The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Dynamics of Particles in Spatially and Temporally Varying Electric Fields near Electrodes
by Junhyung Kim



Two phenomena were studied in this thesis: 1) the relative motion between two or more colloidal particles on an electrode experiencing an ac potential with aqueous electrolyte solutions, and 2) the movement of ions and particles in non-polar liquids in one and two dimensional electric fields. The unifying theme of the work is the use of electric fields to position particles over micron length scales.
In the first part of the thesis, I discuss aggregation or separation of pairs of negatively charged polystyrene latex particles deposited on an electrode under ac polarization. I studied the aggregation experimentally by varying the zeta potential of the particles, the electrolyte composition and concentration, and the frequency of the electrode potential. Trajectories of two adjacent particles were recorded by video-imaging, and analyses were performed to determine the relative velocity between the particles. The relative velocity did not depend on the zeta potential of particles, implying the motion was not electrokinetic in nature. The two-particle dynamics were distinctly different between electrolytes containing bicarbonate ion versus hydroxyl ion; pairs of particles aggregated in bicarbonate solutions at frequencies between 30 and 500 Hz but separated at 1,000 Hz, while the pairs separated with hydroxyl ions at all frequencies up to 1,000 Hz. In all cases the relative velocity between a pair of particles was relatively independent of electrolyte concentration and the cation of the electrolyte. The lack of dependence of the two-particle dynamics on the zeta potential of the particles suggests that the phenomenon of aggregation/separation is driven by electrohydrodynamics resulting from nonuniform electric fields at the electrode surface, but no model is able to quantitatively fit the experimental results.
The second part of the thesis discusses manipulation of particles in non-polar solutions with dc electric fields. The goal was to understand ion conduction in these fluids and characterize particle motion in response to two-dimensional fields established in a thin film of the fluid. First ion conduction was studied with solutions of OLOA 371 (primarily the amphiphile poly(isobutylene succinimde)) and dodecane confined between two parallel electrodes. The conductivity of the solutions was measured with a conductivity probe and found to be proportional to the OLOA concentration. The applied dc potential across the parallel electrodes caused an initial current that was proportional to the applied potential, and the conductivity determined from this proportionality agrees with that measured with the conductivity probe. The current decayed to about 10% of its initial value within 10 seconds of applying the potential and then remained nearly constant up to 10 hours. This long-time residual current could be due to continuous polarization of the electrode if the capacitance of the double layer was to increase with time, but it could also be due to charge transfer across the electrode/fluid interface. I applied the Gouy-Chapman model of the diffuse double layer to analyze the transient current within 1 second of the applied potential and obtained the equivalent ionic strength of the solution. Using this ionic strength and the known solution conductivity, I calculated the size of the charge carrying species to be 10 nm in radius for OLOA concentrations over the range 0.5 – 3.5 weight percent. This result is consistent with the notion that the charge carrying species are micelles of PIBS with only one charge per 105 PIBS molecules. I conclude that OLOA/dodecane solutions represent a very weak electrolyte system with charge carriers about 100 times the size of simple ions in aqueous electrolyte solutions.
Conduction of OLOA/dodecane solutions in two-dimensional fields was also studied in a cell consisting of electrode strips on one of the two glass slides confining the solution. A dc potential was applied between the center strip and the two outer strips, one on each side. Theoretical calculations of the electrode cell constant gave results that agree with the experimental determinations from the initial current versus applied potential difference. As with the planar electrode configuration, the initial current decayed in seconds to about 10% of its initial value, and the residual current persisted for hours. Carbon black particles about 1 μm in diameter were allowed to sediment onto the lower glass slide containing the strip electrodes. Their trajectories were video-taped and analyzed during the residual current phase. The particles moved against the direction of the electric field between the strips, indicating they were negatively charged. In the region midway between two strips, the velocity of the particles was nearly constant as expected since the electric field in this region is very uniform. The electrophoretic mobility determined from these trajectories is in reasonable agreement with data for this system published in the literature. The particles accelerated when they approached the more positive strip, and appeared to stall near the lower potential strip. This anomalous motion cannot be explained by dielectrophoresis nor by the higher fields near the edges of the strips. I suggest that electrohydrodynamic flows might be affecting the particle motion near the edges. These flows could arise, for example, by uneven polarization of the strip electrodes.