title: Bioinspired surfaces for turbulent drag reduction.
abstract: In this review, we discuss how superhydrophobic surfaces (SHSs) can provide friction drag reduction in turbulent flow. Whereas biomimetic SHSs are known to reduce drag in laminar flow, turbulence adds many new challenges. We first provide an overview on designing SHSs, and how these surfaces can cause slip in the laminar regime. We then discuss recent studies evaluating drag on SHSs in turbulent flow, both computationally and experimentally. The effects of streamwise and spanwise slip for canonical, structured surfaces are well characterized by direct numerical simulations, and several experimental studies have validated these results. However, the complex and hierarchical textures of scalable SHSs that can be applied over large areas generate additional complications. Many studies on such surfaces have measured no drag reduction, or even a drag increase in turbulent flow. We discuss how surface wettability, roughness effects and some newly found scaling laws can help explain these varied results. Overall, we discuss how, to effectively reduce drag in turbulent flow, an SHS should have: preferentially streamwise-aligned features to enhance favourable slip, a capillary resistance of the order of megapascals, and a roughness no larger than 0.5, when non-dimensionalized by the viscous length scale.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.
authors.affiliation: Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: Kevin B
authors.initials: KB
authors.lastname: Golovin
authors.affiliation: Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: James W
authors.initials: JW
authors.lastname: Gose
authors.affiliation: Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: Marc
authors.initials: M
authors.lastname: Perlin
authors.affiliation: Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: Steven L
authors.initials: SL
authors.lastname: Ceccio
authors.affiliation: Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA Department of Macromolecular Science and Engineering, University of Michigan, Ann Arbor, MI 48109, USA atuteja@umich.edu.
authors.forename: Anish
authors.initials: A
authors.lastname: Tuteja
keywords: biomimetics; drag reduction; hierarchical; slip; superhydrophobic; turbulence