Study Of The Behaviour Of Droplet Impact Motion On The Wing Surface Of Dragonflies

Dragonflies are not wetted by rain during flight,which involves the impact and transport of raindrops.Studying the impact behaviour of droplets on their superhydrophobic wing surfaces provides ideas for the design of directional surfaces.Also,considering the difference between the study’s stationary and realistic dragonfly wings,droplet impact experiments on planar and cantilevered dragonfly wing surfaces were conducted to explore the droplet impact motion,fluid-solid coupling dynamics,and fluid-solid coupling dynamics and unidirectional transport properties of droplet impact on dragonfly wing surfaces.The droplet impacts the surface of the dragonfly’s wing,showing a complete bounce.The droplet jumps from the position at the base of the dragonfly’s wing to the distal position,showing a directional migration.The droplet impact process consists of three phases: the compress phase,the recover phase and the separate phase.In the compress phase,the droplet forms a expanding sheet by impacted and Gibbs free energy progressively higher.The droplet forms a recoil sheet in the recovery phase,and the Gibbs free energy decreased.In the separate phase,the droplet rebounds when t > 48 ms,reaching a maximum height when the droplet is at 74 ms.Droplet impact on the PDMS imitation dragonfly wing surface then oscillating and staying on the surface except for some droplet will separate which achieve a height,this is related to the effect of elastic modulus,viscous loss,and surface energy consumption.The droplet spreading coefficient increases and then decreases,and the Oh number on the surface of the PDMS dragonfly wings is greater than 0.00186,meaning no bouncing occurs.Droplet impact deforms the wings of the cantilevered dragonfly,and achieve unidirectional transport after droplet impact.During the impact phase,the droplet bulge takes on a straw hat shape,the lateral spread diameter and cross-sectional area increase and the base tilt angle increases.During the diffusion phase,the droplet forms an elongated shape,with the droplet’s lateral spread diameter remaining essentially unchanged,the cross-sectional area decreasing,and the tilt angle slowly increasing.The droplet rolls off at the wing root as it enters the recoil phase.Further theoretical model of the strain response of droplet impact on the wall is developed.Droplet transport time and wing deformation increased with the impact height,and the maximum deformation energy followed the same trend as the actual strain variation.In the droplet-cantilever dragonfly wing coupling,the droplet potential energy promotes the wings’ adaptive deformation and downward movement,and the elastic energy is released,showing the "springboard effect" and returning to the starting position.Bridging of adjacent droplets and droplet fusion occurs when multiple droplets impact.The polymerization of adjacent droplets produces secondary droplets.The droplet spreading coefficient will decreases at first then increases at different horizontal spacings,finally remains at a constant value.A decrease in the horizontal spacing of neighbouring droplets can increase the droplet spreading coefficient.Successive droplet impacts on dragonfly wings cause droplets to bounce back,the bounce height and jump energy changing as the height of impact increases,while droplets move towards smaller contact angles,resulting in migration.The droplets have an agglomeration effect,and the droplets tend to fuse,the surface energy of the fused droplets first decrease rapidly and then increase.Droplet self-driven bounce jumps caused by droplet potential energy into kinetic energy(including droplet kinetic energy and surface bounce energy)and surface energy conversion.The superposition of these energies make the droplet to bounce and jump to achieve a directional transport effect.