Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan.
Langmuir. 2011 Sep 20;27(18):11481-8. doi: 10.1021/la202056b. Epub 2011 Aug 16.
The interaction of colloidal particles with a planar surface (i.e., wall) in the presence of an electric field applied parallel to the planar surface is of interest in various microfluidic devices. Evanescent wave-based particle-tracking velocimetry was used to investigate the dynamics of a dilute suspension of polystyrene and silica particles (radii a = 110-463 nm) in a monovalent electrolyte solution with a Debye length of 6.8 nm driven through a microchannel by external electric fields E = 15-31 V/cm over the first 300 nm next to the channel wall. The particle velocity parallel to the wall due to electrophoresis and electroosmosis was in good agreement with the Helmholtz-Smoluchowski relation, and the hydrodynamic interactions between the wall and the particles were negligible, for all particle types. Measurements of the distribution of particles along the wall-normal coordinate, however, suggest that an additional force as great as 30 fN that repels the negatively charged particles away from the wall is induced by nonzero E. The results suggest that the magnitude of this force scales as E(2) and a(2) but is independent of the particle ζ-potential, in agreement with previous theoretical studies. However, estimates of the force assuming that the particles have a Boltzmann distribution were up to 40 times greater than the theoretical predictions, which only considered "remote" particle-wall interactions. These results are, to our knowledge, the first to observe a repulsive wall-normal force due to an applied electric field for near-wall colloidal particles.
在平行于平面的电场存在下，胶体颗粒与平面表面（即壁面）的相互作用在各种微流控设备中很重要。我们采用消逝波粒子跟踪测速法，研究了在 Debye 长度为 6.8nm 的单价电解质溶液中，由外部电场驱动的直径为 110-463nm 的聚苯乙烯和二氧化硅颗粒（a）的稀悬浮液在靠近通道壁的前 300nm 内的动力学。由于电泳和电渗，颗粒在平行于壁面的速度与亥姆霍兹-斯莫卢霍夫斯基关系吻合良好，对于所有颗粒类型，壁面和颗粒之间的流体动力学相互作用可以忽略不计。然而，沿壁面法向坐标测量颗粒的分布表明，由于非零 E，会产生一个高达 30fN 的排斥力，将带负电荷的颗粒从壁面排斥开。结果表明，该力的大小与 E(2)和 a(2)成正比，但与颗粒 ζ 电位无关，这与之前的理论研究一致。然而，假设颗粒具有玻尔兹曼分布的力的估计值比仅考虑“远程”颗粒-壁相互作用的理论预测值高出 40 倍。这些结果是我们所知的首次观察到近壁胶体颗粒由于外加电场而产生的排斥法向力。
