Turbulent Drag Reduction Using Superhydrophobic ... - CiteSeerX

AIAA 2006-3192

3rd AIAA Flow Control Conference 5 - 8 June 2006, San Francisco, California

Turbulent Drag Reduction Using Superhydrophobic Surfaces C. Henoch* Naval Undersea Warfare Center, Newport, RI 02841USA T. N. Krupenkin,† P. Kolodner, ‡ J. A. Taylor,§ M. S. Hodes**, A. M. Lyons†† Bell Laboratories, Murray Hill, NJ 07974 USA and C. Peguero‡‡ and K. Breuer§§ Brown University, Providence, RI 02912 USA Superhydrophobic surfaces are known to exhibit reduced viscous drag due to "slip" associated with a layer of air trapped at the liquid-solid interface. It is expected that this slip will lead to reduced turbulent skin-friction drag in external flows at higher Reynolds numbers in both the laminar and turbulent regimes. Results are presented from experiments exploring this effect. Large-area Superhydrophobic test surfaces have been fabricated and tested in a water tunnel, measuring drag in both the laminar and transitional regimes at velocities up to 1.4 m/s. Drag reduction of approximately 50% is observed for laminar flow. Lower levels of drag reduction are observed at higher speeds after the flow has transitioned to turbulence.

I.

Introduction

T

he combination of hydrophobicity and micron-scale surface roughness leads to a phenomenon known as superhydrophobicity. Water droplets placed on superhydrophobic surfaces bead up into a nearly spherical shape, exhibiting contact angles that approach 180º. 1 The flow of water over superhydrophobic surfaces exhibits non-zero slip, and this has lead to the observation of enhanced droplet mobility2 and drag reduction in laminar flow through microchannels.3 Experiments in water at higher Reynolds numbers4,5 have also suggested that the structure of the turbulent boundary layer is changed by the presence of a superhydrophobic surface such that the turbulent skin friction is reduced. In addition, numerical experiments6 have indicated that the structure of a turbulent wall-bounded flow in water can also be significantly affected when the solid surface is superhydrophobic. The effect was found to be geometrically anisotropic: micron-sized streamwise ridges were found to reduce skin-friction drag, while spanwise ridges *

Mechanical Engineer, Hydrodynamics Branch, Building 1302 Code 8233, Newport, RI 02841, member of AIAA. Member of Technical Staff, Materials for Communications Research Department, Room 1D-352, 600 Mountain Ave., Murray Hill, NJ 07974-0636. ‡ Distinguished Member of Technical Staff, Optical Technology Research Department, Room 1E-314, 600 Mountain Ave., Murray Hill, NJ 07974-0636. § Member of Technical Staff, Nanofabriaction Research Laboratory, Room 2D-551, 600 Mountain Ave., Murray Hill, NJ 07974-0636. ** Member of Technical Staff, Optical Technology Research Department, Room 1C-462, 600 Mountain Ave., Murray Hill, NJ 07974-0636. †† Member of Technical Staff. Value Chain Research, Blanchardstown Industrial Park, Blanchardstown, Ireland. ‡‡ Graduate research assistant, Division of Engineering, Box D, 182 Hope Street, Providence, RI 02912. §§ Professor, Division of Engineering, Box D, 182 Hope Street, Providence, RI 02912. Senior member of AIAA. Corresponding author. Tel: 401.863.2870; Fax: 401.863.9028; Email: [email protected]. †

1 American Institute of Aeronautics and Astronautics Copyright © 2006 by Kenneth Breuer. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

increased it. An isotropic pattern of surface roughness was found to produce effects of intermediate magnitude. Although these results suggest new and powerful techniques for controlling drag in turbulent flows, verifying these predictions presents many challenges, including • •

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Manufacturing of large areas of tailored surface roughness in a controlled manner, Preparation of robust hydrophobic coatings that can survive extensive testing, and Accurate characterization of turbulent flows and measurement of turbulent drag over test surfaces

In this paper, we describe fabrication techniques and experimental testing of superhydrophobic surfaces. The preliminary results demonstrate large areas of uniformly fabricated hydrophobic surfaces, successful hydrophobic coatings and significant reduction of viscous skin friction in the laminar and transitional regimes. Further experiments, currently underway, will add results from PIV on the detailed structure of turbulent flow with and without the hydrophobic surfaces.

II.

Superhydrophobic surfaces

It has long been known that a high degree of surface roughness often results in a substantial increase in the degree of hydrophobicity of the solid substrate.1 More recently, this phenomenon, coupled with modern self-assembly and microfabrication techniques, has been used to demonstrate so-called superhydrophobic surfaces, which exhibit a number of new and exciting properties such as extremely high contact angles and very low flow resistance.1,2 This kind of behavior makes superhydrophobic surfaces important candidates for a wide range of applications, from microfluidics and lab-on-a-chip devices to drag reduction and self-cleaning coatings. One example of this surface has been developed at Bell Laboratories and is termed “nanograss”, due to its construction from a large array of grass-like silicon structures.7 Fig. 1 shows an SEM image of a typical nanograss structure. Uniform arrays of nanograss can be fabricated over large areas of 200-mm-diameter Si wafers; the height, diameter, and spacing of the structures are freely adjustable. When these surfaces are coated with a hydrophobic polymer, they exhibit superhydrophobic characteristics, with contact angles approaching 180º. Air trapped in between the posts acts to preserve the nonwetting character and to generate a "slip surface" so that low-drag characteristics are generated when the nanograss surface is submerged in water.

Figure 1. SEM image of silicon nanograss. The diameter of the posts is about 400 nm, the height is about 7 µm, and the spacing between the posts is 1.25 µm.

Fig. 2 illustrates another superhydrophobic surface, in a geometry called “nanobricks”. This structure consists of closed cells that trap air, thus resisting hydrostatic pressure more effectively than the open “nanograss” structure. As with nanograss, the geometrical parameters of closed-cell structures like “nanobricks” are determined photolithographically and can be chosen freely, within wide limits.

III.

Fabrication of large-area superhydrophobic surfaces

Fields of 22 mm x 22 mm of appropriate patterns were stitched together to continuously cover 200 mm Si wafers using 248 nm photolithography. Deep reactive ion etching was used to form the

Figure 2. SEM image of silicon nanobricks. The size of each cell is 4 µm x 10 µm, the height of the cell walls is about 1 µm, and the thickness of the walls is 300 nm

2 American Institute of Aeronautics and Astronautics

nanostructures. After oxide growth, the wafers were diced into individual 104.0 mm x 87.3 mm plates, which were then coated with a thin CFx film formed by plasma vapor deposition using C4F8 as a precursor. To produce superhydrophobic surfaces covering a large area, eight tiles were epoxied to an aluminum plate of dimensions 415.9 mm x 174.6 mm. To ensure minimal perturbation of the incident fluid flow field, the tiles were carefully butted together so that the width of the cracks between them was less than about 0.05 mm. Steps in level height between adjacent tiles and between the tiles and the surrounding metal border were also held Figure 3. Nanograss-covered test plate. The overall to similar values. A photograph of one such test plate is size of the nanostrucutured area is 416 mm x 175 mm. shown in Fig. 3. In this picture, the colors in the photograph are due to diffraction of room light by the microstructures. One test plate was made using each of the nanostructures shown in Figs. 1 and 2, and a third reference plate was assembled using flat silicon tiles. A flat plate made from PVC was also used as a reference.

IV.

Water tunnel testing

Experiments were conducted in the NUWC research water tunnel, which has a 0.3 m x 0.3 m test section of length 3.1 m. The test section contains a full-width fiberglass plate with a 4:1 elliptical leading edge, of total length 1.2 m. This plate has a rectangular hole, 370 mm from the leading edge, in which the test plate is suspended by flexible steel strips. Thus, the test plate is free to deflect under hydrodynamic forces. An optical proximity sensor measures this deflection and is calibrated in situ so that the hydrodynamic drag can be determined from the test-plate deflection. Two series of tests were performed – a control experiment in which a PVC dummy plate was tested, and an experiment in which the nanograss plate was tested. Auxiliary experiments were also performed using a flat silicon surface and a surface of nanobricks; these are not reported here. Fig. 4 shows the result of baseline measurements on the flat PVC plate. The open circles represent individual data points, while the smooth black curve is a spline fit to the data. Theoretical estimates for the drag in the laminar and turbulent regimes are represented by colored curves and are consistent with the data. The transition to turbulence is evident at a freestream velocity of 1.0 – 1.1 m/sec. (Rex = 370,000 – 407,000 at the leading edge of the test surface). In Fig. 5, the drag measured using the nanograss plate (red symbols) is compared with the baseline data copied from Fig. 4 (blue symbols). The principal observation is that the drag force on the nanograss plate is substantially lower than that measured using the PVC plate. This is maintained over the entire range of speed tested. As with the baseline flow (Fig 4), the drag rises slowly until a speed of approximately 1 m/s before rising sharply, presumably due to the onset of a turbulent flow. In the laminar regime, the skin friction experienced by the nanograss plate is approximately 50% of that experienced by the uncoated surface, as evidenced by the solid and dotted lines in Fig. 5. At speeds higher than about 1 m/s, the drag reduction is more modest, but is nevertheless significant, and is present over the full range of velocities tested. One should remember that the nanograss surfaces only coats a small

Figure 4. Axial drag force plotted as a function of channel velocity for the flat, PVC test plate. Open symbols: data points. Black curve: spline fit to data. Green curve: estimate of laminar drag. Red and blue curves: estimates of turbulent drag.


piece of the wetted surface, and does not include the leading edge and development length of the plate. The developed boundary layer must therefore adjust following the transition from a hydrophilic to a hydrophobic boundary condition, and hence the local drag reduction (for example, at the downstream end of the nanograss surface) is likely substantially higher than is represented by the integrated drag measurement presented here.

V.

Conclusions and additional experiments

Although these results are preliminary, they are extremely encouraging, suggesting that the nanograss surface is effective in reducing the skin friction over a submerged body over a wide range of speeds. In addition to the results presented in this paper, experiments are currently underway to better quantify drag reduction, and to characterize in more detail the turbulent flow structure due to the superhydrophobic surface. These experiments are being conducted both in the NUWC tunnel and the Brown University Low Speed water channel in which a fully-turbulent channel flow can be established at a turbulent Reynolds number, R* (based on friction velocity and half-channel height) ranging from 150 to 600. High-resolution PIV is used to measure the velocity field in the near-wall region of the flow and will be carried out for both a flat test surface and nanostructured surfaces. This testing condition corresponds to that simulated in Ref. 2, and so direct comparisons will be possible. An example of the near-wall velocity statistics on the control surface is shown in Fig. 6. Additional experiments at higher speeds in the NUWC water tunnel are also planned.

Figure 5. Axial drag force plotted as a function of channel velocity for the flat, PVC test plate (blue points) and for the nanograss plate (red points). The solid blue curve is a quadratic fit to the blue points in the laminar regime. The dashed blue curve shows 50% of this drag force. In the laminar regime, the silicon nanograss plate experiences about half the drag force of the flat plate. The red points below 0.6 m/sec show essentially zero drag.

Figure 6. Measurements of the mean and fluctuating velocities over a control surface (smooth wall) obtained using high-resolution PIV in the Brown University Water Channel Facility. The mean velocity profiles are compared with the profiles generated using DNS simulations at two different Reynolds numbers.


References 1

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