Enabling the enhancement of electroosmotic flow over superhydrophobic surfaces by induced charges

Steffes, Clarissa; Baier, Tobias; Hardt, Steffen

doi:10.1016/j.colsurfa.2010.09.002

Cited by 21 publications

(17 citation statements)

References 7 publications

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“…In section III C, we will compare these results with the more exact ones based on expressions (11), (14), (21) and (23). Additionally we also compare with numerical calculations, using the nonlinear Poisson-Boltzmann equation instead of relying on the linearised Debye-Hückel approximation.…”

Section: Resultsmentioning

confidence: 97%

“…This latter condition seems nontrivial at first, since any ions that are preferentially attracted to the surface are expected to attract an equal amount of opposite charge in a diffuse cloud just as on a solid surface. However, a net charge is a) Corresponding author: baier@csi.tu-darmstadt.de attracted to the surface when an electric field is applied normal to the fluid-air interface 20 , for example by embedding electrodes within the grooves 21 .…”

Section: Introductionmentioning

confidence: 99%

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Effect of electro-osmotic flow on energy conversion on superhydrophobic surfaces

Gowrishankar

Baier²

2013

Physics of Fluids

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It has been suggested that superhydrophobic surfaces, due to the presence of a no-shear zone, can greatly enhance transport of surface charges, leading to a considerable increase in the streaming potential. This could find potential use in micro-energy harvesting devices. In this paper, we show using analytical and numerical methods, that when a streaming potential is generated in such superhydrophobic geometries, the reverse electro-osmotic flow and hence current generated by this, is significant. A decrease in streaming potential compared to what was earlier predicted is expected. We also show that, due to the electro-osmotic streaming-current, a saturation in both the power extracted and efficiency of energy conversion is achieved in such systems for large values of the free surface charge densities. Nevertheless, under realistic conditions, such microstructured devices with superhydrophobic surfaces have the potential to even reach energy conversion efficiencies only achieved in nanostructured devices so far.

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Section: Resultsmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

Effect of electro-osmotic flow on energy conversion on superhydrophobic surfaces

Gowrishankar

Baier²

2013

Physics of Fluids

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“…Instead of air being enclosed in the corrugations, other materials like for instance oil could be employed, which promises an improved stability of the Cassie state against pressure (Wong et al 2011) or enhanced electrical properties (Steffes, Baier & Hardt 2011). The choice of an appropriate groove-filling medium provides the chance to specifically tailor the surface properties, for examples in terms of phobicity and philicity, not only with regard to water, but to any desired fluid.…”

Section: Introductionmentioning

confidence: 99%

Influence of the enclosed fluid on the flow over a microstructured surface in the Cassie state

2014

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Analytical expressions for the flow field as well as for the effective slip length of a shear flow over a surface with periodic rectangular grooves are derived. The primary fluid is in the Cassie state with the grooves being filled with a secondary immiscible fluid. The coupling of the two fluids is reflected in a locally varying slip distribution along the fluid-fluid interface, which models the effect of the secondary fluid on the outer flow. The obtained closed-form analytical expressions for the flow field and effective slip length of the primary fluid explicitly contain the influence of the viscosities of the two fluids as well as the magnitude of the local slip, which is a function of the surface geometry. They agree well with results from numerical computations of the full geometry. The analytical expressions allow an investigation of the influence of the viscous stresses inside the secondary fluid for arbitrary geometries of the rectangular grooves. For classic superhydrophobic surfaces, the deviations in the effective slip length compared to the case of inviscid gas flow are pointed out. Another important finding with respect to an accurate modelling of flow over microstructured surfaces is that not only the effective slip length, but also the local slip length of a grooved surface, is anisotropic.

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“…Next, the velocity field w q |i (originating from the charge accumulated at the gas-liquid interface) is described. In the limit of an infinitely thin EDL as treated herein, the overall electro-osmotic force per volume on the ion cloud accumulating in the vicinity of the free surface can be replaced [38,39] by an effective stress condition η(∂ y w q |i ) |i = −q |i ∂ z φ st = −τ ∞,q , while in the bulk (∂ 2…”

Section: Velocitymentioning

confidence: 99%

Energy conversion by surface-tension-driven charge separation

Pini

Baier

Dietzel

2015

Microfluid Nanofluid

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In this work, the shear-induced electrokinetic streaming potential present in free-surface electrolytic flows subjected to a gradient in surface tension is assessed. Firstly, for a Couette flow with fully resolved electric double layer (EDL), the streaming potential per surface stress as a function of the Debye parameter and surface potential is analyzed. By contrast to the Smoluchowski limit in pressure-driven channel flow, the shear-induced streaming potential vanishes for increasing Debye parameter (infinitely thin EDL), unless the free surface contains (induced) surface charge or the flow at the charged, solid wall is permitted to slip. Secondly, a technical realization of surface-tension induced streaming is proposed, with surface stress acting on the free (slipping) surfaces of a micro-structured, superhydrophobic wall. The streaming potential is analyzed with respect to the slip parameter and surface charge. Finally, the surface tension is assumed to vary with temperature (thermocapillarity) or with surfactant concentration (destillocapillarity). The maximal thermal efficiency is derived and compared to the Carnot efficiency. For large thermal Marangoni number, the efficiency is severely limited by the large heat capacity of aqueous solvents. By contrast, destillocapillary flows may reach conversion efficiencies similar to pressure-driven flow.

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Enabling the enhancement of electroosmotic flow over superhydrophobic surfaces by induced charges

Cited by 21 publications

References 7 publications

Effect of electro-osmotic flow on energy conversion on superhydrophobic surfaces

Effect of electro-osmotic flow on energy conversion on superhydrophobic surfaces

Influence of the enclosed fluid on the flow over a microstructured surface in the Cassie state

Energy conversion by surface-tension-driven charge separation

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