RESEARCH ARTICLE Experimental investigation of inclined liquid water jet flow onto vertically located superhydrophobic surfaces Ali Kibar • Hasan Karabay • K. Su ¨ leyman Yig ˘it • Ikrime O. Ucar • H. Yıldırım Erbil Received: 27 November 2008 / Revised: 5 March 2010 / Accepted: 14 March 2010 / Published online: 1 April 2010 Ó Springer-Verlag 2010 Abstract In this study, the behaviour of an inclined water jet, which is impinged onto hydrophobic and superhydro- phobic surfaces, has been investigated experimentally. Water jet was impinged with different inclination angles (15°–45°) onto five different hydrophobic surfaces made of rough polymer, which were held vertically. The water contact angles on these surfaces were measured as 102°, 112°, 123°, 145° and 167° showing that the last surface was superhydrophobic. Two different nozzles with 1.75 and 4 mm in diameters were used to create the water jet. Water jet velocity was within the range of 0.5–5 m/s, thus the Weber number varied from 5 to 650 and Reynolds number from 500 to 8,000 during the experiments. Hydrophobic surfaces reflected the liquid jet depending on the surface contact angle, jet inclination angle and the Weber number. The variation of the reflection angle with the Weber number showed a maximum value for a constant jet angle. The maximum value of the reflection angle was nearly equal to half of the jet angle. It was determined that the viscous drag decreases as the contact angle of the hydro- phobic surface increases. The drag force on the wall is reduced dramatically with superhydrophobic surfaces. The amount of reduction of the average shear stress on the wall was about 40%, when the contact angle of the surface was increased from 145° to 167°. The area of the spreading water layer decreased as the contact angle of the surface increased and as the jet inclination angle, Weber number and Reynolds number decreased. 1 Introduction When a liquid jet impinges on a solid surface, it starts to flow radially outward in the form of a thin layer at the impingement zone by changing its direction. While the liquid spreads on the surface as a thin layer, the liquid is stretched over the surface by inertia. At some radial dis- tance from the impingement zone, the film thickness abruptly increases while gaining potential energy. This event is called as hydraulic jump and studied by many scientists such as Watson (1964), Bohr et al. (1993) and Kate et al. (2007a, b). Normal impingement of a circular liquid jet on a smooth horizontal surface leads to an axi- symmetric jump (Fig. 1). However, if the angle of the liquid jet is not vertical to horizontal surface, or vice versa, the axisymmetric structure of the jump is corrupted (Fig. 2a). Watson (1964) was the first scientist who examined the influence of the liquid viscosity in a circular hydraulic jump. He presented both experimental and theoretical results by direct adaptation of the momentum balance theory to impinging jet. Bohr et al. (1993) developed a model which predicted hydraulic jump on a surface around the diameter of a circle and then compared it with their experiments, similar to Godwin (1993). Brechet and Ne ´da (1999) also developed a new equation and compared this equation with their experimental results and Bohr’s and Godwin’s experiments. Bush and Aristoff (2003) experi- mentally investigated the influence of surface tension on a laminar situation. Miller et al. (2005) studied the collision of opposite jets of viscoelastic liquid and described the A. Kibar (&)  H. Karabay  K. S. Yig ˘it Department of Mechanical Engineering, Kocaeli University, 41040 Kocaeli, Turkey e-mail: alikibar@kocaeli.edu.tr I. O. Ucar  H. Y. Erbil Department of Chemical Engineering, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey 123 Exp Fluids (2010) 49:1135–1145 DOI 10.1007/s00348-010-0864-6