase the speed of biomedical analysis [6]. Indeed, precise control and manipulation of micro/nano/bio par- ticles inside those miniaturized devices depend greatly on our understanding of the phenomena induced by AC electric field inside microchannels and how we take ad- vantage of them. It is well known that an electric field induces a dipole in an uncharged particle and the interaction between the dipole and a nonuniform electric field generates a force on the particle. The resultant motion is called dielec- trophoresis. Dielectrophoresis is an efficient and increas- ingly popular method for separating particles according to their electrical properties, one of the generic operation needed for biochemical analysis. In this paper, we will show experiments where the use of multi-frequency DEP permitted the separation of two entities when single- frequency DEP could not. The use of multi-frequency DEP to improve separation techniques was suggested in [2] through a systems theory approach of dielectrophore- sis. This intriguing idea is thanks to this paper vali- dated by a specific experimentation. However, knowing the theory of dielectrophoresis is not enough to precisely control particles by an AC electric field. We also need to study the induced fluid flow: AC electroosmosis and thermal effect. Theoretical predictions of AC electroos- mosis in the case of electrodes separated by small gap has been carried out, for example by Ramos et al in [3]. Nevertheless the available results are not valid for our device which consists of arrays of electrodes separated by a large gap. We present here numerical results for the AC electroosmosis characteristics of our device. This paper is organized as follows. In §II, we develop basic theoretical ideas and give some numerical results. In §III, we present the experimental set-up and results. 2 BASIC THEORY AND NUMERICAL RESULTS 2.1 Electrical force on particles: dielectrophoresis The essential idea begins with the observation that the induced dipole moment, m(x,t), in a particle due to an external electric field of frequency ω, E(x,t), depends linearly on the electric field [4], where x is a spatial variable. This relation can be written as: m(x,t)= G(ω)E(x,t) (1) For example, when a spherical particle with the per- mittivity ǫ p , the conductivity σ p and radius r, lies in a medium with the permittivity ǫ m and the conductivity σ m , the function G(iω) is given by G(iω)=4πr 3 ǫ m ( ǫ p + σp iω ) − ( ǫ m + σm iω ) ( ǫ p + σp iω ) +2 ( ǫ m + σm iω ) (2) where G(iω)/(4πr 3 ǫ m ) is called the Clausius-Mossotti function [4]. The dielectrophoretic force, F dep , on the particle due to the interaction between the induced di- pole and the electric field, is given by F dep (x,t)=(m(x,t) ·∇)E(x,t). (3) NSTI-Nanotech 2005, www.nsti.org, ISBN 0-9767985-2-2 Vol. 3, 2005 483