Quantifying effective slip length over micropatterned hydrophobic surfaces

Abstract

We employ micro-particle image velocimetry (µ-PIV) to investigate laminar micro-flows in hydrophobic microstructured channels, in particular the slip length. These microchannels consist of longitudinal micro-grooves, which can trap air and prompt a shear-free boundary condition and thus slippage enhancement. Our measurements reveal an increase of the slip length when the width of the micro-grooves is enlarged. The result of the slip length is smaller than the analytical prediction by Philip et al. [1] for an infinitely large and textured channel comprised of alternating shear-free and no-slip boundary conditions. The smaller slip length (as compared to the prediction) can be attributed to the confinement of the microchannel and the bending of the meniscus (liquid-gas interface). Our experimental studies suggest that the curvature of the meniscus plays an important role in microflows over hydrophobic micro-ridges.

Superhydrophobic drag reduction in laminar flows: a critical review

“…However, the numerous experimental studies in the literature have reported (many erroneously, as discussed in Sects. 3.2.5 and 3.3.5) a wide range of slip lengths spanning from tens of nanometers to even millimeters on SHPo surfaces consisting of regular (periodic) structures (Ou et al 2004;Ou and Rothstein 2005;Choi et al 2006;Davies et al 2006;Truesdell et al 2006;Maynes et al 2007;Steinberger et al 2007;Byun et al 2008;Lee et al 2008;Tsai et al 2009;Jung and Bhushan 2010;Lee and Kim 2011a;Kashaninejad et al 2012;Kim and Hidrovo 2012;Maali et al 2012;Karatay et al 2013;Bolognesi et al 2014;Lee and Kim 2014) or random structures (Watanabe et al 1999(Watanabe et al , 2003Gogte et al 2005;Choi and Kim 2006a;Joseph et al 2006;Bhushan et al 2009;Govardhan et al 2009;Shirtcliffe et al 2009;Wang et al 2009;Kim and Hwang 2010;Li et al 2010;Wang and Bhushan 2010;Ming et al 2011;Srinivasan et al 2013). Sometimes orders-of-magnitude differences in the measured slip lengths were reported even on structurally similar SHPo surfaces (e.g., Lee et al 2008vs.…”

“…4b, micro-particle image velocimetry (micro-PIV) has been widely used as a direct way to measure a velocity profile close to the wall and extract a slip length from the measured velocity profile (Gogte et al 2005;Ou and Rothstein 2005;Joseph et al 2006;Byun et al 2008;Tsai et al 2009;Karatay et al 2013;Bolognesi et al 2014). In this method, fluorescent particles dispersed in water are used as a tracer for the liquid velocity.…”

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

“…using µ-PIV measurements (Ou & Rothstein 2005;Joseph et al 2006;Truesdell et al 2006;Tsai et al 2009), the slip length at the wall can be computed using relation (3.9) based on the wall velocity The diamonds show the optimum gas layer thickness/maximum drag reduction, whereas the circles and squares indicate the gas layer thicknesses needed to achieve 90 and 50 % of the maximum drag reduction.…”

“…The second approach is to use the top of the roughness supporting the gas layer, i.e. the liquid-gas interface, for the wall location in the computation of the apparent slip length (Ou & Rothstein 2005;Tsai et al 2009). In this case, an extension of the velocity profile is not necessary.…”

“…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.…”