Effects of Particle Size and Distance between Electrodes on Dielectrophoretic Phenomena of Micro Particles.
Date Issued
2014
Date
2014
Author(s)
Weng, Ping-You
Abstract
Dielectrophoresis (DEP) is an important application for micro particle manipulation, microfluidic system and lab on-chip which can be used for trapping, separating and manipulating of micro particles suspended in a liquid medium. However, the most widely used analytic mode, dipole model, has several assumptions that will cause errors when calculating crossover frequency in the case of large particles. Thus, we propose another analytic model, Maxwell stress tensor (MST), combined with finite-element method to study the DEP phenomenon without the assumptions on particle sizes and shapes.
We conduct two-dimensional (2D) axial symmetric frequency domain electric field finite-element analysis to study the electric field distribution of the system in an alternating current (AC) electric field and use MST to calculate the DEP force exerted on the micro particle. We compare the relationship between crossover frequency and particle diameter obtained by the dipole mode, MST model and published experimental data from the literature. We find that the crossover frequency calculated by MST shows a similar trend as that measured experimentally, in particular the existence of dual slopes and a transitional diameter. On the contrary, dipole model fails to predict the crossover frequency for larger particles. As a result MST model is more applicable in general cases than the conventional dipole model, and may provide valuable insights into some puzzling experimental findings reported in the literature.
To understand the physics and mechanism that determine the crossover frequency curve as a function of particle diameter, we conduct two-dimensional (2D) axial symmetric simulation which has the same relative geometry and avoid the influence of the relative geometric size change. Studying the electric field distribution of various particle sizes, we find that the crossover frequency curve is highly correlated to the ratio of complex permittivity of the particle and medium. Moreover, we use the finite-element model to study of the sensitivity of DEP properties to medium material properties by adjusting the medium conductivity and Ks, included to account for the electric double layer in the modified equation, for particle conductivity.
Lastly, we conduct simulations to analyze ellipsoid particle, and give guidelines on the applicability of conventional analytic dipole model about the particle size. We find that the dipole model is more applicable to an ellipsoid than a spheroid, i.e. the transitional diameter for an ellipsoid is larger than that of a spheroid, which is then explained by the distribution of electric field.
Subjects
介電泳
馬克斯威應力張量
電雙層
表面電導
微流道
微粒子操控
Type
thesis
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