Direct Measurements of Hydrophobic Slippage Using Double-Focus Fluorescence Cross-Correlation

Vinogradova, Olga I.; Koynov, Kaloian; Best, A.; Feuillebois, François

doi:10.1103/physrevlett.102.118302

Cited by 127 publications

(118 citation statements)

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“…However, recently direct high-precision measurements at the nanoscale have been performed with a new optical technique, based on a DF-FCS (double-focus spatial fluorescence cross-correlation) [30,29] (as is schematically shown in Fig. 3).…”

Section: Experimental Methodsmentioning

confidence: 99%

Wetting, roughness and flow boundary conditions

Vinogradova

Belyaev

2011

J. Phys.: Condens. Matter

152

191

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Abstract. We discuss how the wettability and roughness of a solid impacts its hydrodynamic properties. We see in particular that hydrophobic slippage can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well-controlled micro-or nanotextures on a solid, these effects are being exploited to induce novel hydrodynamic properties, such as giant interfacial slip, superfluidity, mixing, and low hydrodynamic drag, that could not be achieved without roughness.

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Section: Experimental Methodsmentioning

confidence: 99%

Wetting, roughness and flow boundary conditions

Vinogradova

Belyaev

2011

J. Phys.: Condens. Matter

152

191

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“…correlation [20] in other geometries, the slip length resolution is higher in this work, mainly 10 due to the use of a thin lubricant film. In regards to stimulated emission depletion 11 photobleaching anemometry [18] the spatial resolution in the x-direction and the temporal 12 resolution for this work is relatively low at the moment but can be improved as discussed …”

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confidence: 94%

Through-Thickness Velocity Profile Measurements in an Elastohydrodynamic Contact

2013

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7594 8991This work presents for the first time through-thickness velocity profiles obtained in an EHL contact by photobleached imaging. The velocity profile was inferred by following the evolution of the shape of a photobleached plug formed through the thickness of the fluorescently-doped lubricant, oligomer polybutene (PB), in the contact when shear was applied. The proposed methodology was validated by successfully obtaining the expected linear profile with PB experiencing Couette flow. The methodology was then applied to PB in an EHL contact. The variation of the profiles within the contact area was also investigated.The velocity profile of PB in an EHL contact severely deviates from the common linear assumption and exhibits inhomogeneous shear: three regions of varying shear rate have been observed. The phenomenon is shown to be neither due to thermal nor diffusion effects. PB also shows significant slip at the glass-liquid interface. The amount of slip varies with position in the contact. Possible causes, such as pressure induced viscosity enhancement, as well as the significance of the findings and the benefits of the technique are discussed.The linear velocity profile in an EHL contact is usually assumed for both the film thickness and friction predictions. The profile has however never been measured experimentally until now. This work enables the validation of conventional assumptions and the study of flow heterogeneity of lubricants in a contact. This facilitates an improved understanding of the rheology of confined lubricant and hence more accurate predictions of tribological properties.

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“…Since the EO flow amplification scales as (1 + bκ), and b can be of the order of tens of nanometers [9,10,11,12], for typically nanometric Debye length an order of magnitude enhancement might be expected.…”

Section: Electro-osmotic Velocity In Eigendirectionsmentioning

confidence: 99%

Electro-osmosis on Anisotropic Superhydrophobic Surfaces

Belyaev

Vinogradova

2011

Phys. Rev. Lett.

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Surface conduction in presence of slip is characterized by full Dukhin number, which is given by [1]:where m = 2ε(k B T /ze) 2 /(ηD), and D is the ion diffusivity, which is assumed equal for both types of ions. The first term in (1) is the Dukhin number for the no-slip surface, Du b=0 , while the second one, Du b , is due to hydrodynamic slip. In the Debye-Hückel modelThis restriction (2) allows the simplification:The parameter m ≈ 100/z 2 , and κL ≫ 1 since EDL is thin. Whence Du b=0 ≈ 1/(z 2 κL) ≪ 1 provided the potential is low. For a "slippery part" of Du we evaluate01. Therefore for increasing surface charge (potential) and b/L, the conductivity of the diffuse layer can become comparable to the bulk, and surface condition must be considered. The Péclet number, P e = U L/D , in presence of slip can be evaluated asTypically, electroosmotic velocity is of order is of order micrometers per second for no-slip surfaces [2]. For nanoscale patterns L < 1 µm and typical ion diffusivities D ≈ 10 −6 cm 2 /s this gives P e b=0 < 0.01 ≪ 1. The slip implies a correction factor (1 + bκ) , which suggests that the convective ion transport can safely be neglected only for bκ < 10. Larger values of bκ should relax this standard approximation of small P e. ELECTRO-OSMOTIC VELOCITY IN EIGENDIRECTIONSLongitudinal stripes.-In this configuration only x−velocity component remains, and the Stokes equation takes the formWe expand surface charge density in a Fourier series, and the potential is thenwhere ξ n = κ 2 + λ 2 n , λ n = 2nπ/L , q = q 1 φ 1 + q 2 φ 2 is the mean surface charge, andThe general solution to (6) for u(y, z) has the formwhere U n and U are determined by the slip boundary conditions. Imposing them on (9) in a thin EDL limit yields a dual serieswhich can be solved exactly by using a technique [3] to obtain the thin-EDL electro-osmotic velocity:Transverse stripes.-Although an external pressure gradient is equal to zero, local pressure variations contribute into a non-zero term ∇p in the Stokes equation, so that the flow is essentially two-dimensional. We first introduce a stream function f (x, y)2 which obeys inhomogeneous biharmonic equation:Here u and v are x and y velocity components, correspondingly. The general periodic solution to (14) has the formE t q n κ 2 η + g n y e −λny cos λ n x + + E t ε κ 2 η ∂ψ ∂y (15)Here the potential ψ(x, y) has exactly the form (7) with z replaced by x. The dual series problem in a thin EDL limit can be written aswhere a n = g n + E t q n ηκ 2 (ξ n − λ n ).These dual series can be solved exactly to obtainWe emphasize that a comparison of Eqs. (12) and (18) indicates that the EO flow is generally anisotropic, so that our results do not support an earlier conclusion [4] that the electro-osmotic mobility tensor is isotropic in the thin EDL limit. This inconsistensy [4] (due to an erroneous expression for a transverse electro-osmotic velocity, where factor of 2 was lost) has been corrected for a case b 2 = ∞ in [5]. Sciences, 31 Leninsky Prospect, 119991 Moscow, Russia 3 ITMC and...

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Direct Measurements of Hydrophobic Slippage Using Double-Focus Fluorescence Cross-Correlation

Cited by 127 publications

References 24 publications

Wetting, roughness and flow boundary conditions

Wetting, roughness and flow boundary conditions

Through-Thickness Velocity Profile Measurements in an Elastohydrodynamic Contact

Electro-osmosis on Anisotropic Superhydrophobic Surfaces

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