Drag reduction in turbulent flows over superhydrophobic surfaces

Daniello, Robert; Waterhouse, Nicholas E.; Rothstein, Jonathan P.

doi:10.1063/1.3207885

Cited by 561 publications

(418 citation statements)

References 28 publications

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“…The recent numerical (Martell et al 2009(Martell et al , 2010Park et al 2013) and experimental (Daniello et al 2009) results under turbulent flow conditions have shown that SHPo surfaces with even a moderate slip length (~10 µm) can result in significant turbulent drag reduction, as the turbulent structures become weakened near the SHPo surface and a thin viscous sublayer becomes the main characteristic fluidic length scale in the turbulent flow. Recently, using regular-structured SHPo surfaces of large slip lengths, drag reductions as much as 75 % have been reported in turbulent boundary layer flows (Park et al 2014).…”

Section: Discussionmentioning

confidence: 99%

Superhydrophobic drag reduction in laminar flows: a critical review

2016

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Section: Discussionmentioning

confidence: 99%

Superhydrophobic drag reduction in laminar flows: a critical review

2016

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“…3 A-C, SI Experimental Protocols, and Tables S3 and S4). Similar to earlier work (10,13,24), our SHSs were made of hydrophobic polydimethylsiloxane (PDMS), using photolithography techniques. The SHSs consisted of stream-wise parallel rectangular lanes or gratings, as shown in Fig.…”

Section: Experiments Show Reduced Slipmentioning

confidence: 99%

“…6-11) and rheometer tests reported slip lengths of up to 185 µm (12). Turbulent flow experiments have reduced drag by up to 75% (13)(14)(15)(16). However, a wide range of experiments have provided inconsistent results, with several studies reporting little or no drag reduction (16)(17)(18)(19)(20)(21)(22)(23)(24)(25).…”

mentioning

confidence: 99%

Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

Peaudecerf

Landel

Goldstein

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

162

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Superhydrophobic surfaces (SHSs) have the potential to achieve large drag reduction for internal and external flow applications. However, experiments have shown inconsistent results, with many studies reporting significantly reduced performance. Recently, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible by creating adverse Marangoni stresses. However, testing this hypothesis is challenging. Careful experiments with purified water already show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases. To test the surfactant hypothesis while controlling surfactant concentrations with precision higher than can be achieved experimentally, we perform simulations inclusive of surfactant kinetics. These reveal that surfactant-induced stresses are significant at extremely low concentrations, potentially yielding a no-slip boundary condition on the air-water interface (the "plastron") for surfactant concentrations below typical environmental values. These stresses decrease as the stream-wise distance between plastron stagnation points increases. We perform microchannel experiments with SHSs consisting of streamwise parallel gratings, which confirm this numerical prediction, while showing near-plastron velocities significantly slower than standard surfactant-free predictions. In addition, we introduce an unsteady test of surfactant effects. When we rapidly remove the driving pressure following a loading phase, a backflow develops at the plastron, which can only be explained by surfactant gradients formed in the loading phase. This demonstrates the significance of surfactants in deteriorating drag reduction and thus the importance of including surfactant stresses in SHS models. Our time-dependent protocol can assess the impact of surfactants in SHS testing and guide future mitigating designs.superhydrophobic surface | drag reduction | surfactant | Marangoni stress | plastron S uperhydrophobic surfaces (SHSs) combine hydrophobic surface chemistry and micro-or nanoscale patterning to retain a network of air pockets when exposed to a liquid (e.g., reviews in refs. 1-3). Because a large portion of the interface between the solid wall and the liquid is replaced by an air-liquid interface, which can be considered almost as a shear-free surface (known as a "plastron"), SHSs could be used to obtain significant drag reduction in fluid flow applications (4, 5). Microchannel tests have recorded drag reductions of over 20% (e.g., refs. 6-11) and rheometer tests reported slip lengths of up to 185 µm (12). Turbulent flow experiments have reduced drag by up to 75% (13-16). However, a wide range of experiments have provided inconsistent results, with several studies reporting little or no drag reduction (16)(17)(18)(19)(20)(21)(22)(23)(24)(25).A key step toward solving this puzzle has come with the realization that surfactants could induce Marangoni stresses that impair drag reduction. This was first hypothesized to account for experiments that rev...

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“…In particular, the formation of hierarchical structures is integral to forming superhydrophobic surfaces [10]. The ridge type of structures on shark skin aid their drag reduction [11] and similar ridge type structures have suggested by Rothstein et al [12,13] as possible drag reducing artificial surfaces. Therefore, a subsidiary aim in this article is to introduce approach to forming ridge-type drag reducing SU8 structures.…”

Section: Introductionmentioning

confidence: 99%

Lithographically fabricated SU8 composite structures for wettability control

Hamlett

McHale

Newton

2014

Surface and Coatings Technology

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SU8 is a negative resist which is widely used for the fabrication of micron scale lateral features over a wide range of heights using photolithographic methods. This has been extensively used as a method to produce surface structure to which hydrophobicity can be added and a model super-water repellent surface of achieved. However, such an approach requires at least two-steps and does not embed the desired properties as part of the structure itself or create multiple levels of topographical structure. In other applications, a variety of inclusions have previously been studied to tailor the properties of SU8. In this work we report an approach to, and results from, incorporating inclusions, in our case glass beads, of different wettabilities into the SU8 structures produced by photolithography. In particular, we focus on ridge structures expected to be of use in flow systems as drag reducing surfaces. We used scanning electron microscopy and profilometry to investigate how the inclusion of either hydrophobic or hydrophilic glass beads (sieve size of 20-30 μm) affects the definition of the structures formed and contact angle goniometry to define what effect such inclusions has on the wettability of the SU8 composite structure. It was found that the inclusion of hydrophobic glass beads in the SU8 resist resulted in poorly defined structures compared to both SU8 on its own and SU8 to which hydrophobic glass beads were added. In contrast, inclusion of hydrophilic glass beads resulted in a well-defined ridge structure with the majority of the beads located in the 'valleys'. Both EDX analysis and contact angle data indicated that the surface chemistry of the beads themselves (both the hydrophobic and hydrophilic beads) were masked by the SU8. Counter-intuitively, the inclusion of hydrophobic beads in the SU8 composite resin resulted in ridges with increased wettability, compared to SU8 ridges, as opposed to the inclusions of hydrophilic beads that resulted in surfaces with increased effective hydrophobicity .2

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Drag reduction in turbulent flows over superhydrophobic surfaces

Cited by 561 publications

References 28 publications

Superhydrophobic drag reduction in laminar flows: a critical review

Superhydrophobic drag reduction in laminar flows: a critical review

Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

Lithographically fabricated SU8 composite structures for wettability control

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