| Droplet impact solid surface phenomenon exists widely in nature and our daily life,such as inkjet printing,self-cleaning,anti-icing,pesticide spraying,etc.In the past twenty years,scientists have conducted in-depth research and achieved fruitful results in many areas,but there are still many challenges.First,although the study about droplets impacting rigid substrate is already very intensive and extensive,but the research on droplet impacting flexible substrate is just beginning and remains questions.Secondly,the universe wired and curved superhydrophobic leaves in nature induce more reduced contact time and easier splitting of the impacting droplets,which in turn leads to serious deposition problems the field of pesticide spraying.Thirdly,the underlying physics of directional droplet bouncing after impacting different roughness substrate is not clear enough.In order to solve the above problems,the following three research contents were mainly carried out:(1)Study on dynamic behavior and mechanism of droplet impacting flexible superhydrophobic cantileverThe phenomenon of droplets hitting a flexible cantilever is widely present in nature,such as raindrops hitting insect wings and plant leaves.The related research is of great significance to application fields such as the transmission of information,materials,and energy.However,at present,the underlying physics of the highly-coupled solid-droplet motion upon impact is not clear enough,which greatly affects the predictability and maneuverability of the impact outcome.This work is inspired by the flexible adjustment of the running direction of the water droplets after impacting the leaves of Pogonatherum crinitum(Thunb.)Kunth.We first adjusted the experimental conditions before impact(impact velocities and impact positions of droplets and the composition,wettability and flexibility of cantilevers,etc.).Secondly,we observed and calculated the effect of impact conditions on the coupling motion parameters of droplet-solid system during impact and the motion processes of droplet and cantilever after impact.Thirdly,we combined the theoretical derivation of Euler-Bernoulli equations and two-way coupled fluid-solid interaction(FSI)3D numerical simulation,to elucidate the underlying physics of the highly-coupled soliddroplet motion and realize the mutual deduction and precise linkage of the three parts of behaviors before,during and after collision.Finally,we achieved the purpose of programmatic prediction and manipulation of droplet-cantilever impact.This work opens a new avenue for delicately controlling liquid motion in space by a flat and flexible cantilever,and is valuable for a plethora of applications like soft robots to transport materials and energies,monitor plant growth as well as predict pathogen transmission in plants.(2)Enhancing droplet deposition on macro-structured wired and curved superhydrophobic leavesEnhancing droplet deposition is extremely important to agrochemical spraying.The superhydrophobic surface facilitate the droplets bouncing and splashing due to the short contact time of impacting droplet.Especially,improving deposition on the ubiquitous wired and curved superhydrophobic surfaces remains challenging as their surface structures promote asymmetric impacts,thereby shortening the contact times and increasing the likelihood of droplet splitting.Here,we propose a simple strategy to solve the deposition problem by applying binary additives consisting of an extremely small amount of high molecular weight poly(ethylene oxide)(PEO)and a small amount of surfactant bis(2-ethylhexyl)sulfosuccinate sodium(AOT).Combining the prominent extension property of flexible polymer with surface tension reduction of surfactant,the well-chosen binary additives simultaneously solve retention and coverage problems by limiting fragment and enhancing local pinning and wetting processes at a very low usage.Leaves of rice,cauliflower,cabbage,scallion and artificial single wired,parallel wired,cross wired and curved superhydrophobic are chosen as the representative of wired and curved superhydrophobic surfaces.The results show that the problem of water droplet deposition on superhydrophobic stripes and curved superhydrophobic leaves can be solved by using very low concentration of surfactant and polymer,which can not only reduce the amount of pesticide,but also increase the utilization rate of pesticide.This work provides a theoretical basis for the guidance of agrochemical spraying and other applications.(3)Study on the directional bouncing behaviors of water droplets at macro-structured superhydrophobic interfaces with different roughnessDirecting a liquid droplet after or upon impacting a solid is crucial in many biological,sustainable,environmental,and engineering applications.Despite significant progress in the translational motion of droplets on surfaces,a fundamental dynamic understanding of directional bouncing of an impinging droplet needs to be developed and improved.Here,we investigate the projectile motion of a droplet after impacting the superhydrophobic interfaces with various roughness gradients at room temperature.The results reveal that an initial oblique velocity and the reactive force generated in intrinsic gyration(seesaw effect)engenders the preferential bouncing motion of a droplet towards a region with lower roughness.An arbitrary planar force is used to describe the basic mechanical model.New anti-gravity pendulum dynamics are deduced and demonstrated by tuning the model and adjusting the substrate gradient and impact position.The direction of droplet transport is also flexibly adjusted by changing the structure parameter and the inclined angle of the substrate.This acts as a tri-directional rectifier and might be useful for creating a miniature musical fountain.This work advances the understanding of directed droplet transportation and provides a promising approach to delicately control liquid motion by actuating a force model upon a designed interface. |
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