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

Carnegie Mellon University, Pittsburgh PA

Investigators

Abstract

Abstract CTS-0338089 Paul Sides, Carnegie Mellon University The 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 from electrokinetic, 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.