Surface roughness effect on droplet impact characterization: Experimental and theoretical study

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.