Ali KİBAR, Kadri Suleyman YİĞİT
Introduction: The impingement of liquid jet on the solid surfaces is commonly used in industrial applications and processes such as ink-jet printing, spraying, cooling, heating, steel making (Ashgriz, 2011). In these applications, the spreading behavior of the jet on the surface is an important factor. One of the properties that affect the spreading behavior of jet is the wettability of the surface. The surface wettability is determined by its contact angle. There are three main classifications for the wettability of the surface, including hydrophilic (θ<90°), hydrophobic (90°<θ<150°) and superhydrophobic (θ>150°). The droplet with less size than capillary length stands as an almost spherical shape on the superhydrophobic surface. Therefore, the spreading of the droplet/liquid jet is affected by the surface wettability. The spreading behavior of the impinging liquid jet on the superhydrophobic surfaces varies greatly with regard to hydrophilic surfaces. Purpose: The purpose of this study is to present the effect of the wettability, impingement angle and inertia of the impingement of the liquid jet on superhydrophobic and hydrophobic surfaces. Scope: The spreading behavior such as spreading area, shape and thickness is important parameters for applications of the liquid jet. In this study, the behaviors of the impinging liquid jets on the solid surfaces are classified depends on the wettability, impingement angle and physical properties of the liquid jet. Limitations: The experiments should be done in a certain Reynolds numbers range. Because, the coated surface could be damaged by the liquid jet with high Reynolds numbers. Method: In this study, flat superhydrophobic, hydrophobic and hydrophilic surfaces, which have 154°, 110°and 75° apparent contact angles, respectively were used for experiments. The superhydrophobic and hydrophobic surfaces were obtained by using the spray coating and the Teflon sheet, respectively. The plexiglass sheet was used for hydrophilic surface. The liquid jet was pumped by using the centrifugal pump throughout a glass tube with 4mm inner diameter. The inclination angles of the surfaces were altered in the range of 0−90°. Results: When the liquid jet impinges on a horizontal hydrophilic surface perpendicularly, it spreads as a thin film around the impingement point and then film thickness increases abruptly at a certain distance from the impingement point. This axial symmetric spreading is called as a circular hydraulic jump. If the target solid surface is superhydrophobic, the impinging liquid jet breaks up into droplets after a certain spreading radially outward on surface (Kibar, 2017). When the liquid jet impinges on the hydrophobic and superhydrophobic surface with an angle, the liquid spreads and afterwards combines again at a certain distance from the impingement point. Afterwards, the combining liquid is oriented outward from the surface for both of surfaces. However, while it is spread as a second spreading on the hydrophobic surface that results braiding types flow (Mertens et al., 2005), it is separated from the superhydrophobic surface that results reflection types flow (Kibar et al., 2010). If the liquid jet impinges on a superhydrophobic surface in a high Weber number, liquid splashes outward of the spreading boundary. The rivulet type flow can be observed on the inclined hydrophilic and hydrophobic surfaces. When a liquid flows and its width decreases over a surface, this type flow is called rivulet flow (Wilson et al., 2012). The impinging liquid jet spreads smaller area on the superhydrophobic surface than hydrophobic one due to low molecular attraction, which is determined by free surface energy, between the liquid and the superhydrophobic surface. The spreading and reflection of liquid is also governed by inertia, surface tension and viscosity of the liquid apart from contact angle. Conclusions: The impinging liquid jet spreads larger area on the hydrophilic surface than superhydrophobic and hydrophobic surfaces. The capillary force prevents the spreading on the superhydrophobic surface. Therefore, the impinging liquid jet obliquely on a superhydrophobic surface spreads in a small area and then bounces off the surface with a reflection angle. If the surface is a hydrophobic, the liquid jet spreads in the form of squeezing shrinking area called as braiding flow. Consequently, the spreading behavior of the impinging liquid jet on the solid surface is mostly affected by the wettability of the surface.

Anahtar Kelimeler: Liquid jet, Superhydrophobic, Wettability