Motion Of Droplets And Dropwise Condensation On The Gradient Surface

The boiling and condensation heat transfer, as one of the most efficient cooling method, have been paid more attention. Dropwise condensation is a most efficient heat transfer because of its higher surface heat transfer coefficient. Wettability is one of the most important properties of a solid surface and can be applied to propel drops to spread or move on the solid surface without external force. The characteristics of the droplet movement on the gradient surface energy supply a method of eliminating the condensation expect for the gravity. Therefore, the gradient surface energy could be capable of eliminating the condensation in time and improve the condensation heat transfer as for horizontal surface or other gravity free state surface. It can be expected that the interesting and inspiring research on liquid drops motion on the gradient energy surface will significantly contribute to condensation heat transfer enhancement of heat exchanger, biological applications involving cell cytometry studies as well as design and operation of microfluidic devices. Therefore, this research possesses high learning value and engineering applicable value. Presently, research on this aspect is just in the first step in the international academic, and the experiment and mechanism research on this field is limited.In the present study, we adopted the visualized experiments to investigate the coalescence of droplets on the homogenous surface under ambient conditions. The evolution of the three-phase contact line, liquid bridge and contact angle of the coalescing droplets with time were analyzed. The results showed that coalescing drops behaved as a typical damped oscillation after coalescence as a result of differential pressure in concave convex liquid level of the two drops. The energy consumption resulted from viscous dissipation in process of internal flow within liquid drop was compensated by the released surface energy due to the decrease of drop interface area in coalescing. The property of the solid surface had obvious influence on the characters of the coalescence. When the coalescence on the same solid surface, the higher viscosity fluid exhibited the lower oscillation amplitude and frequency of liquid bridge as will as the contact angle, and oscillated for a short time, and decreased the amplitude of the shrinkage of contact lines. Finally, we used a function to fit the evolution of the liquid bridge at the initial rapid growth of a meniscus between the droplets( 0 <τ<τ0), and established experimental correlative equation. The evolution of the liquid bridge with time satisfied with the function, R y= atb. It conformed to the law R y∝tb which could be conveniently governed by adjusting the parameters a, b, and the value of a and b were related with diameter of drops, viscosity, character and inclination of surface, and 0