The role of electrochemistry in the 2D assembly of colloidal particles on a planar electrode.

摘要

The development of a quick, cheap, and reversible method for the synthesis of colloidal crystals would benefit the emerging field of photonic materials. An external electric field was used to assemble colloidal particles into a colloidal crystal onto a proximate electrode. The steady structure of the assembly depended on a number of system parameters, including the electric field strength and frequency, the presence of confining boundaries, and the dispersing electrolyte. For example, particles dispersed in NaHCO3 attracted each other to form a close-packed structure, but particles in KOH formed a diffuse structure, indicating a repulsive inter-particle interaction.;Net motion (and the subsequent assembly) among particles excited by an AC electric field occurred because of a break in symmetry between the motion of a single particle normal to the electrode and the electric field. The break in symmetry was parameterized with the phase angle between the oscillatory motion of a particle (as measured with Total Internal Reflection Microscopy (TIRM)) and the electric field. The experimentally measured phase angle was crucial to predicting whether particles attracted (as in NaHCO3) or repelled each other (as in KOH). The hypothesis for the origin of these effects was that faradaic current arising from electrochemical reactions on the electrode provided an out-of-phase force; the details herein describe the testing of this hypothesis.;The hypothesis was tested by measuring the phase angle of a particle proximate to an ideally polarizable electrode in a variety of electrolytes. In addition, the separation distance between particles near an ideally polarizable electrode was measured. No faradaic reactions are possible on an ideally polarizable electrode. Particle motion remained electrolyte dependent in both experiments despite the absence of electrochemical reactions on the electrode. These results indicated our hypothesis was incorrect.;The model for the motion of a single particle was revisited and updated in response to these results. Newton's second law was solved with inclusion of a colloidal force field, dynamic drag, and an electrophoretic driving force. The dynamic drag comprised three parts, namely boundary corrected Stokes drag, added inertia, and the Basset or "history" force. The electrophoretic driving force included a full description of the dynamic electrophoretic mobility of the particle at a finite value of kappaalpha. Theoretical predictions of the phase angle were in fair quantitative agreement with experimental measurements. In addition, these calculations revealed two origins of electrolyte dependence of the motion of a single particle. The first was the effect of electrolyte conductivity and the second was the electrolyte dependence of the dynamic electrophoretic mobility; both of these effects arise from the transport properties of the dispersing electrolyte.;In a parallel (but different) project, the motion of a single particle normal to a powered electrode was used as a probe of local electrochemical current. The technology, named the Imaging Ammeter, transduces the local electrochemical current from a light signal scattered from the particle (as measured with TIRM). Initial experiments demonstrated that the Imaging Ammeter does a good job of measuring local electrochemical current. Future work will scale the technique to a ∼1 cm2 sample with a "pixel" width of 100 mum or 10,000 measurements of local electrochemical current on a single sample.