The Study Of The Enhance Electrowetting-on-dielectric Responses By Space Charge Polarization Based On RGO@SiO₂ And La₂O₃@TiO₂ Embedded Composite Structures

In this paper,the basic concepts and theoretical models of surface wetting and Electrowetting-on-dielectric（EWOD）properties of solid materials are briefly introduced,and the development history,research status,main challenges and problems to be improved of EWOD are summarized.EWOD has the advantages of simple operation,fast response,intelligence and controllability.However,there are still several obstacles to improve the performance of EWOD devices,such as breakdown,contact angle saturation,contact angle hysteresis,wettability reversibility,ohmic warming,electrolyte electrolysis,large threshold voltage and so on.By analyzing the Young-Lippmann equation,the EWOD property of the solid material surface is affected by the dielectric constant of the dielectric layer.On the one hand,the dielectric wetting property is improved by selecting a material with a large dielectric constant or a multi-component compound.For example,dielectric materials such as silicon dioxide（SiO₂,_d??.39）,aluminum oxide（Al₂O₃,?_d?5.9）,tantalum pentoxide（Ta₂O₅,?_d?5.9）,and barium titanate（BST,[（Ba,Sr）TiO₃],?_d?180）have been used.Increasing the capacitance of the dielectric layer enhances its EWOD properties.On the other hand,the moving ions are doped,and prevented by lattice distortion,and grain boundaries,causing charges to accumulate at the hetero interfaces,forming space charge polarization,changing the potential distribution of the external electric field in the dielectric layer,and increasing the dielectric.The dielectric constant of the layer enhances the EWOD properties.Therefore,in this paper,we focus on the topic of"The study of the enhanced Electrowetting-on-dielectric responses by Space Charge Polarization Based on RGO@SiO₂ and La₂O₃@TiO₂Embedded Composite Structures".We have completed the following two aspects of research work,and obtained the relevant research results:（1）The EWOD of high conductivity material RGO embedded in silica（SiO₂）sol with good dielectric properties was studied.RGO@SiO₂ composite structure was prepared by sol-gel spin coating process which is simple,has small energy consumption,and good environmental protection.The thickness of the prepared samples can be controlled by the number of spin coatings,and the surface roughness can be adjusted by doping concentration of RGO.In this paper,space charge polarization effect is proposed to improve the relative dielectric constant of the dielectric layer.When RGO@SiO₂ composite structure is injected into the electric field,space charge polarization is formed by accumulating charge at the heterogeneous RGO/SiO₂ interface.Thus,the relative dielectric constant of RGO@SiO₂ composite film can be improved by doping RGO.The results of EWOD test show that the initial contact angle is about 102°,the EWOD can be adjusted at low pressure of 0-15V,and the range of the contact angle can be up to 79°.（2）The rare earth oxide La₂O₃ nanoparticles were embedded in TiO₂ thin films by sol-gel spin coating method,and La₂O₃@TiO₂ composite structures with large number of nano-cracks were successfully prepared.By adjusting the number of spin coatings and the doping concentration of La³⁺,the thickness of the composite film and the content of La₂O₃ in the composite structure can be controlled,the dielectric constant of the composite structure can be changed,and then the EWOD can be controlled.The characterization of chemical composition and crystal structure shows that La₂O₃ nanoparticles with polycrystalline properties have been embedded tightly and distributed evenly in the TiO₂ film,forming a large number of dense contact heterogeneous La₂O₃/TiO₂ interface,which provides an ideal space for charge accumulation and storage.When applied voltage,charges accumulated on the heterogeneousLa₂O₃/TiO₂ interface,resulting in space charge polarization,which improved the polarizability of La₂O₃@TiO₂ composite film and further improved the electrical properties of the film.In addition,the optimum doping concentration of La³⁺in La₂O₃@TiO₂ composite films is 0.9%,and the number of spin coatings is 8layers.The results of EWOD test show that the adjusting range of droplet contact angle can reach 43°,and very low threshold voltage and high breakdown voltage are obtained.