Laminar drag reduction in microchannels using ultrahydrophobic surfaces

Ou, Jia; Perot, Blair; Rothstein, Jonathan P.

doi:10.1063/1.1812011

Cited by 922 publications

(707 citation statements)

References 26 publications

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“…It is safe to assume that in the case of this rough surface, the solid fraction of the spreading lamella was at an intermediate value between that of its smooth and textured counterparts and, therefore, that the value of m decreased monotonically for surfaces exhibiting lower solid fractions in our experiments. A low solid fraction reduces the magnitude of m because the conventional no-slip boundary condition for fluid flow over a solid is no longer valid on an air-trapping surface [36]. On such surfaces, only the solid fraction of the lamella is subject to the no-slip condition (and is hence dragged by the tangential motion of the surface), while the remaining fraction flows over trapped air, experiencing minimal drag, which explains why the tangential velocity of the surface had a diminished effect on the impact behaviour for the rough and textured surfaces tested.…”

Section: Modelling the Splashing Thresholdmentioning

confidence: 99%

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

Aboud

Kietzig

2015

Langmuir

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Oblique drop impacts were performed at high speeds (up to 27 m/s, We>9000) with millimetric water droplets, and a linear model was applied to define the oblique splashing threshold. Six different sample surfaces were tested: two substrate materials of different inherent surface wettability (PTFE and aluminum), each prepared with three different surface finishes (smooth, rough, and textured to support superhydrophobicity). Our choice of surfaces has allowed us to make several novel comparisons. Considering the inherent surface wettability, we discovered that PTFE, as the more hydrophobic surface, exhibits lower splashing thresholds than the hydrophilic surface of aluminum of comparable roughness. Furthermore, comparing oblique impacts on smooth and textured surfaces, we found that asymmetrical spreading and splashing behaviours occurred under a wide range of experimental conditions on our smooth surfaces, however impacts 2 occurring on textured surfaces were much more symmetrical, and one-sided splashing occurred only under very specific conditions. We attribute this difference to the air-trapping nature of textured superhydrophobic surfaces, which lowers the drag between the spreading lamella and the surface. The reduced drag affects oblique drop impacts by diminishing the effect of the tangential component of the impact velocity, causing the impact behaviour to be governed almost exclusively by the normal velocity. Finally, by comparing oblique impacts on superhydrophobic surfaces at different impact angles, we discovered that although the pinning transition between rebounding and partial rebounding is governed primarily by the normal impact velocity, there is also a weak dependence on the tangential velocity. As a result, pinning is inhibited in oblique impacts. This led to the observation of a new behaviour in highly oblique impacts on our superhydrophobic surfaces which we named the stretched rebound, where the droplet is extended into an elongated pancake shape and rebounds while still outstretched, without exhibiting a recession phase.

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Section: Modelling the Splashing Thresholdmentioning

confidence: 99%

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

Aboud

Kietzig

2015

Langmuir

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“…In this case the slip length can be based on the change in the shear rate on the upper wall or the change in the pressure drop or mass flow rate (Ou et al 2004;Ou & Rothstein 2005;Govardhan et al 2009) by finding the analytic solution for a velocity profile that gives the same effect with the assumption of a slip length boundary condition on the superhydrophobic walls. In the Couette flow case the resulting slip is equal to the slip length based on the local velocity profile at the wall, since the velocity profile is linear.…”

Section: Slip Length Based On Mean Flow Quantitiesmentioning

confidence: 99%

“…Due to the lower dynamic viscosity of air compared to water the trapped air layer on a superhydrophobic surface has a lubricating effect on the flow over it. Drag reducing properties of superhydrophobic surfaces have been observed experimentally in microfluidic devices (Choi, Westin & Breuer 2003;Ou, Perot & Rothstein 2004;Ou & Rothstein 2005;Joseph et al 2006;Daniello, Waterhouse & Rothstein 2009;Govardhan et al 2009;Tsai et al 2009;Rothstein 2010) and for coated objects, such as hydrofoils (Gotge et al 2005), settling spheres (McHale et al 2009) and cylinders (Muralidhar et al 2011), covering flow regimes from laminar to turbulent. In a stable configuration, i.e.…”

Section: Introductionmentioning

confidence: 98%

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

et al. 2013

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Analytic results are derived for the apparent slip length, the change in drag and the optimum air layer thickness of laminar channel and pipe flow over an idealised superhydrophobic surface, i.e. a gas layer of constant thickness retained on a wall. For a simple Couette flow the gas layer always has a drag reducing effect, and the apparent slip length is positive, assuming that there is a favourable viscosity contrast between liquid and gas. In pressure-driven pipe and channel flow blockage limits the drag reduction caused by the lubricating effects of the gas layer; thus an optimum gas layer thickness can be derived. The values for the change in drag and the apparent slip length are strongly affected by the assumptions made for the flow in the gas phase. The standard assumptions of a constant shear rate in the gas layer or an equal pressure gradient in the gas layer and liquid layer give considerably higher values for the drag reduction and the apparent slip length than an alternative assumption of a vanishing mass flow rate in the gas layer. Similarly, a minimum viscosity contrast of four must be exceeded to achieve drag reduction under the zero mass flow rate assumption whereas the drag can be reduced for a viscosity contrast greater than unity under the conventional assumptions. Thus, traditional formulae from lubrication theory lead to an overestimation of the optimum slip length and drag reduction when applied to superhydrophobic surfaces, where the gas is trapped.

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“…Tretheway and Meinhart [2002] showed that the no-slip boundary condition was valid for a clean hydrophilic microchannel but slip occurred over a hydrophobic surface. Ou et al [2004] demonstrated a significant drag reduction, consequently resulting in slip at a solid-fluid boundary, for laminar flow of water through micron-sized channels with hydrophobic surfaces. Slip increases flow rate of fluid, which can be analytically quantified by introducing a concept of slip length [Ou et al, 2004].…”

Section: Introductionmentioning

confidence: 99%

Water flow and slip on NAPL‐wetted surfaces of a parallel‐walled fracture

Lee¹,

Yeo²,

Lee³

2007

Geophysical Research Letters

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The effects of various wetting conditions and aperture sizes on water flow and slip in a parallel‐walled fracture were investigated. Water flow experiments showed that a larger slip occurred as the surface became more hydrophobic. For a creosote‐wetted surface of the fracture with an aperture of 508 μm, the increase in the water flow rate due to the slip was as much as 10.0% compared to that in a water‐wetted surface. Similarly to a water‐wetted surface, no slip took place at a gasoline‐wetted surface, which was weakly hydrophobic. The slip lengths, a notional distance from the wall to a point inside the wall where fluid velocity extrapolates to zero, for light oil‐ and creosote‐wetted surfaces were constant over a range of flow velocities in the laminar flow regime. The study indicates that no slip boundary condition‐based equations are not adequate for quantifying water flow through NAPL‐wetted fractures.

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Laminar drag reduction in microchannels using ultrahydrophobic surfaces

Cited by 922 publications

References 26 publications

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

Splashing Threshold of Oblique Droplet Impacts on Surfaces of Various Wettability

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

Water flow and slip on NAPL‐wetted surfaces of a parallel‐walled fracture

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