Separation of latex spheres using dielectrophoresis and fluid flow B. Malnar, B. Malyan, W. Balachandran and F. Cecelja Abstract: The authors present a method for separation of two latex spheres populations using dielectrophoresis (DEP) and the fluid drag force. Microelectrodes of a suitable layout are used to trap one population of spheres, while the other one is dragged away from the electrodes by the generated fluid flow. The finite difference method is implemented in C++ to calculate the potential distribution by solving Laplace’s equation. From the potential distribution, the DEP force on particles is calculated. The drag force on particles due to the liquid motion is calculated from the observed fluid velocity. The experimental results are shown to be in good agreement with the numerical solution. 1 Introduction Dielectrophoresis (DEP) is a well known effect that occurs when a polarisable particle is subjected to a nonuniform AC electrical field [1]. The dipole moment is induced across the particle and the interaction between the moment and the field causes the particle to move. The magnitude and the direction of the DEP force on the particle is a function of several parameters, such as the dielectric properties of the particle and the suspending medium, the frequency and magnitude of the electrical field, and the size of the particle [1–4] . This makes dielectrophoresis a powerful tool for separation of micron and sub-micron particles, with great promise in future microsystem technology with applications in medical diagnostics, food processing, microbiology, etc. [5]. The technique based on dielectrophoresis and fluid flow fractionation has been reported as a successful tool for separation of different particle populations [2] . Microelec- trodes are used to generate the electrical field within a closed microchannel. The particles suspended within the liquid experience the negative DEP force and are levitated to a certain height above the electrode plane [2, 3] . The fluid motion is generated across the electrodes and used for the separation of the particles. The fluid velocity and therefore the drag force on the particles have a parabolic distribution relative to the height above the electrodes. The particles closest to the centre of the microchannel will experience the strongest drag force and be the first to leave the channel [2, 6]. In this paper, we present a different method that allows only one population of particles to be dragged away from the channel, while the other one remains trapped within the channel. Both populations experience the negative DEP force and are therefore levitated above the electrodes. One population of particles is immobilised against the top of the microchannel by the strong DEP force. For this purpose, the microchannel is made only 10 mm deep. The other population is also subjected to the negative DEP force, but the particles leave the channel carried away by the liquid, because the drag force overcomes the DEP force. The numerical calculation of the DEP force is performed using the code written in C++. The system is described in the input file of a suitable format. The results are presented, discussed and compared with the experimental observations. 2 Theory The DEP force on the spherical particle suspended in a homogeneous medium and subjected to the electrical field generated by microelectrodes utilising the single frequency sine wave source is given in a time-averaged form by the following equation: ~ F DEP ¼ 2pe m r 3 Reðf CM ÞrjE RMS j 2 ð1Þ Here e m is the permittivity of the suspending medium, r is the radius of the particle, Re(f CM ) is the real part of the Clausius–Mossotti factor, and E RMS is the root-mean- square of the electrical field. The Clausius–Mossotti factor is given by the following equation: f CM ¼ e  p  e  m e  p þ 2e  m ð2Þ where e  p and e  m are the complex permittivities of the particle and the medium, respectively. The complex permittivity is given as e * ¼ eis/o, where e is the permittivity, s is the conductivity, o is the angular frequency and i ¼ ffiffiffiffiffiffi 1 p . As the first step towards the solution for the DEP force on the particle, we calculate the potential distribution above the electrodes by solving the Laplace equation, given as [7] r 2 j ¼ 0 ð3Þ Here we assume that the suspending liquid is linear and homogeneous. Equation (3) is solved with the appropriate boundary conditions. The layout of the electrodes that we use for the particles separation is shown in Fig. 1. The electrodes are fabricated on the glass slide and the 10 mm thick insulating tape is used to form the microchannel, The authors are with Brunel University, Cleveland Road, Uxbridge UB8 3PH, UK r IEE, 2003 IEE Proceedings online no. 20031079 doi:10.1049/ip-nbt:20031079 Paper first received 2nd April 2003 and in revised form 30th September 2003 66 IEE Proc.-Nanobiotechnol., Vol. 150, No. 2, November 2003