Technical Papers and Presentations

In this paper, the topic of frequency-controlled manipulation of sinking particles in a stationary liquid column based on AC dielectrophoresis is studied in depth. Particles sink downwards within a liquid column due to the gravitational force. In doing so, they are subjected to Stokes friction, which leads to vertical separation depending on the particle size. Large particles sink much faster than small particles. Here, the sinking velocity is proportional to the square of the particle radius. Subsequently, the so vertically separated particles can be deflected horizontally by an applied AC electric field. Figure 1 shows an equation set of the dielectrophoretic phenomenon which is modeled and simulated within COMSOL Multiphysics® utilizing the AC/DC Module and the Particle Tracing Module. In particular, the influence of the surface conductance as a function of the particle size is considered. This effect is often neglected and leads to misconceptions about the electrical conductivity of particles and thus to a falsified influence of the dielectrophoretic force. In addition, for particles with negligible surface conductance, it can be shown that for a given electric field strength, the horizontal deflection is almost independent of the particle size. This is not observed in typical horizontal lab-on-chip applications, where the particles move with the velocity of the flowing liquid, and allows another degree of freedom of manipulation. In the presented experimental setup, the electric field is capacitively coupled into the liquid by means of two insulated electrodes. It should be noted that a sufficient electric field strength within the liquid requires a minimum cut-off frequency. Figure 2 shows that the higher the electrical conductivity of the liquid phase, the higher the cut-off frequency. The Clausius-Mossotti factor describes whether the dielectrophoretic force accelerates the particles in the direction of an increasing electrical field (positive DEP) or in the opposite direction (negative DEP). This depends on the electrical conductivities respectively permittivities of liquid and particles and the applied frequency of the electric field. This allows the particles to be attracted or repelled in the horizontal direction depending on the applied frequency (cf. Figure 3). In the region of the crossover frequency, the dielectrophoretic force disappears. Figure 4 illustrates an example of the particle trajectories for the three cases less than, equal to, and greater than the crossover frequency. Consequently, it is possible to selectively manipulate the vertically separated particles of different sizes by applying a suitable time-dependent and frequency-varying electric field in the horizontal direction. A potential application is the separation or sorting of particle mixtures according to size. Furthermore, the paper presents a detailed model that simulates the possibility of sorting by particle size and elaborates the possibilities and limitations with respect to different material properties of liquid and particles.