Wettability And Unidirectional Self-actuation Transport Behavior Of Liquid Metals

Interface phenomena have always attracted great attention from scientists.Wettability,as one of the important properties of the solid material surface,has gradually become the focus of interface research.It has been found that the surface microstructure,geometric morphology and surface free energy can significantly affect the wettability.By adjusting the surface structure,the wettability and the dynamic behavior of the liquid would like to change,such as the directional motion of the droplet.Droplet directional transport is a ubiquitous process in industrial production and daily life.Fundamentally,droplet transport involves interface science and fluid dynamics.Inspired by natural phenomena,researchers have realized self-actuation of droplets without extra force through wetting effect,resulting from the difference of wettability on both sides of droplet.Compared with the wetting effect which can provide the driving force of transport,the lubrication effect aims to reduce the interfacial resistance and improve the transport capacity.Although wetting and directional transport have been widely studied and applied,most of them focus on water droplets or droplets with low surface tension,and the research on liquid metal droplet with high surface tension is not enough.The room-temperature liquid metal has a wide application prospect because of its excellent properties.Compared to mercury,gallium and its alloys stand out for their low toxicity,no vapor pressure,high electrical conductivity and heat transfer properties.Nondestructive transfer of liquid metals is an urgent problem in biological drug loading,microfluidic,fluid sensor,direct writing.Based on the above research background,this paper studies the relationship between wetting and directional transport of liquid metal droplet from the perspective of wetting effect and lubrication effect.The main research results are as follows:(1)The directional transport dynamics and kinematics of liquid metal droplets on radial textured gradient surfaces are studied.Firstly,a series of radial textured surfaces with different densities were set up,and the wetting state of gallium droplets transitions from the Wenzel state to the Cassie state with the increase of density.Then,we combined surfaces of different densities in pairs to create shape asymmetry on both sides of gallium droplets,providing the driving force for directional transport of gallium droplets.Under the action of surface tension gradient,gallium droplets will be oriented to the side with a small contact Angle.Through the study of gallium droplet self-actuation kinematics,we believe that the droplet self-actuation can be divided into four stages,among which the precursor film formed in the second stage plays a key role in the movement.Furthermore,the effect of droplet impact on the self-actuation process has been analyzed.We explain the reasons why gallium droplets will experience larger reverse displacement and longer retraction time at moderate impact velocity from the perspectives of energy and motion behavior.(2)To get closer to the real environment of gallium droplet directional transport,this paper studied the directional transport behavior of liquid metal droplet in confined space.Gallium droplets can be self-driven in two plate confinement microchannels by the continuous wetting gradient.The self-actuation in the channels is the result of the competition between the driving force generated by the Laplace pressure difference and the energy dissipation generated by viscous resistance.In addition,different response behaviors of droplet formation can be controlled by contractive cross-section microchannel.We specify two parameters(height difference △H,wetting gradient △ε)that significantly affect the droplet movement process.The channel parameters can be used to successfully regulate the droplet kinematic behavior.(3)The organic liquid film as the second interface could realize lossless long-distance transport.Firstly,the wetting behavior of gallium droplets on organic liquid film was studied by molecular dynamics simulation and experiment.The organic liquid film can significantly change the wetting state of gallium droplets.Subsequently,thermodynamic stability models were applied to different organic liquid films by spreading parameters to predict a priori whether any combination of solid roughness and organic liquid is suitable for designing lubricant injection surfaces(LIS)for gallium droplets.We believe that a lubricant injection surfaces with a small polar component of organic liquid and a high roughness are more suitable for liquid metal.More interestingly,the cloaking phenomenon was found to delay the formation of surface oxides,which is beneficial for manipulating liquid metal droplets.In summary,this paper studies the dynamic process of directional transport of liquid metal from the perspective of wettability.Then puts forward some measures to accurately control the surface behavior of liquid metal droplet.The conclusions provide a theoretical basis and guidance for the control and application of liquid metal droplet in the microfluidic,flexible robot,fluid sensor and other fields.