A Visual Experimental Study On A Droplet Impacting Onto Solid Surfaces

The free surface flow problems are commonly found out in the fields of chemical, metallurgy, aviation and aerospace and material science. In this process, the position of the free surface changes along with the movement of liquid, moreover, the movement of the free surface has a great influence on the system. These problems which are inherent in the nonlinear changes coupled with phase change that may occur in the process of impact make it extremely difficult to be dealt with, so it has become a great concern in the field of fluid dynamics.In this paper, theoretical analysis and experimental investigation combining with computer image processing are employed to study the behavior of a single droplet impact onto a solid surface, which are one of the typical cases in the free surface flow problems. The impacts between droplet and homogeneous surface in the atmosphere under the uniform temperature condition and non-uniform temperature condition are observed and studied by using a high speed photographic camera. The process of is analyzed qualitatively and quantitatively by using computer image processing techniques, then the evolution of the spreading diameter, droplet height and contact angle after the droplet impacted onto the solid surface are obtained. The effect factors of spreading process, such as solid surface temperature, the droplet size, droplet kinds and the impact of different factors such as the impact angle on the process of spreading after the droplet impacted onto solid surface are analyzed as well. The effects of the solid surface material and the surface roughness are studied systematically, the mechanism is also analyzed.The results show that a single droplet impacts a solid surface; It forms a rim and spreads radially until it reaches a certain maximum diameter. During the kinetic phase and spreading phase, various factors have little influence on the process of spreading. When the droplet reaches the maximum diameter, it then retracts. The influence becomes obvious. Image processing analysis of evolution of spreading behavior of a droplet is consistent with the theoretical analysis. It explains the feasibility of the experimental technique. It also proves that computer image processing technology can be applied to the analysis of the free surface position in free surface flow.