Kim Junhyung, Garoff Stephen, Anderson John L, Schlangen Luc J M
Department of Chemical Engineering, Center for Complex Fluid Engineering, and Physics Department, Carnegie Mellon University, Pittsburgh, Pennsylvania, USA.
Langmuir. 2005 Nov 22;21(24):10941-7. doi: 10.1021/la051233c.
We characterize the movement of carbon black particles in inhomogeneous, two-dimensional dc electric fields. Motivated by display applications, the particles are suspended in a nonpolar solvent doped with a charge control agent. The two-dimensional fields are generated between strip electrodes on a glass slide spaced 120 microm apart with field strengths up to 10(4) V/m. Such fields are insufficient to drive either electrohydrodynamic instabilities or natural convection due to ohmic heating, but they move the particles between the electrodes in about 30 s. In the center region between the strip electrodes, the particles move by electrophoresis; that is, the particle velocity is proportional to the electric field. However, when imposing a constant-potential or constant-current boundary condition at the electrodes to derive the electrical field, the electrophoretic mobility calculated from the measured particle velocities is outside the range of mobilities predicted from the theory of O'Brien and White. Near the electrodes the particles either speed up or slow down, depending on the polarity of the electrode, and these changes in velocity cannot be explained simply by electrophoresis in a spatially varying electric field. We suggest that this anomalous motion arises from electrohydrodynamic flows originating from the interaction between the space charge of the polarized layers above the electrodes and the electric field. Approximate calculations indicate such flows could be sufficiently strong to explain the anomalous trajectories near the edges of the electrodes.
我们描述了炭黑颗粒在非均匀二维直流电场中的运动。受显示应用的启发，颗粒悬浮在掺杂有电荷控制剂的非极性溶剂中。二维电场在玻璃载玻片上间距为120微米的条形电极之间产生，场强高达10⁴V/m。这样的场不足以驱动由于欧姆加热引起的电流体动力学不稳定性或自然对流，但它们能在大约30秒内使颗粒在电极之间移动。在条形电极之间的中心区域，颗粒通过电泳移动；也就是说，颗粒速度与电场成正比。然而，当在电极处施加恒电位或恒流边界条件来推导电场时，根据测量的颗粒速度计算出的电泳迁移率超出了奥布赖恩和怀特理论预测的迁移率范围。在电极附近，颗粒要么加速要么减速，这取决于电极的极性，而这些速度变化不能简单地用空间变化电场中的电泳来解释。我们认为这种异常运动源于电极上方极化层的空间电荷与电场相互作用产生的电流体动力学流动。近似计算表明，这种流动可能足够强以解释电极边缘附近的异常轨迹。
