Ali KİBAR, Mehmet BAYRAKTUTAR, Kadri Suleyman YİĞİT
Abstract: Introduction: The hydrophobic surfaces have attracted great attentions during the last decades. These surfaces are commonly used in industrial and domestic applications including; low friction applications, medical tools, self-cleaning surfaces, antifogging/anticorrosive surfaces, microfluidic channels, filtration, phase separation etc. Alternatively, the liquid jet is extensively used industrial processes such as cooling, cleaning, coating and in-jet printing. The impingement of the liquid jet doesn’t spread much more area on the hydrophobic surface than hydrophilic surface because of the low free surface energy of the hydrophobic surface. This difference offers alternative using to hydrophobic surfaces in liquid jet applications. Purpose: The purpose of this study is to investigate the behavior of the impinging liquid jet on the hydrophobic surfaces. Scope: The physics of the impinging liquid jet is very complex phenomena. In this study, the impingement, spreading and leaving of the liquid jet on the hydrophobic surfaces are examined thanks to numerical results in detail. Limitations: The durability of the surface was important during the experiments in order to ensure the hydrophobicity. Therefore, we did experiments in the certain Reynolds numbers range. Method: In this study, we used flat Teflon sheet surface with 110° apparent contact angles a hydrophobic surface for the experiments. The liquid jet was obtained by using a glass tube. The inclination angles were in the range of 15-60°.The Reynolds numbers of the jet were in the range of 1400-5600. The experiments were recorded using a CCD camera. The numerical results were used to explain the complex spreading phenomena. Results: When the liquid jet impinges on the hydrophobic surface obliquely, the liquid spreads with increasing its free surface area on the surface (Kibar, 2016). This spreading ends laterally at the maximum width and then the liquid starts to gathering after this point. The incoming liquid jet has a kinetic energy. This energy is converted into stored surface energy, which is the ratio of the free surface area to the wetting area of the spreading liquid. The behavior of the gathering liquid after the spreading is related mainly the stored surface energy. If the stored surface energy is sufficient, the liquid is reflected away from the hydrophobic surface. Otherwise it turns back to the surface after perpendicular expanding to the surface. Then, the liquid spreads as a second spreading. This second spreading area is smaller than the first one due to the frictional heating. The spreading scenario, which is called as braiding flow, continuously repeats several times before it fully damps out. When there is no sufficient stored free energy left, the liquid flows as a rivulet type flow on the surface (Kibar, 2016). Conclusions: In this study, the impingement of the liquid jet on the hydrophobic surface is examined experimentally and numerically. The kinetic energy of the impinging liquid jet is transformed to the surface energy. The spreading and leaving of an impinging liquid jet on the hydrophobic surface are determined by this surface energy of the spreading liquid. The surface energy is determined mainly by the ratio of the free surface to the wetting area of the liquid on the hydrophobic surface.

Anahtar Kelimeler: Liquid jet, Hydrophobic Surface, Jet Impingement, CFD