| The behavior and phenomenon of droplets impacting a moving superhydrophobic solid surface are ubiquitous in nature and have important applications in many biological,environmental and industrial fields.For example,raindrops hit swinging leaves in the wind,cars,trains and planes running in the rain,dedusting,inkjet printing,separating the liquid medicine and other scenes.At present,there are relatively few studies on the impact behavior of droplets on moving substrates and many problems and challenges are remained.In this paper,the phenomenon and mechanism of droplet impact on horizontally moving superhydrophobic substrates are investigated and their relative mechanisms and applications are explored as follows.(1)Orientational transport behaviors after droplets impacting a low-speed horizontal moving superhydrophobic substrate.Manipulating the orientation bounce behaviors of droplets after collision has important applications in biology,medicine,engineering and other fields.At present,there are few studies on the solid-liquid coupling mechanism of the orientation bounce of water droplets after impact with a moving superhydrophobic substrate,and the regulation strategy of the orientation velocity of droplets after impact is still unclear.In this work,by investigating the impact processes and post-impact trajectories of water droplets hitting a horizontally moving superhydrophobic substrate,adjusting the experimental conditions(substrate velocity,droplet impact velocity,substrate wettablity,etc.),the state in the process of droplet impacts and the orientation velocity in the horizontal direction after impact were analyzed,combined with three-dimensional diffusion interface(DI)numerical simulation,The process and mechanism of the momentum transfer between the droplet and the substrate during the impact process are elucidated.It is revealed that the tilt state of the droplet at the moment leaving after the impact is determined by the speed of the momentum transfer via the viscous force,which also determines the horizontal orientation velocity of the droplet after the impact.An empirical formula for regulating the horizontal orientation velocity of a droplet after impact is obtained.The effect of predicting and controlling the horizontal velocity of a droplet after impact is achieved by the impact condition.It is proved that the orientation bounce behavior can greatly affect the flight speed and momentum of a small aircraft.This work not only deepen the understanding of the motion behavior of droplet impact motion after substrate impact,but also reveal the underlying mechanism.It has universal guiding significance for the solid-liquid impact behavior involving relative horizontal motion(such as pesticide spraying by UAV),and provides basic data and theoretical support for the application of orientational droplet transportation of matter,energy and momentum.(2)Enhancing the deposition of aqueous droplets on the high-speed moving superhydrophobic substrates.It is important to enhance the deposition behavior of droplets on superhydrophobic surface for pesticide spraying,inkjet printing and other applications.At present,there are many deposition methods and strategies on static superhydrophobic surface,but none of these strategies can make water droplets deposit on high-speed superhydrophobic surface.The impacts of water droplets hitting the high-speed superhydrophobic surface show the balloon-linear impact dynamics,which is not only asymmetrical in the transverse and longitudinal directions,but also in the upstream and downstream parts.The high asymmetry results in the rapid separation of water droplets from the surface,and the contact time is greatly reduced(>63%).In this work,by learning the deposition behavior of oil droplets such as ethanol and paraffin on the high-speed moving substrate,we add binary compound additives,surfactant bis(2-ethylhexyl)sodium succinate sulfonate(AOT)and polymer polyvinyl alcohol(PEG)additives to the water phase and change the physical and chemical properties of water droplets to achieve the deposition effect.It is found four d distinct impact dynamics: balloon-linear non-deposition,balloon-pencil downstream deposition,filamentous non-deposition and spindle complete deposition.Based on the statistical characteristic information,we deduced the impact process and analyzed the critical velocity of air entrainment,etc.,and revealed the formation mechanisms of four kinds of impact dynamics on the super-hydrophobic substrate moving at high speed.It was found that increasing viscosity and reducing surface tension were crucial for the deposition to reduce the asymmetry of the droplet impact morphology.Finally,when the process was simulated which the UAV sprayed pesticide and even when there was a relative horizonal motion speed between the spray droplets and the leaves,the deposition rate was also greatly improved.This work provides basic data and theoretical guidance for revealing the dynamic formation process and deposition mechanism of droplets on high-speed moving superhydrophobic substrates,and also provides a new idea for the study of liquid-solid impact behavior with relative horizontal motion.(3)The orientation transportation behavior of the droplet impinging on the striped moving superhydrophobic surface.When a water droplet hits a striped substrate,the orientation bounce behavior of water droplets on the superhydrophobic substrate is regulated by the angle between the stripes direction and the substrate moving direction,so as to achieve the purpose of inhibiting or promoting the orientation bounce behavior of water droplets after impact.In this work,we analyzed the process of a droplet hitting a vertical stripe structure(the direction of the stripe is perpendicular to the direction of the running substrate),and found it is controlled by the Laplace force,surface tension,capillary force,inertial forces during the impact.Stripes with different parameters(different stripe spacing and stripe height)and experimental conditions(substrate running speed and droplet impact speed)were used to regulate the corresponding orientation bounce behavior of droplets.The results show that the striated structure perpendicular to the moving direction of the substrate promotes the horizontal transport of droplets,while the striated structure parallel to the moving direction of the substrate inhibits the horizontal transport behavior of droplets after collision.The striated structure on the substrate inhibits the vertical bounce of droplets after collision.The spacing,width,and height of the stripes all affect the magnitude of the orientation velocity obtained by the droplet after impact.In addition,when the stripe height is appropriate,stripe bouncing similar to pancake bouncing can occur when the substrate runs at a large speed,that is,there is no receding phenomenon observed from the side view.The permeation of water droplets on a vertically striped substrate is similar to the state transition from Cassie state to Wenzel state,resulting in longer contact time between droplets and moving substrates and larger droplet horizontal velocity.The behavior and phenomenon of orientational transport of droplets on a moving superhydrophobic solid surface are ubiquitous in nature and have important applications in many biological,environmental and industrial fields. |
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