scholarly journals The local slip length and flow fields over nanostructured superhydrophobic surfaces

2020 ◽  
Vol 126 ◽  
pp. 103258 ◽  
Author(s):  
Luyao Bao ◽  
Nikolai V. Priezjev ◽  
Haibao Hu
Keyword(s):  
Slip Length ◽  
Flow Fields ◽  
Superhydrophobic Surfaces ◽  
Local Slip

scholarly journals MODELLING AND INVESTIGATION OF THE DEPENDENCE OF SUPERHYDROPHOBIC PROPERTIES OF NANOSURFACES ON THE TOPOLOGY OF MICROCHANNELS

2019 ◽  
Vol 3 ◽  
pp. 52
Author(s):  
Sharif E. Guseynov ◽  
Jekaterina V. Aleksejeva
Keyword(s):  
Contact Angle ◽  
Interphase Boundary ◽  
Slip Length ◽  
Superhydrophobic Surfaces ◽  
Equilibrium Contact Angle ◽  
Three Phase ◽  
Quantitative Understanding ◽  
Air Cushion ◽  
Local Slip ◽  
Solid Liquid

In the Cassie-Baxter state anisotropic superhydrophobic surfaces have high lubricating properties. Such superhydrophobic surfaces are used in medical implants, aircraft industry, vortex bioreactors etc. In spite of the fact that quantitative understanding of fluid dynamics on anisotropic superhydrophobic surfaces has been broadened substantially for last several years, there still are some unsolved problems in this field. This work investigates dynamics of a liquid on unidirectional superhydrophobic surfaces in the Cassie-Baxter state, when surface texture is filled with gas and, consequently, the liquid virtually is located on some kind of an air cushion. Energy of the interphase boundary liquid-gas is much smaller than energy of the interphase boundary solid-liquid, that is why the contact angle at wetting such surfaces differs a lot from the Young contact angle and depends on contact area ratio of liquid-gas and liquid-solid in visible contact of liquid and surface. Considering difference in energy obtained if we slightly shift the three-phase contact line, expression for macroscopic equilibrium contact angle, which describes the Cassie-Baxter state, can be deduced. In the work the design formula for computing local-slip length profiles of liquid on the considered superhydrophobic surfaces is obtained.

scholarly journals Flow past superhydrophobic surfaces with cosine variation in local slip length

2013 ◽  
Vol 87 (2) ◽  
Author(s):  
Evgeny S. Asmolov ◽  
Sebastian Schmieschek ◽  
Jens Harting ◽  
Olga I. Vinogradova
Keyword(s):  
Slip Length ◽  
Superhydrophobic Surfaces ◽  
Flow Past ◽  
Local Slip

Turbulent flow over superhydrophobic surfaces with streamwise grooves

2014 ◽  
Vol 747 ◽  
pp. 186-217 ◽  
Author(s):  
S. Türk ◽  
G. Daschiel ◽  
A. Stroh ◽  
Y. Hasegawa ◽  
B. Frohnapfel
Keyword(s):  
Boundary Conditions ◽  
Turbulent Flow ◽  
Drag Reduction ◽  
Secondary Flow ◽  
Slip Length ◽  
Laminar Flows ◽  
Superhydrophobic Surfaces ◽  
Mean Velocity ◽  
Effective Slip Length ◽  
Effective Slip

AbstractWe investigate the effects of superhydrophobic surfaces (SHS) carrying streamwise grooves on the flow dynamics and the resultant drag reduction in a fully developed turbulent channel flow. The SHS is modelled as a flat boundary with alternating no-slip and free-slip conditions, and a series of direct numerical simulations is performed with systematically changing the spanwise periodicity of the streamwise grooves. In all computations, a constant pressure gradient condition is employed, so that the drag reduction effect is manifested by an increase of the bulk mean velocity. To capture the flow properties that are induced by the non-homogeneous boundary conditions the instantaneous turbulent flow is decomposed into the spatial-mean, coherent and random components. It is observed that the alternating no-slip and free-slip boundary conditions lead to the generation of Prandtl’s second kind of secondary flow characterized by coherent streamwise vortices. A mathematical relationship between the bulk mean velocity and different dynamical contributions, i.e. the effective slip length and additional turbulent losses over slip surfaces, reveals that the increase of the bulk mean velocity is mainly governed by the effective slip length. For a small spanwise periodicity of the streamwise grooves, the effective slip length in a turbulent flow agrees well with the analytical solution for laminar flows. Once the spanwise width of the free-slip area becomes larger than approximately 20 wall units, however, the effective slip length is significantly reduced from the laminar value due to the mixing caused by the underlying turbulence and secondary flow. Based on these results, we develop a simple model that allows estimating the gain due to a SHS in turbulent flows at practically high Reynolds numbers.

Modeling of liquid–gas meniscus for textured surfaces: effects of curvature and local slip length

2015 ◽  
Vol 25 (12) ◽  
pp. 125002 ◽  
Author(s):  
Anvesh Gaddam ◽  
Mayank Garg ◽  
Amit Agrawal ◽  
Suhas S Joshi
Keyword(s):  
Slip Length ◽  
Textured Surfaces ◽  
Local Slip

scholarly journals SUPERHYDROPHOBIC SURFACES FOR DRAG REDUCTION

2014 ◽  
Author(s):  
Alessandro Bottaro
Keyword(s):  
Drag Reduction ◽  
Slip Length ◽  
Superhydrophobic Surfaces ◽  
Transition To Turbulence ◽  
Effective Coupling ◽  
Instability Waves ◽  
Low Surface Energy ◽  
Apparent Slip ◽  
Energy Material ◽  
Relevant Length Scale

Properties of superhydrophobic materials are examined in light of their possible use for drag reduction in naval applications. To achieve superhydrophobicity a low-surface-energy material must be structured so as to minimize the liquid-solid interactions. The crucial aspect is that of maintaining a layer of gas in between the (rough) wall and the liquid, and this can be achieved by hierarchical micro- and nano-structuring of the solid surface, to ensure a sufficiently large apparent slip of the fluid at the wall, thus reducing skin friction. The behavior of the liquid is quantified by a slip length; recent results have shown that this length can be as large as 400 μm. As far as transition to turbulence is concerned, we show that superhydrophobic surfaces are effective (i.e. they delay the onset of travelling instability waves) only in channels with characteristic dimensions of a few millimeters. Conversely, when the fluid flow has already attained a turbulent state, the gain in term of drag reduction can be very significant also in macroscopic configurations. This occurs because the relevant length scale of the boundary layer is now the thickness of the viscous sub-layer, which can be of magnitude comparable to the slip length, so that an effective coupling emerges. Finally, some procedures to produce superhydrophobic surfaces are examined, in light of the possible application of such innovative coatings on the hull of ships.

scholarly journals Local Flow Field and Slip Length of Superhydrophobic Surfaces

2016 ◽  
Vol 116 (13) ◽  
Author(s):  
David Schäffel ◽  
Kaloian Koynov ◽  
Doris Vollmer ◽  
Hans-Jürgen Butt ◽  
Clarissa Schönecker
Keyword(s):  
Flow Field ◽  
Slip Length ◽  
Superhydrophobic Surfaces ◽  
Local Flow

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

2014 ◽  
Vol 740 ◽  
pp. 168-195 ◽  
Author(s):  
Clarissa Schönecker ◽  
Tobias Baier ◽  
Steffen Hardt
Keyword(s):  
Flow Field ◽  
Slip Length ◽  
Two Fluids ◽  
Cassie State ◽  
Effective Slip Length ◽  
Primary Fluid ◽  
Effective Slip ◽  
Local Slip ◽  
Analytical Expressions ◽  
Secondary Fluid

AbstractAnalytical 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.

scholarly journals Effective slip in pressure-driven flow past super-hydrophobic stripes

2010 ◽  
Vol 652 ◽  
pp. 489-499 ◽  
Author(s):  
A. V. BELYAEV ◽  
O. I. VINOGRADOVA
Keyword(s):  
Slip Length ◽  
Mean Flow ◽  
Flow Past ◽  
Relevant Situation ◽  
Effective Slip ◽  
Trapped Gas ◽  
Local Slip ◽  
Texture Size ◽  
Pressure Driven Flow ◽  
Pressure Driven

A super-hydrophobic array of grooves containing trapped gas (stripes) has the potential to greatly reduce drag and enhance mixing phenomena in microfluidic devices. Recent work has focused on idealized cases of stick-perfect slip stripes. Here, we analyse the experimentally more relevant situation of a pressure-driven flow past striped slip-stick surfaces with arbitrary local slip at the gas sectors. We derive approximate formulas for maximal (longitudinal) and minimal (transverse) directional effective slip lengths that are in a good agreement with the exact numerical solution for any surface slip fraction. By representing eigenvalues of the slip length tensor, we obtain the effective slip for any orientation of stripes with respect to the mean flow. Our results imply that flow past stripes is controlled by the ratio of the local slip length to texture size. In the case of a large (compared to the texture period) slip at the gas areas, surface anisotropy leads to a tensorial effective slip, by attaining the values predicted earlier for a perfect local slip. Both effective slip lengths and anisotropy of the flow decrease when local slip becomes of the order of texture period. In the case of a small slip, we predict simple surface-averaged isotropic flows (independent of orientation).

scholarly journals Laminar–turbulent transition in channel flow with superhydrophobic surfaces modelled as a partial slip wall

2019 ◽  
Vol 881 ◽  
pp. 462-497 ◽  
Author(s):  
Francesco Picella ◽  
J.-Ch. Robinet ◽  
S. Cherubini
Keyword(s):  
Channel Flow ◽  
Initial Conditions ◽  
Slip Length ◽  
Low Noise ◽  
Superhydrophobic Surfaces ◽  
Partial Slip ◽  
Turbulent Transition ◽  
Onset Of Turbulence ◽  
Fully Developed Turbulent Flow ◽  
Laminar Turbulent Transition

Superhydrophobic surfaces are capable of trapping gas pockets within the micro-roughnesses on their surfaces when submerged in a liquid, with the overall effect of lubricating the flow on top of them. These bio-inspired surfaces have proven to be capable of dramatically reducing skin friction of the overlying flow in both laminar and turbulent regimes. However, their effect in transitional conditions, in which the flow evolution strongly depends on the initial conditions, has still not been deeply investigated. In this work the influence of superhydrophobic surfaces on several scenarios of laminar–turbulent transition in channel flow is studied by means of direct numerical simulations. A single phase incompressible flow has been considered and the effect of the micro-structured superhydrophobic surfaces has been modelled imposing a slip condition with given slip length at both walls. The evolution from laminar, to transitional, to fully developed turbulent flow has been followed starting from several different initial conditions. When modal disturbances issued from linear stability analyses are used for perturbing the laminar flow, as in supercritical conditions or in the classical K-type transition scenario, superhydrophobic surfaces are able to delay or even avoid the onset of turbulence, leading to a considerable drag reduction. Whereas, when transition is triggered by non-modal mechanisms, as in the optimal or uncontrolled transition scenarios, which are currently observed in noisy environments, these surfaces are totally ineffective for controlling transition. Superhydrophobic surfaces can thus be considered effective for delaying transition only in low-noise environments, where transition is triggered mostly by modal mechanisms.

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