Experimental Study Of Surface Wettability Effect On Droplet Dynamics During Impacting On Tubular Wall

Phenomenon of droplets impacting on solid often occurs in natural and engineering field, and in engineering field, numerous precise relevant knowledge is required. Surface wettability takes an important role on dynamics of droplets after impacting. Wettability has direct influence on heat and mass transfer efficiency in project. Comprehensive study of droplet impacting has been done, but while surface wettability effect on droplet impacting on curved surface is less concerned. In this work, dynamics of droplet was explored by experiment when impacted on tubular wall with different wettability. Surface wettability effect on droplet dynamics during impacting on tubular wall was analyzed.Five different wettability surfaces were prepared by experiment, using method of dislocation-selective chemical etching and surface modification. Under the situation where diameter of tubular wall was less, lager, or much larger than the diameter of the droplet, droplet with the same initial velocity impact on hydrophilic, hydrophobic and super-hydrophobic surfaces was observed. Wettability effect on droplet dynamics characteristics after impacting with different curvature ratio was analyzed. It was found that when diameter of tubular wall was less than that of droplet, water film partially detached. Water film covered the entire circumference of the hydrophilic tube, and the falling location was just under the tube. While liquid film draped on both sides of hydrophobic or super-hydrophobic tube, and partial liquid would fall down. When diameter of tubular wall was lager or much lager than that of droplet, Droplet would reach steady quickly after spreading on hydrophilic surface, while retraction would happen after reaching maximum spread on hydrophobic surface, and rebound with film shape happened on super-hydrophobic surface. When surface wettability got worse, it would hinder droplet spreading and promote retraction. And when curvature ratio become larger, retraction process would be hindered, and breaking trend was more obvious during retraction and rebound process.Droplet with the different initial velocity impact on hydrophilic, hydrophobic and super-hydrophobic surfaces was observed. Wettability effect on droplet dynamics with different initial velocity was analyzed based on this experiment. The wettability effect on spread and retraction trend was consistent on the whole. While contact angle is less than 107° when the initial impact velocity became smaller, droplet retraction and concussion became more obvious. On superhydrophobic surface, when initial impact velocity was greater than 0.94m/s, fragmentation occurred after impact, and the greater the initial impact velocity was, the sooner period the fragmentation occurred. When the initial impact velocity wasl.95m/s, film fracture occurred on the hydrophobic wall, furthermore, the worse the wettability was, the sooner period the fragmentation occurred.