March 9, 2009Aytug Gencoglu
| Aytug Gencoglu - Surprising Consequences of Ion Conservation in Electro-osmosis Over a Surface Charge Discontinuity
A. S. Khair, T. M. Squires
Journal of Fluid Mechanics, v615, pp323-334 (2008).
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The paper reviews situations where surface charge is inhomogeneous and develops equations describing the disruption of the electric field lines and fluid flow for the case of a surface charge discontinuity on a flat plate over which electro-osmosis occurs. Later on, they discuss the expected differences for different surface conductivity models and different sets of surface charge and electric field conditions. The developed equations are argued to be universal for high zeta potentials, and it is shown that nanoscale inhomogeneities in the surface charge can cause micro-scale disruptions of the electric field and fluid flow. Finally, the authors discuss the chemi-osmosis effect and speculate on its impact on this phenomenon.
Introduction
First paragraph would be trivial in a publication more specific to the field of electrokinetics, but provides good background in the Journal of Fluid Mechanics.
The second paragraph is a review of cases where the surface charge is inhomogeneous, including some due to the electrode configuration.
A flat plate where the surface charge is zero before a point and highly negative after that point is considered. The high zeta potential assumption here is the opposite of the model presented by Yariv (2004), where the bulk electric field is equal to the applied field (Detailed in paragraph 4).
Basic Physical Picture
The system is further defined: The equation has been developed for a binary, symmetric electrode, although that only affects the definition of the Debye length, so the equation is not restricted to this case.
Figure 1 and the text both clearly demonstrate the physical phenomenon causing the disruption. The Guoy-Chapman model is used for the specific conductivity, but other models can be used with the same equation, with different results with respect to the healing length.
It is shown that at low zeta potentials, the healing length is very small and the prediction of Yariv (2004) holds.
Bulk Field and Flow
In this section, the equation for the healing length is developed. As I understand, (3.4) is used to describe bulk lines and healing length is deduced from the result.
In addition, the equation for the fluid flow is developed in this section.
Discussion
It is stated that the GC model may be overestimating.
The cases where the surface charge is positive instead of negative and where field is reversed are discussed and the cases where the salt concentration increases and decreases near the surface are identified. This brings up another flow mechanism, chemi-osmosis and it is speculated that its contribution to the described phenomenon may be significant.
Presentation and Recommendation
The paper is written in such a way that it can be understood fairly well even without extensive knowledge of the mathematical concepts. It has a good introduction and thoughtful discussion throughout. However, the sense of suspense in the paper is somewhat amusing. The figures are simple and can be followed with ease.
I would recommend accepting this paper.
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