Weng Ping-You, Chen I-An, Yeh Che-Kai, Chen Pin-Yi, Juang Jia-Yang
Department of Mechanical Engineering, National Taiwan University , Taipei 10617, Taiwan.
Biomicrofluidics. 2016 Feb 11;10(1):011909. doi: 10.1063/1.4941853. eCollection 2016 Jan.
Dielectrophoresis (DEP) has been extensively used in lab-on-a-chip systems for trapping, separating, and manipulating of micro particles suspended in a liquid medium. The most widely used analytic model, the dipole model, provides an accurate prediction on the crossover frequency of submicron particles, but cannot explain the significant drop in crossover frequency of larger particles. Here, we present numerical simulations using the Maxwell stress tensor (MST) and finite element method to study the size effect of the DEP crossover frequency of spherical polystyrene particles suspended in de-ionized water. Our results show that the surface conductance due to the electrical double layer plays a key role, and the size dependency of crossover frequency obtained by the MST method agrees reasonably well with published experimental data. The exponents of the power law are approximately -1.0 and -4.3 for smaller (diameter < 4.6 μm) and larger particles (diameter > 4.6 μm), respectively. The free surface charge distribution reveals that the charge begins accumulating on the particle equator for particle diameters larger than a critical diameter of 4.6 μm, a result not captured by the dipolar approximation. This method may be extended to analyze bioparticles with complex shapes and composition, and provides new insights into the interpretation of dielectrophoresis applications using lab-on-a-chip systems.
介电泳（DEP）已在芯片实验室系统中广泛用于捕获、分离和操纵悬浮在液体介质中的微粒。最常用的分析模型——偶极子模型，能准确预测亚微米级颗粒的交叉频率，但无法解释较大颗粒交叉频率的显著下降。在此，我们使用麦克斯韦应力张量（MST）和有限元方法进行数值模拟，以研究悬浮在去离子水中的球形聚苯乙烯颗粒的介电泳交叉频率的尺寸效应。我们的结果表明，由于双电层产生的表面电导起关键作用，通过MST方法获得的交叉频率的尺寸依赖性与已发表的实验数据相当吻合。对于较小（直径<4.6μm）和较大颗粒（直径>4.6μm），幂律指数分别约为-1.0和-4.3。自由表面电荷分布表明，对于直径大于4.6μm的临界直径的颗粒，电荷开始在颗粒赤道上积累，这一结果未被偶极近似所捕获。该方法可扩展用于分析形状和组成复杂的生物颗粒，并为使用芯片实验室系统解释介电泳应用提供新的见解。
