Measurement of the Dielectrophoretic Properties of Microparticles Using Optical Tweezers and Effects of Particle Size and Electrode Design

Micro particle manipulation based on dielectrophoresis (DEP) is of great importance and commonly applied in lab-on-a-chip systems. Depending on the applied alternating current (AC) electric field frequency and electrode design, Micro particles, suspended in a liquid medium, experience DEP force in a particular direction which can be used for separation, trapping, and manipulation. Accurate measurement of the DEP force (on the order of several piconewtons) and crossover frequency of particles becomes challenging in many applications. Here, we use optical tweezers to directly measure the DEP force on a micro particle at different applied frequencies, and compare the results with published experimental data by other researchers and our numerical simulations. The particle is initially trapped at the beam waist center between electrodes when the external electrical field is off, and it is displaced due to the DEP force when the electrical field is on. The direction and magnitude of the displacement can be detected by the quadrant photodiode (QPD) at different frequencies. Around the crossover frequency, the DEP force becomes very weak and the signal/noise ratio (S/N ratio) is too small. In this study, we use amplitude modulation (AM) input and lock-in amplifier to detect the micro particle vibration signal phase. The noise is not coherent, so it cannot pass though the lock-in amplifier. At the crossover frequency, the signal phase shifts 180° relative to the phase of the AM input signal. Therefore, we can obtain the precise crossover frequency. In most research and applications, the well-known analytical dipole model is used to estimate the crossover frequency. However, its accuracy drops significantly for larger particles due to the basic assumptions of the theory. In the simulation case, we consider the electric double layer (EDL) which increases the effective particle conductivity,. and use Maxwell stress tensor (MST) to calculate the DEP force without any assumption on the particle size. By comparing the analytical model and simulation model, we determine a critical diameter at which the crossover frequencies begin deviating from the analytical dipolar prediction.