(3ev) Electrokinetic Motion of Particles near An Electrode: Theory, Experiments, and Applications

The motion of particles proximate to an electrode stimulated by an electric field has been investigated for nearly two decades because of its potential impact on the assembly of colloidal crystals and on the high throughput evaluation of electrocatalysts. A rich mixture of electrokinetic phenomena has motivated an abundance of theoretical and experimental work in this area.¹ Despite the wealth of knowledge built over the last twenty years, a number of questions remained that required answers to leverage this technology. At the start of my PhD, I was charged with answering these questions; this poster will display snapshots of this undertaking.

1.       What causes symmetry breaking in a single particle’s longitudinal motion? The equilibrium spacing of a colloidal crystal depends on a broken symmetry between a single particle’s longitudinal motion and the electric field. Symmetry breaking arises from the dynamic forces acting on the particle during AC electrophoresis and not on an additional out of phase element as previously expected. New modeling suggests that inertial effects and dynamic drag on the particle play a crucial role in determining the phase angle (i.e. evidence of the broken symmetry) between the electric field and the resulting particle motion.

2.       Are electrode reactions responsible for electrolyte dependent particle motion? No. Using a new tool called the Electrochemical Total Internal Reflection Microscope, I measured single particle longitudinal and pairwise lateral motion engendered by an electric field while the electrode was ideally polarizable (IPE).² Despite removing any possibility of electrode reactions on the IPE, particle motion depended on the choice of dispersing electrolyte. Isolated particle pairs aggregated in 0.15 mM NaHCO₃ and separated in 0.15 mM KOH with every other experimental condition kept identical.

3.       Can single particle motion be an effective tool for probing local current density on an electrode? Yes. Instead of considering the particles to be objects of manipulation, I employed the particles as probes of local current density.^3,4 Local current density as predicted by the particle’s motion was within a factor of two of the nominal current density even with a simple model using no adjustable parameters. In principle, this allows for the local measurement of current density on spatially heterogeneous electrocatalysts, such as composition spread alloy films, paving the way for high throughput electrochemistry.

        In the future, I will build on my experience in electrokinetics and complex fluids to conduct research in responsive fluids, namely electrorheological, magnetorheological, and electrowetting fluids. I’m also interested in extending Total Internal Reflection Microscopy to a broader experimental space, particularly to anisotropic particles.

1.       DC Prieve, PJ Sides, CL Wirth; 2D particle assembly of colloidal particles on a planar electrode, Current Opinion in Colloid & Interface Science (2010) 15 (3).

2.       CL Wirth, RM Rock, PJ Sides and DC Prieve; Single and pairwise motion of particles near an ideally polarizable electrode, in preparation.

3.       CL Wirth, PJ Sides, DC Prieve; The imaging ammeter, Journal of Colloid and Interface Science (2011) 357 (1).

4.       PJ Sides, CL Wirth, DC Prieve; An imaging ammeter for electrochemical measurements, Electrochemical and Solid-State Letters (2010) 13 (8).