Отправлено: 18.08.08 03:08. Заголовок: Volume 325, Issue 2, Pages 301-602 (15 September 2008)

Theoretical model for the wetting of a rough surface
Pages 472-477

K.M. Hay, M.I. Dragila, J. Liburdy
By what mechanism is water propelled along rough surfaces? A theoretical relationship describing the wetting of a rough surface is derived based on capillary and frictional forces
Рисунок не импортируется, жаль, красиво. ;-)
[img]http://www.sciencedirect.com/cache/MiamiImageURL/B6WHR-4SP3SPV-2-1/0?wchp=dGLzVlz-zSkWb[/img]


Electrokinetics in nanochannels: Part I. Electric double layer overlap and channel-to-well equilibrium
Pages 526-538
Fabio Baldessari

In this paper a new model is described for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. Electrolyte concentration in the nanochannel is predicted self-consistently via equilibrium between ionic solution in the wells and within the nanochannel. Differently than published models that require detailed iterative numerical solutions of coupled differential equations, the framework presented here is self-consistent and predictions are obtained solving a simple one-dimensional integral. The derivation clearly shows that the electric potential field depends on three new parameters: the ratio of ion density in the channel to ion density in the wells; the ratio of free-charge density to bulk ion density within the channel; and a modified DebyeЁCHЁ№ckel thickness, which is the relevant scale for shielding of surface net charge. For completeness, three wallЁCsurface boundary conditions are analyzed: specified zeta-potential; specified surface net charge density; and charge regulation. Predictions of experimentally observable quantities based on the model proposed here, such as depth-averaged electroosmotic flow and net ionic current, are significantly different than results from previous overlapped electric double layer models. In this first paper of a series of two, predictions are presented where channel depth is varied at constant well concentration. Results show that under conditions of electric double layer overlap, electroosmosis contributes only a small fraction of the net ionic current, and that most of the measurable current is due to ionic conduction in conditions of increased counterion density in the nanochannel. In the second of this two-paper series, predictions are presented where well-concentration is varied and the channel depth is held constant, and the model described here is employed to study the dependence of ion mobility on ionic strength, and compare predictions to measurements of ionic current as a function of channel depth and ion density.
Article Outline
1. Introduction
2. Theoretical formulation
2.1. Distribution of ions (ni(r)) and free-charge (¦СE(r))
2.2. The potential distribution in a wide, shallow channel (¦Ч(y))
2.2.1. Boundary condition I (BC I): specified wall-potential (¦Ч(0)=¦Ж)
2.2.2. Boundary condition II (BC II): specified wall-charge density
2.2.3. Boundary condition III (BC III): charge regulation
2.3. Ionic mobility dependence on ionic strength and pH
2.4. Electroosmotic flow
2.5. Net ionic current
3. Parameter estimates in thin EDL regime: zeta potential, surface charge density, and fraction of chargeable sites
4. Theoretical results for constant BGE well concentration, and varying channel depth
5. Conclusions and recommendations
Appendix A. Appendix
Appendix B. Appendix
Appendix C. Appendix
References

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Electrokinetics in nanochannels: Part II. Mobility dependence on ion density and ionic current measurements
Pages 539-546
Fabio Baldessari

Abstract
In the first of this two-paper series, a new model was developed for calculating the electric potential field in a long, thin nanochannel with overlapped electric double layers. The model takes into account the dependence of ion mobility on local ion densities and pH. This model is used here to study and demonstrate the effect of ion density and pH on ionic current measurements. A comparison is shown of predictions based on each of three boundary conditions, as studied in Part I. The model developed in Part I is validated by comparing simulations with measurements of ionic current as a function of sodium borate concentration. Results show that predictions based on extended Debye–Hückel theory for ion mobility significantly improve the accuracy of simulations, but that these do not predict exact scaling behavior. A simple bulk conductivity measurement used as input parameter for the simulations, in place of the predicted bulk conductivity (K0), guarantees agreement with data in the thin EDL region. Results also indicate that the charge regulation boundary condition, complemented with an adequate bulk electrolyte model, provides better agreement with experimental trends than the specified zeta potential or specified surface net charge boundary conditions. Further, it is shown that currents due to advection (by electroosmotic flow) are in all cases studied less than 25% of the total current in the system.
Article Outline
1. Introduction
2. Overview of channel-to-well equilibrium model for ionic current
3. Materials and experimental methods
3.1. Nanochannel fabrication
3.2. Electrolyte solutions
3.3. Current and conductivity measurements
4. Bulk electrolyte model for pH and experimental validation
5. Results for varying BGE well concentration and constant channel depth
6. Conclusions and recommendations
References


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