Study Of Bioinspired One-Dimensional Cone Structure With Helical Micro-Grooves For Directional Liquid Transport

Droplet directional motion has important application value in fog water collection,microfluidics,selfcleaning and other fields,which has attracted a lot of attention from the scientific community at home and abroad.The directional motion of droplets on the substrate surface is mainly influenced by the chemical composition of the material and the surface morphology and microstructure.The main challenge is how to achieve rapid directional motion of droplets without the use of external forces such as gravity.Cactus spine with micro-grooved cone structure provides an ideal model for the design of directional fluid transport.Inspired by cactus spine,a lot of artificial liquid transport systems have been reported.Compared to no-grooved conical spines,grooved conical spine could amplify the Laplace pressure difference to which the liquid droplets on them are subjected,thus providing more efficient liquid transport performance.In fact,the array of micro-grooves on natural cactus needles is helical rather than straight.In addition,helical structures are also found in plant xylem tissue and animal heart tissue,which are closely related to liquid transport.Up to now,the influence of helical microstructures on liquid transport and the mechanism of action have not been explored.In this context,this thesis is mainly inspired by the helical micro-groove structure on the surface of natural cactus needles,to explore the law and mechanism of action of helical micro-grooves on the surface of one-dimensional asymmetric micro-structures on droplet directional motion performance,and to try to investigate the advantages of helical micro-groove needles in the application of fog water collection.The helical structure of the cone is changed to compare the droplet directional motion velocity on the cone surface of different helical structures and to optimise and improve the size of the helical structure.The specific research is as follows:(1)Droplet directional motion properties of a one-dimensional cone surface with helical microgrooves and the fog collection behaviour of a fog collection system with its core.Inspired by the structural characteristics of microgrooves on the surface of natural cactus needles,cones with helical microgrooves were prepared using 3D printing combined with template replication to compare the liquid transport capacity of cones with arrays of straight microgrooves.The experimental results show that the critical volumes of liquid droplets starting to move on the one-dimensional cone surfaces of the helical and straight microgrooves are 0.17 μL and 0.41 μL respectively,and the corresponding average velocities are 96.73 μm/s and 29.72μm/s,i.e.the helical microgroove cone has better liquid transport capability.The analysis suggests that the helical microgroove makes it easier for the droplet to shift from clamshell to barrel morphology,with the droplet being longer in the radial direction along the cone,while the three-phase vapour-liquid-solid contact line(TPCL)morphology is transformed,resulting in an overall increase in the kinematic driving force on the liquid caused by the Laplace pressure difference,facilitating the directional motion of the droplet.The helical microgroove cone is further combined with a highly absorbent wood block treated with delignification to create a fog collection system that can be rapidly absorbed after directed droplet transport,demonstrating continuous and efficient fog water collection capability.This new helical microgroove cone structure will provide new ideas for the design of liquid manipulation systems.(2)Modulation of helical microstructure parameters on one-dimensional cone surfaces to further optimise the directional transport properties of liquids.Based on the previous part of the study,the development of a simple and efficient strategy for the controlled modulation of helical microstructures can help to systematically investigate the facilitation of helical grooves on the directional motion of liquid droplets.In this chapter,a series of helical microstructures with controllable pitch and helix angle were obtained by varying the entanglement parameters with a triangular cross-section of micron wires helically entangled on the surface of a tapered copper wire,and a one-dimensional conical structure with helical microgrooves was successfully prepared by compounding the subsequent template replication approach.The velocity of the droplet on the cone can be adjusted from 24.46 μm/s to 87.42 μm/s,which confirms the role of the helical microstructure parameters on the cone surface in regulating the directional motion of the droplet.Subsequently,the helical microstructure parameters will be further optimised to improve the kinematic performance of the liquid.This part of the work will help to promote the application of one-dimensional cones with helical microgrooves in practice.