Effects of Zeta Potential and Electrolyte on Particle Interactions on an Electrode under ac Polarization Junhyung Kim, John L. Anderson,* Stephen Garoff, † and Paul J. Sides Department of Chemical Engineering, and Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received February 28, 2002 The relative motion between two colloidal particles loosely deposited on an electrode passing alternating current was investigated. Parameters such as zeta potential, electrolyte composition, electrolyte concentration, and frequency were varied. At a low frequency (100 Hz), the particles aggregated in both sodium bicarbonate and sodium chloride solutions but separated in sodium hydroxide solutions. At 1000 Hz, the particles separated in both bicarbonate and hydroxide solutions, and the rate of separation was slower than at 100 Hz for the hydroxide solutions. The effect of zeta potential was negligible, indicating a convective mechanism causing the relative motion between the particles. Electrolyte concentration had no appreciable effect on the motion. These results are qualitatively consistent with predictions of a theory based on electrohydrodynamic flow induced by the interaction between a space charge in the liquid adjacent to the electrode’s surface, generated by concentration gradients of the ions, and an electric field tangent to the electrode which is caused by deflection of current around each particle. The interparticle separation velocity in hydroxide solutions predicted from the theory without adjustable parameters is comparable to the experimental values. Introduction Colloidal particles loosely deposited on an electrode have been observed to move laterally and form clusters in both direct current (dc) and alternating current (ac) electric fields. 1-8 In both cases, the interaction between particles occurred on a length scale comparable to the particle size; however, the electric fields in the ac mode were 100 times those in the dc mode to achieve the same relative velocity between pairs of particles. Experimental studies of the interactions between two particles in dc fields 5,6 have demonstrated that electroosmotic flow 9,10 about each particle is responsible for the relative motion between particles, and the relative velocity between the particles is proportional to E where  is the zeta potential of the particles and E is the electric field at the electrode’s surface in the absence of the particles. The relative motion of particles in ac fields is not as well understood. The objective of this paper is to present data that can distinguish between electrokinetic and electrohydrodynamic mechanisms for particle motion on electrodes in ac fields. In particular, we test certain features of a recently published electrohydrodynamic theory 11 based on fluid convection generated by the ac electric field interacting with a diffusion layer of ions created by electrode reactions. In a previous paper, 8 we presented data for the relative motion of two charged latex particles on a tin-doped indium oxide (ITO) electrode undergoing ac polarization. The electrolyte was sodium bicarbonate, and the root-mean- square (rms) field was about 30 V/cm. At frequencies in the range of 30-500 Hz, pairs of particles approached each other at a speed that decreased as frequency increased, and at 1000 Hz the particles moved apart. Another interesting observation in ac fields was that the particles stopped moving together when the gap between them was about 1/2 the particle radius. (In dc fields, the particles approach each other until they come essentially into contact. 5,6 ) This steady-state gap in ac fields was somewhat dependent on the field strength and frequency. While these results 8 provide some insight into the dynamics in ac fields, they are not sufficient to test competing theories for the mechanism behind the two- particle interactions. Two important system properties that have not yet been examined are the zeta potential of the particles and the type of ions composing the electrolyte. If the mechanism behind the particle interactions in ac fields is dominated by electrokinetics, as it is with dc fields, then the zeta potential would be an important parameter and the relative velocity between particles should be proportional to . In addition, an electrokinetic mechanism should depend only on the Debye screening length and hence would not distinguish between similar-valence electro- lytes, that is, between sodium bicarbonate and sodium hydroxide. On the other hand, if the relative two-particle velocity is independent of  but dependent on the type of ion in the electrolyte, then electrohydrodynamic convec- tion 3,11 might be the cause. Experiments The experimental cell appears in Figure 1. The two ITO electrodes consisted of 100 nm thick films of ITO on glass substrates 25.4 mm in diameter having a sheet resistance of 16 * To whom correspondence should be addressed. † Department of Physics. (1) Bohmer, M. Langmuir 1996, 12, 5747. (2) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (3) Trau, M.; Saville, D. A.; Aksay, I. A. Langmuir 1997, 13, 6375. (4) Yeh, S. R.; Seul, M.; Shaiman, B. I. Nature 1997, 386, 57. (5) Guelcher, S. A. Investigating the Mechanism of Aggregation of Colloidal Particles during Electrophoretic Deposition. Ph.D. Thesis, Carnegie Mellon University, Pittsburgh, PA, 1999. (6) Guelcher, S. A.; Solomentsev, Y.; Anderson, J. L. Powder Technol. 2000, 110, 90. (7) Sarkar, P.; De, D.; Yamashita, K.; Nicholson, P. S.; Umegaki, T. J. Am. Ceram. Soc. 2000, 83, 1399. (8) Kim, J.; Guelcher, S. A.; Garoff, S.; Anderson, J. L. Adv. Colloid Interface Sci., in press. (9) Solomentsev, Y.; Bohmer, M.; Anderson, J. L. Langmuir 1997, 13, 6058. (10) Solomentsev, Y.; Guelcher, S. A.; Bevan, M.; Anderson, J. L. Langmuir 2000, 16, 9208. (11) Sides, P. Langmuir 2001, 17 (19), 5791. 5387 Langmuir 2002, 18, 5387-5391 10.1021/la025682d CCC: $22.00 © 2002 American Chemical Society Published on Web 06/13/2002