Surface Charge Accumulation Effect Induced By Solid-liquid Interface Separation And Its Research On Droplet Transport

The directed transport of droplet is crucial for many applications from microfluidic devices to condensation and heat transfer.Although droplet transport can be realized by the input of external energy,self-propelling without continuous supply of external force has obvious advantages in practical application.Typically,the self-propelled and directed droplet transport is achieved by topographic or chemical modulation of surface wetting gradients that break the asymmetric contact line and overcome the resistance force to move droplets along a particular direction.Nonetheless,despite extensive progress,directional droplet transport is limited to small transport velocity or short transport distance.Although the droplets in the Leidenfrost state can move rapidly with the help of temperature,the extra high temperature brings some limitations to the practical application.How to realize the spontaneous,rapid and directional transport of droplets in an ambient environment,and even to overcome the gravity to realize the movement from bottom to top,is still a pending challenge.A new strategy based on surface charge is developed in this dissertation,which solves the problems of slow velocity and short distance of droplet transport.The directed,long-range and self-propelled transport of droplets at room temperature is realized by the surface charge density gradient.At the same time,droplet transport based on surface charge density gradient shows the wide applications.The main research results and conclusions are listed as follows:(1)This dissertation reveals the surface charge retention effect after water drop impact on the superamphiphobic surface.It is found that the generated surface charge is erasable,cumulative and affected by humidity.Erasability is the basis of printing different surface charge paths on the same superamphiphobic surface and improving the utilizing efficiency of the superamphiphobic surface.By heating the surface in advance or using the dielectric material,the stability of surface charge in high humidity environment can be greatly improved.This provides a guarantee for the reliability of the surface charge density gradient method.Besides,the mechanism of surface charge generation is explored.Firstly,the solid-liquid interface is characterized by the sum frequency spectrum.It is found that there is a strong interaction at the solid-liquid interface,which is the basis of the interface charge separation.The mechanism of surface charge generation is then discussed based on the existing literature and relevant experimental results,which helps people to understand the solid-liquid contact electrification.(2)This dissertation also studies the effect of the substrate on the surface charge,which is often ignored.We find that the charge on the superamphiphobic surface is easily affected by the properties of the substrate material,and the substrate can even completely determine the expression of surface charge.The short-range effect of substrate and the long-range effect on surface charge expression are discussed in this dissertation.It is found that the surface charge is mainly affected by the dielectric constant,conductivity and thickness of the substrate based on the theoretical analysis.Thus,reversible control of the net charge on surface is realized by only changing the substrate.We also find that the surface charge can greatly increase the adhesion of the superamphiphobic surface,and the net charge determines the surface adhesion at the same time.We realize the reversible in-situ adjustment of the adhesion on superamphiphobic surface by controlling the substrate.Based on this principle,we developed a tip-free liquid transfer pipette for no mass loss transfer of the low surface energy and high viscosity liquid.(3)Based on the full understanding of the surface charge generation and its adjustment law,we can tune the surface charge density by controlling the hydrodynamics of water drop.This process is analyzed theoretically.The surface charge density gradient for droplet transport is successfully developed,which solves the problem of droplet transport efficiency.The mechanism of droplet transport by surface charge density gradient is analyzed.The surface charge density gradient can drive droplets 10 times faster than the Leidenfrost effect.The theoretical limit of droplet transport distance is broken by designing the alternative transport units with surface charge density gradient and without surface charge.The new surface charge density gradient method developed in this dissertation has good universality.It can not only transport all kinds of low surface energy liquids but also be printed into any path on various superhydrophobic surface.It can even overcome the gravity to drive droplets vertically from bottom to up.The excellent performance of this method makes it possible for a wide range of applications.This transport strategy finally demonstrates the droplet car which can realize cargo handling and an open liquid transport platform which has obvious advantages over the traditional microfluidic.