8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P7.112Thu 811:10-14:00Water chamber and drop tank measurements onsuperhydrophobic spheresSimon Stanley 11 Nottingham Trent University, College of Arts & Science, Clifton Lane NG11 8NS,Nottingham, United KingdomA range of authors have reported possible drag reduction as water flows across a superhydrophobicsurface. Experimental systems have included plates, hydrofoils, microfluidic channels, millimetrictubes and settling spheres [1-4]. Drag reduction has been reported for both laminar and turbulentflow regimes and is believed to be caused by the ability of an immersed superhydrophopbic surfaceto retain a layer of air and, hence, a shear-free air-water interface between peaks of the surfacemicro-topography. In this work, we report measurements on a range of surfaces that are shownto retain an air-layer (i. e. a plastron [5]) when immersed in water. For the case of a roughsand surface with a hydrophobic coating, as used in previous terminal velocity measurements [3],examination under confocal microscopy indicates that the rough surface features protrude throughthe plastron into the surrounding liquid. This effect is believed to contribute towards drag andsuggests that further improvements in terms of the terminal velocity of settling superhydrophobicspheres could be achieved with more regular surface features. We report new terminal velocitymeasurements and complementary flow measurements using a circulating water chamber withsuperhydrophobic surfaces comprising regular sized surface features for comparison to previousdata. Acknowledgement The authors’ acknowledge the financial support of EPSRC under grantEP/G057265/1.[1] J. P. Rothstein, Annu. Rev. Fluid Mech. , 42, 89 (2010).[2] N. J. Shirtcliffe, G. McHale, M. I. Newton and Y. Zhang, ACS Appl. Mater. Interf. 1, 1316(2009).[3] G. McHale, N. J. Shirtcliffe, C. R. Evans and M. I. Newton, Appl. Phys. Lett. , 94, art. 064104(2009).[4] G. McHale, M. I. Newton and Neil J. Shirtcliffe, Soft Matter, 6, 714 (2010).[5] N. J. Shirtcliffe, G. McHale, M. I. Newton, et al. , Appl. Phys. Lett. , 89 art 104600 (2006).112