In this paper, the impact of distilled water drops on hydrophobic cylinders is characterized using both experiments and numerical simulations. Water drops of 2.54 mm in diameter impact with a velocity of 1 m/s on hydrophobic cylinders. The corresponding Reynolds and Weber numbers are 2800 and 34, respectively. Three different stainless steel cylinders with diameters of 0.48 mm, 0.88 mm, and 1.62 mm were used. The surfaces of the cylinders were made hydrophobic using a special coating spray. An experimental setup consisting of a drop generator, a high-speed camera, a lighting system, and a photoelectric sensor was used to capture images of the impact with a time-step of 1 ms. The images were then analyzed using an image processing technique implemented in the matlab software. Both the centric and off-centric impacts were studied for each cylinder diameter. A numerical simulation of the impact was also obtained using an open-source code called OpenFOAM by employing its InterFoam solver. The numerical scheme used by the solver is the volume-of-fluid (VOF) method. The predicted images of the simulations were compared well with those of the captured photographs both qualitatively and quantitatively for the entire experiments. The behavior of the drop after the impact and the subsequent deformation on hydrophobic cylinders including flow instabilities, liquid breakup, and secondary drops formation were observed from both simulations and experiments. By decreasing the cylinder diameter, the breakup occurs sooner, and a smaller number of secondary drops are formed.

In this paper, the effect of surface roughness on the both normal and inclined droplet impact is investigated experimentally by image processing.  The impingement of water droplets with 2.9 mm diameter and 1m/s velocity impacting on three types of stainless steel surfaces with respective arithmetic average surface roughness values of 2.24 μm (Smooth), 6.04 (Medium) and 30.2 (Rough) is examined using a high-speed camera. The dynamic behavior of the impact including droplet deformation, the maximum spreading diameter and length, contact angle and the number of fingers are studied. Experimental results demonstrate that rough surfaces not only prevent secondary droplet formation but also decrease the number of fingers formed around the droplet in the normal droplet impact. Considering the inclined droplet impact scenarios, the asymmetric spreading of droplet on inclined surfaces avoids the secondary droplet formation by decreasing the fluid kinetic energy. In the inclined impact, fingers are formed around the droplet perimeter like the normal impact. The only difference between impacts onto the inclined surfaces is a gradual decrease of the number of fingers by increasing the incline angle. The experimental results are compared with those of the analytics available in the literature for the normal droplet impacts. Next, a simple analytical model for droplet impact on an inclined surface is developed; the predictions from this model was also compared to those of measurements. Calculated values from the analytical models agreed well with experimental data for both normal and inclined impact scenarios.