(272h) Modeling of Two-Phase Fluid Flow Over Superhydrophobic Surfaces

Superhydrophobic surfaces, which combine the chemical composition of a hydrophobic surface with microscopic roughness elements to achieve an apparent contact angle between a drop of fluid and a surface of at least 150°,  have attracted increasing attention in recent years for their potential use in a number of applications, including separation processes, bioadhesion, and microfluidic devices.  These surfaces result in a reduced contact area between the liquid and solid by trapping air between the roughness elements, causing the flowing liquid to form a meniscus that may extend into the space between the roughness elements.  One of the most potentially transformative uses of these surfaces is for sustainable drag reduction in laminar and in turbulent flow regimes.   One common approach to modeling these surfaces with computational fluid dynamics assumes the meniscus formed between the roughness elements is flat and involves the single phase simulation of liquid flowing over a surface with alternating free shear and no slip boundary conditions [1, 2], or single phase flow with specified slip [3].  The exclusion of the vapor phase in these simulations, however, could prevent a robust theoretical understanding of drag reduction over superhydrophobic surfaces from being developed.  This study evaluates the validity of the flat free surface assumption with emphasis in turbulent drag reduction  and compares the results obtained using a volume of fluid multiphase model with previously published direct numerical simulation data.  The dependence of the slip length of the liquid on the Reynolds number and shape, size, and configuration of the superhydrophobic roughness elements is also examined in order to better understand the mechanism by which slip results in flow over superhydrophobic surfaces.

ACKNOWLEDGEMENTS

The financial support of the National Science Foundation (CBET-0853657) is acknowledged.

REFERENCES

1. Daniello, R.J., N.E. Waterhouse, and J.P. Rothstein, “Drag Reduction in Turbulent Flows Over             Superhydrophobic Surfaces,” Phys. Fluids, 21(8), 085103 (2009).
2. Rothstein, J.P., “Slip on Superhydrophobic Surfaces,” Annu.Rev. Fluid Mech., 42, 89-109 (2010).
3. Spencer, N.B., L.L. Lee, R.N. Parthasarathy and D.V. Papavassiliou, “Turbulence Structure for    Plane Poiseuille-Couette Flow and Implications for Drag Reduction Over Surfaces with             Slip,” Can. J. Chem. Eng., 87(1), 38-46 (2009).