Study On Tribological And Adhesive Behaviors Of Micro-droplets On Super-hydrophobic Surface Under Electric Field

The microfluidic control system can realize operations such as transportation,separation,and merging of micro-droplets on a microscopic scale,which can be applied to the fields of lab-on-a-chip,biotechnology,and liquid lens.The main contact interface of the microfluidic control system is the solid-liquid interface,so the friction and adhesion between the solidliquid interface play a vital role in the realization of the function of the microfluidic control system.Previous studies have shown that the friction and adhesion behavior of the solid-liquid interface are closely related to the morphology and wetting properties of the solid surface.Therefore,various types of microstructures can be constructed on the solid surface to adjust the solid-liquid interface behavior.However,the real-time adjustment of the solid-liquid interface behavior cannot be achieved through the microstructure design of the solid surface.Studies have shown that external stimuli such as temperature and infrared light can induce the transition of the wet/wetting state of the solid surface,but the wetting response time of the solid-liquid interface induced by such external stimuli is very slow,usually in a few hours or several days.It is suitable for real-time control of solid-liquid interface behavior.Electrowetting on dielectric is a physical phenomenon in which the solid-liquid interfacial tension is changed by applying a voltage,so that the apparent contact angle of the microdroplets and the three-phase contact line are changed.In the electrowetting system on the dielectric,the apparent contact angle of the micro-droplets and the length of the three-phase contact line will change in real time with the applied voltage,and the response time is fast.Therefore,the electrowetting technology on the dielectric has potential applications in realtime control of the friction and adhesion behavior of the solid-liquid interface.At present,the research on the electrowetting behavior on the dielectric mainly focuses on the optimization of the electrical properties of the dielectric material,while the research on the friction and adhesion behavior of the solid-liquid interface under the electric field is relatively small,especially the real-time control of the solid by the electrowetting on dielectric.The research on the friction and adhesion of the liquid interface has not been reported yet.The goal of this project is based on the real-time control of the solid-liquid interface friction and adhesion force of electrowetting on dielectric,and the friction and adhesion behavior of micro-droplets on superhydrophobic surfaces under an electric field have been systematically studied through the instrument independently developed by the research group.First,analyze the electrowetting behavior of micro-droplets on the superhydrophobic surface.According to the change trend of the three-phase contact line and the apparent contact angle,the change interval of the micro-droplet morphology is divided into static zone,transition zone,and saturated zone;To test and analyze the solid-liquid interface friction and adhesion in different areas.The results show that friction and adhesion are closely related to the electrowetting area on the dielectric.In the static region,the electrowetting effect has little influence on the wetting state of the solid-liquid interface,and the solid-liquid friction and adhesion force do not change with the increase of voltage;In the transition zone,the solidliquid interface changes from the Cassie state to the Wenzel state,the friction force increases from 0.1 μN to 60 μN,and the adhesion force increases from 30 μN to 250 μN.Both friction and adhesion are increased by two orders of magnitude.And the response of adhesion force to voltage is real-time;In the saturation region,the micro-droplets are in the Wenzel state under the action of high voltage,and the friction and adhesion remain relatively stable,and they do not increase further with the increase of voltage.The above results all show that the friction and adhesion behavior of the solid-liquid interface can be controlled in real time by applying an external voltage.In addition,this study also conducted a systematic analysis of the relationship between the friction force,adhesion force,three-phase contact line,apparent contact angle,and radius of curvature of the micro-droplet under the electric field,and further studied the volume of the micro-droplet.And the effect of moving speed on the friction and adhesion behavior of the solid-liquid interface.This subject proposes a method for real-time control of the friction and adhesion of the solid-liquid interface with the help of the electrowetting effect on the dielectric.The research results can not only realize the real-time control of the solid-liquid interface behavior,but also help to further enrich the solid-liquid interface under the electric field.Basic theory of friction and adhesion.