AC Electric Field Driven Motion of Colloidal Particles near an Electrode

James Hoggard, Dennis Prieve, and Paul Sides

Carnegie Mellon University

Controlling the aggregation or separation of colloidal particles is of interest in many fields, including display technologies, many biotechnical applications, and in photonic materials.  In the last decade, it has been observed that applying an electric field between parallel plate electrodes can lead to the aggregation or separation of colloidal particles, even though this motion is normal to the applied electric field.  While this motion is understood for dc fields,1,2 the motion is much more complex when ac fields are applied and not understood.  Investigations by several groups have shown that many different colloidal phases are possible depending on the frequency and magnitude of the applied electric field, electrode spacing , particle density, and even the electrolyte.3-19  Observed phases include hexagonal crystals of various spacing, 2- and 3-D wormlike structures, honeycomb structures, and fluid-like phases with no apparent structure.  We are interested in frequencies between approximately 50 and 1000 Hz where reactions on the electrode surface are important.  Some structures that we have observed using video microscopy include:

0.15 mM KCl on Indium-Tin-Oxide (ITO) Electrodes (100 Hz, 7158 V/m):

View Video of KCl on ITO

This video shows aggregation similar to that observed by others.

0.15 mM NaHCO3 on Platinum Electrodes (100 Hz, 7158 V/m):

View Video of NaHCO3 on Platinum

                This is a really neat video of what we believe to be a "condensation" process in which the particles slowly aggregate after initially being repelled.

0.15 mM KOH on Platinum Electrodes (100 Hz, 7158 V/m):

View Video of KOH on Platinum

Depending on the concentration of particles, 3-D worms, or a highly spaced hexagonal array can form.

 

While many groups are looking at the 2-D motion parallel to the electrode surface, we have the capability of also observing the vertical (surface normal) motion of the particles using Total Internal Reflection Microscopy (TIRM).  Using TIRM, Fagan, Sides and Prieve13-18 showed that at low frequencies (50 - 500 Hz), the phase of the applied electric field and the vertical oscillatory motion are of necessarily 90° out of phase as would be expected from the electrophoretic particle motion alone.  Because of this phase shift, the net force on the particle over one complete cycle is not zero.  His model showed that phase angles greater than 90° lead to aggregation, while phase angles less than 90° result in separation.  He compared his model to doublet results of Kim.12  His model accurately predicted doublet aggregation and separation times in NaHCO3 and KOH, respectively.  Our experimental setup is shown below.

Currently, we are testing Fagan's model and trying to bridge the gap between systems used by other groups.  In order to do this, phase angle measurements are made on single particles, aggregation (or separation) times are measured on pairs of particles in close proximity, and multiparticle effects are observed using video microscopy.  Experiments have been done on several electrolyte/electrode systems .  To date, all phase angle and doublet aggregation results agree with Fagan's assertions.  Our phase angle and doublet aggregation/separation experiments are shown below.  In each experiment, 5.7 ± 0.5 micron diameter negatively charged stabilized polystyrene particles are used.  The electrode spacing is 1.4 mm so the particles are not confined.  The phase angles are each measured at 1790 V/m where the particle trajectories are symmetric and a legitimate phase angle can be reported.  The doublet experiments are done at 100 Hz and the same field strength. 

 

The results of the experiments show that KOH has a phase angle less than 90° and the doublets always separate on both ITO and Pt.  Phase angles in NaHCO3 and KCl are each greater than 90° resulting in doublet aggregation on both ITO and Pt.

The multi-body effects observed in the pictures and videos above are made much more interesting in light of the knowledge as to if the system tends to aggregate or separate in a given electrolyte.

Future Work

In the near future, we are shall be doing the same experiments described above in HNO3 and H2CO3.  Fagan measured a phase angle less than 90° for HNO3, but no aggregation data is available.  It is expected that doublets will separate, similar to KOH.

A discrepancy that exists in the literature is in the KOH experiments.  Several other groups have done multiparticle experiments in which aggregation has resulted.  This contradicts our experiments in which particles are always observed to always separate.  The reason for this may be that other groups did not remove the carbonic acid from their cells.  We sparge our solution for several hours with nitrogen before injecting our fluid into the cell.  Liu, et. al.19 measured the pH of their cell and found it to be acidic with a pH of 6.0, which indicates that KOH is not the dominate electrolyte in solution.  The pH of our cells is about 9.0.  We believe that it is the electrode reaction that determines the phase angle, thus determining whether particles will aggregate or separate in a given field.  Different electrode reactions could account for the observed differences.  Sparging with nitrogen may be important in order to assure separation in KOH.  One might expect to see aggregation in carbonic acid if this hypothesis is true.

Because we believe that the aggregative behavior of particles is determined by the electrochemical reaction on the bottom electrode, we would like to better characterize these reactions using models and experiments. We plan to measure the zeta potentials of electrodes while passing faradaic current.  The information obtained from these experiments will be added to Fagan's doublet model.

Links:

Carnegie Mellon University, Department of Chemical Engineering

Dennis Prieve's Website

Paul Sides' Website

Email James Hoggard

References:

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  13. Fagan, J. A. Ph. D. Thesis 2005, Carnegie Mellon University.

  14. Fagan, J. A.; Sides, P. J.; Prieve, D. C. Langmuir 2002, 18, 7810-7820.

  15. Fagan, J. A.; Sides, P. J.; Prieve, D. C. Langmuir 2003, 19, 6627-6632.

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  18. Fagan, J. A.; Sides, P. J.; Prieve, D. C. To be published 2006.

  19. Liu, Y.; Narayanan, J.; Liu, X. J. of Chem. Phys. 2006, 124, 124906.