| Summary: | 碩士 === 國立中興大學 === 機械工程學系所 === 96 === This study investigates the temporal characteristics of an impinging droplet onto dry surface of various properties via a high speed camera. The dynamic contact angle, the spreading diameter and the central height of the deformed droplet are the subjects of primary interest. Two impinging velocities, three different surface properties and droplets of seven different ingredients are employed to realize the transient behavior of the impinging droplet onto dry surface.
Experimental results show that: (a) during the kinematics stage, all the spreading diameters and the central heights of the deformed droplet are independent of the surface properties, the droplet material and the impinging velocity. However, the variations of the dynamic contact angle depend strongly on these properties. During this stage, three different variations of dynamic contact angles are found relating to the formation of lamella. (b) During the spreading stage, the more viscous of the droplet, the slower is the spreading speed and the thinner is the deformed droplet. The contact angle decreases from an obtuse angle and fluctuates slightly until the maximum spreading diameter is reached. For the less viscous droplet (water), the contact angle starts from an acute angle, increases abruptly and fluctuates slightly until the maximum spreading diameter is reached. (c) Within the receding stage, the more hydrophobic surfaces, the faster is the receding speed, the thicker is the deformed droplet, and vice versa. The contact angle increases drastically due to the eruption of the central liquid column and then decreases significantly and monotonously in an oscillatory manner. (d) During the wetting stage, the more viscous droplet and the more hydrophilic surface, the faster is the wetting speed, and vice versa. During the wetting stage, the contact angle continues to decrease to asymptotically approach the value of static contact angle.
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