Study On Dynamics Of Droplets Impacting On Superhydrophobic Surfaces

Droplets impacting on solid surfaces is a common phenomenon widely existing in daily life and industrial field,especially droplets impacting superhydrophobic surfaces have important effect on applications,such as self-cleaning,drag reduction and anti-icing.At present,most researches of droplets impacting on superhydrophobic surfaces are limited to horizontal surfaces,and the study about inclined surfaces are rarely reported.However,in specific applications,droplets impinge the irregular surfaces mostly.So droplets impacting on inclined surfaces deserve to be studied.Therefore,in this paper,experiments of droplets impacting on the inclined superhydrophobic surface are conducted,which are mainly divided into two parts:The first part of the experiment studied the dynamic characteristics of the spreading,contact time and sliding for droplets impacting on superhydrophobic inclined droplets with different inclined angle.It's found that a droplet impacting on the inclined superhydrophobic surfaces exhibits asymmetric bouncing with an asynchronous spreading and retraction along the tangential and lateral direction.During the spreading process,the spreading speed as well as the maximum spreading diameter of the droplet in the tangential direction are greater than that in the lateral direction.However,retraction speed in the lateral direction is greater than that in the tangential direction.We measured the maximum diameter in two directions and obtained the calculation model of the maximum spreading diameter in tangential and lateral directions by regression fitting.Meanwhile,droplets impacting on the inclined superhydrophobic surfaces can reduce the contact time compared to conventional case.We demonstrate that contact time reduction results from asymmetric spreading and retraction.The internal mechanism of the contact time reduction revealed by the theoretical analysis indicates that the contact time decreases as normal Weber number and inclined angle increase.In addition,the sliding characteristic of a droplet impacting on the inclined superhydrophobic surface was studied in the experiment.The sliding distance of the droplet on the surface increases as the inclined angle and the normal Weber number.Especially,the front and tail of the droplet move in the opposite direction at the initial collision.Furthermore,the sliding distance of the tail decreases when the tilt angle increases.The second part of the experiment was inspired by the first part of the experiment.The grooved surface of the sub-millimeter macro-scale was fabricated by wire EDM technology.The spreading and contact time dynamics of droplets impacting on the superhydrophobic grooved surface at different inclined angles were studied.The study is divided into two parts according to the angle between the groove direction and the tangential direction of the inclined surface: lateral impacting and longitudinal impacting.There were 7 different bouncing types at the different inclined angle and normal Weber number in the lateral impacting,but only 3 types in the longitudinal impacting.At higher Weber numbers,different spreading types were found at different angles in the lateral impacting(? = 0 °: petal spreading;? = 15 ° and 30 °: claw spreading;? = 45 °: shield spreading;? = 60 °: stretched spreading).However,only petal spreading occurs in the longitudinal impacting.Different spreading conditions correspond to different contact time.The variation of the lateral impacting contact time is affected by the ridges of the grooved surface and the inclined angle,while the longitudinal impacting contact time is only affected by the ridges of the grooved surface by a simple theoretical analysis.