Direct velocity measurements of the flow past drag-reducing ultrahydrophobic surfaces

Abstract

A series of experiments are presented which study the flow kinematics of water past drag-reducing superhydrophobic surfaces. The ultrahydrophobic surfaces are fabricated from silicon wafers using photolithography and are designed to incorporate precise patterns of micrometer-sized ridges aligned in the flow direction. The ridges are made hydrophobic through a chemical reaction with an organosilane. An experimental flow cell is used to measure the velocity profile and the pressure drop as a function of the flow rate for a series of rectangular cross-section microchannel geometries and ultrahydrophobic surface designs. The velocity profile across the microchannel is determined through microparticle image velocimetry ͑-PIV͒ measurements capable of resolving the flow down to lengthscales well below the size of the surface features. Through these detailed velocity measurements, it is demonstrated that slip along the shear-free air-water interface supported between the hydrophobic micrometer-sized ridges is the primary mechanism responsible for the drag reduction observed for flows over ultrahydrophobic surfaces. A maximum slip velocity of more than 60% of the average velocity in the microchannel is found at the center of the shear-free air-water interface whereas the no-slip boundary condition is found to hold along the surface of the hydrophobic ridges. The experimental velocity and pressure drop measurements are compared to the predictions of numerical simulations and an analytical theory based on a simple model of an ultrahydrophobic surface composed of alternating shear-free and no-slip bands with good agreement.

Nano-scale superhydrophobicity: suppression of protein adsorption and promotion of flow-induced detachment

“…The hypothesis is that proteins will probably adhere to superhydrophobic surfaces, but several additional factors may contribute to their effective removal under flow, particularly if micro-metre scale roughness is replaced with nano-metre scale roughness. Interfacial slip between the liquid and solid would cause an increase in liquid flow rate near the surface 20,21,22 ; adsorbed molecules would then experience greater shear forces and are therefore more likely to be swept away. On nano-scale roughness the curvature of the surface approaches protein molecular dimensions, reducing the contact area unless the protein molecules deform.…”

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

“…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.…”

“…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.…”

“…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.…”