Recent studies disclose that electrohydrodynamic flow is often present in electrochemical systems. Nevertheless, our understanding of electrohydrodynamics
(EHD) is largely confined to situations involving fluid interfaces with apolar liquids
or electrokinetics in aqueous systems. Neither addresses events involving electrodes. This work centers on this new class of flows – electrohydrodynamic motion in systems
with electrochemical reactions
Experiments on well-characterized systems are proposed along with theoretical work on the relevant model equations. In addition to their scientific value, the
results will have technological applications since they provide insight into ways of
controlling smallscale fluid motion* – EHD pumping or patterning, for example. A two-part program is envisioned. In the experimental part, flows in the region between two electrodes will be studied with homogeneous and patterned electrodes. With homogeneous electrodes, motions arising from electrohydrodynamic instabilities
will be investigated. With patterned electrodes, flow is engendered by lateral current inhomogenities on an electrode. Because currents are low and the cells thin,
buoyancy driven convection is weak and the experimental set-up is relatively simple. Flow visualization techniques will be used to image the flows. Preliminary
experiments to establish the feasibility of the approach have been completed.
A parallel component of the program involves solving the relevant equations to describe: (i) the hydrodynamic stability of the homogeneous electrode configuration and (ii) the flow structure in a patterned electrode set-up. Because the model
equations are “stiff” a new methodology is proposed to handle matters in the thin layers near the electrodes. The technique is based on well-tested ideas used effectively in electrokinetics. It promises to simplify the numerical work considerably.
Results from the two parts complement one another and provide a definitive test of electrohydrodynamic models, especially the roles of current and charge
induced near electrodes. Since the experimental set-up is simple, flow visualization will provide qualitative and quantitative tests of the theory.
*See “Downsizing Chemistry” by M. Freemantle in Chemical & Engineering News, February 22, 1999 77 pp. 27-36 or “Microfluidics – a review” by P. Gravesen, J. Branebjerg, & O.S. Jensen J. Micromech. Microeng. 3 168-82 (1993) for a discussion of applications to miniaturizing chemical processes with microfluidic systems.
Electrohydrodynamics offers exciting possibilities for applications. For example, electroconvection can be used in deposition-patterning processes. Since ITO is a
semiconductor photochemical/photo electric effects enhance some electrochemical processes. Accordingly, electrohydrodynamic flows present in colloidal deposition can be
modulated by UV light. The figure shows a recent example [Hayward, Saville & Aksay “Electrophoretic assembly of colloidal crystals with optically tunable
micropatterns” Nature 405 56-59, 2000].
An SEM image of an ITO electrode with a pattern of colloidal crystals (intersecting stripes) induced by the interaction of UV light with the semi-conductor surface
during electrodeposition. A mask was used to filter UV light from the diamond-shaped
regions to alter the deposition and assembly processes. These regions are almost entirely free of colloidal particles having been swept clear by an electrohydrodynamic flow. The intersecting stripes are polycrystalline. The density of the crystal images varies from point to point due to the density of particles and SEM imaging processes.
Saville, D.A., Electrohydrodynamic Flows in Electrochemical Systems, Fifth Microgravity Fluids Physics and Transport Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1263-1274, August 9, 2000.