Effect of Electro-osmotic Forces on the Behavior of a Colloidal Particle Near an Electrode During ac Excitation

摘要：卡内基梅隆大学的保罗·赛德斯研究的领域是电化学刺激的自组装。关于实验观察到的胶体颗粒在电极附近异常运动背后的机制的假说将得到检验。实验观察到，在交流极化过程中，当电解液为KOH时，胶体颗粒平均比其平衡位置更靠近电极；相反，当电解液为NaHCO3时，胶体颗粒平均离其平衡位置更远。这种依赖于解决方案的行为的原因仍不清楚。由于电刺激流动而产生的三种可能的候选力来自电动、电流体动力和电渗透效应。动电效应的定义是由电场与粒子上的电荷相互作用引起的。电流体效应的定义是由电场与法拉第电流通过时产生的扩散层中不平衡电荷的相互作用引起的。电渗效应被定义为由电场与电极双层扩散电荷的相互作用引起的。初步计算表明，电动效应是存在的，但不能解释观测结果。电流体效应原则上可以解释粒子平均向下或向上移动，但强度不足以解释观测结果。电渗效应足够强，在某些情况下可以解释实验观察到的结果；因此，要检验的假设涉及与这种效应相关的建模和实验。这项工作将包括四个推力。第一个推力是用有限元分析对电渗效应进行彻底的建模。第二个重点是在全内反射显微镜实验中引入了严格的电化学技术，通过它我们可以观察粒子的行为。第三个推力是一个新颖的实验，它将测量扩散层电位，即电极的Zeta电位。由于扩散层的移动电荷，以几千赫兹振荡(不旋转)的圆盘电极产生准超导交流片。这片电流将通过其周围线圈中的感应电动势来检测。这种设计允许测量电极对的单个表面的Zeta电位，即使在法拉第电流通过期间也是如此。第四个重点是阐述扩散层电势与法拉第电流的关系模型。这项研究将提高对在表面形成有序粒子层的重要作用力的认识。潜在的应用领域是传感器和光学技术。

AbstractCTS-0338089Paul Sides, Carnegie Mellon UniversityThe field of this research is electrochemically stimulated self-assembly. A hypothesis about the mechanism behind an experimentally observed anomalous motion of a colloidal particle near an electrode will be tested. The experimental observation is that a colloidal particle resides on average closer to an electrode than its equilibrium position during ac polarization when the electrolyte is KOH; conversely, the particle resides on average farther away from its equilibrium position when the electrolyte is NaHCO3. The reason for this solution dependent behavior remains unknown. Three possible candidate forces due to electrically stimulated flow arise fromelectrokinetic, electrohydrodynamic, and electroosmotic effects. Electrokinetic effects are defined as arising from the interaction of electric fields with the charge on the particle. Electrohydrodynamic effects are defined as arising from the interaction of electric fields with unbalanced charge in a diffusion layer generated by the passage of faradaic current. Electroosmotic effects are defined as arising from the interaction of electric fields with the diffuse charge of the double layer of the electrode. Preliminary calculations have shown that electrokinetic effects are present but cannot account for the observations. Electrohydrodynamic effects can in principle account move the particle downward or upward on average but are not strong enough to account for the observation. Electroosmotic effects are strong enough and under some circumstances can account for the experimentally observed results; therefore the hypothesis to be tested involves modeling and experimentation related to this effect. The work will consist of four thrusts. The first thrust is thorough modeling of the electroosmotic effect with a finite element analysis. The second thrust is introduction of rigorous techniques of electrochemistry into the Total Internal Reflection Microscopy experiments by which we observe the behavior of the particle. The third thrust is a novel experiment by which the diffuse layer potential, i.e. the zeta potential of the electrode, will be measured. A disk electrode oscillating (not rotating) at several kilohertz produces a quasi-superconducted sheet of alternating current due to the mobile charge of the diffuse layer. This sheet of current will be detected by its induced electromotance in a surrounding coil. This design allows the measurement of the zeta potential of a single surface of an electrode pair even during passage of faradaic current. The fourth thrust is elaboration of a model for the dependence of the diffuse layer potential on faradaic current.The research will improve knowledge of forces important in the formation of ordered layers of particles on surfaces. The potential application areas are sensor and optical technology.