Study Of Hydrodynamic Stability Mechanisms In The Electrohydrodynamic Ejection

Electrohydrodynamic printing(EHDP)is a newly patternable technology of printed electronics with numerous advantages,including high resolution,wide viscosity compatibility,flexible substrate,etc.It is widely applied in the fields of display,sensor,biomedicine,national defense,and military industry.However,there are lack of comprehensive and in-depth investigations on hydrodynamic mechanisms and working diagrams in the EHDP process,although it is one of the high-precision additive manufacturing technologies.All of these limit the manufacturing capacity of EHDP in printed electronic devices.Given the above problems,it is necessary to understand hydrodynamic behaviors under various scales of EHDP and to construct optimized diagrams associating process parameters with jetting modes.Hence,the dissertation develops mathematical models and numerical methods to accurately simulate the hydrodynamic characteristics and conduct theoretical and experimental analyses to verify these simulations.Numerical simulations,theoretical analyses,and experimental observations are carried out as follows:(1)To reveal the ejection mechanisms,the EHDP process is divided into three regions in different spatial scales:Taylor cone,jet breakup,and droplet deformation.Based on the EHDP observation platform,level set(LS)and many-body dissipative particle dynamics(MDPD)simulations,the mechanisms of cone-jet formation,tip breakup of Taylor cone and ion issuing are revealed.LS simulations show that the interfacial shear stress is the main cause of cone jet,and MDPD simulations demonstrate that the electric field force,Coulombic force and surface tension leads to the pulsating breakups of cone jet.(2)Based on arbitrary Lagrangian-Eulerian(ALE)simulation,linear stability and selfsimilarity theory,and the observation apparatus of electrified jets,the characteristics of charged jets at initial and breaking times are analyzed systematically.There are three nonlinear breakup modes in leaky-dielectric jets,viz.,end,ligament,and transition breakups.It is concluded that the dimensionless number a and εr related to electrical conductivity and permittivity are the key factors affecting the breakup mode.The scaling laws of 2/3 and 1 remain unchanged when CaE<<α2/Oh1/2 and CaE<<1/Oh1/2.(3)The droplet breakup and ion models at the mesoscopic scale are built using the MDPD method,which reflects the effects of thermal fluctuation and the ion migration.Breakups of falling nanodroplets and deformation of charged droplets are studied.The effects of fluid properties and initial velocity on droplet breakup are discussed,and the instability mechanism of nanodroplets is revealed.Besides,electric field and ion charge influences on the deformation of charged droplets are investigated.It is found that the droplet is deformed by the convection of the surface charges in four modes,namely,tip streaming,lobe formation,finger stretching,and dumbbell stretching.(4)The dissertation analyzes the effects of five dimensionless numbers(Oh,We,CaE,α and εr)related to operating parameters,physical parameters,and geometric parameters on the jet mode,and provides the property and operating diagrams for EHDP.It is found that the cone-jet mode occurs at 0.05 ≤ Oh ≤ 0.3,α>5,εr≥ 5,10-3 ≤ We≤5 ×10-2 and 0.5≤CaE≤2.7.The above study not only develops the numerical model at various scales in the EHDP process,but also provides the working diagrams for the cone-jet mode,which can lay the foundation of theory and method for mass and efficient EHDP manufacturing.