Study Of The Behavior And Mechanism In Droplet Impact Without Air Entrapment Regulated By Electric Fields

The phenomenon of droplet impact on solid surfaces is prevalent in nature and engineering applications.As the droplet approaches the wall,a central air bubble is trapped at the bottom due to the squeezed air film and increased lubrication pressure.In droplet deposition applications such as Inkjet Printing,Electrohydrodynamic printing/spray,and Molten metal 3D printing,the entrapped air bubble can lead to a local vacancy defect,which affects the consistency of the structure and function.To address this issue,the aim of this dissertation is to eliminate air entrapment in droplet impact.In order to achieve this objective,a comprehensive and systematic investigation of the impact of deionized water droplets on smooth solid surfaces under atmospheric conditions is conducted through a combination of numerical simulations,theoretical analysis,and experimental observations.The main research contributions and novel findings of this dissertation are outlined below:(1)A novel approach for regulating external electric fields to prevent air entrapment during droplet impact is proposed.The electrohydrodynamic behavior of millimeter-sized neutral droplets impacting solid surfaces under such regulation is systematically analyzed,including flight deformation,initial contact,spreading,retraction,ejection,and fragmentation.Furthermore,the regulatory function of the external electric field,and a phase diagram that specifies the range of electric field intensities that can prevent air entrapment or droplet fragmentation during impact are elucidated.(2)The elimination mechanism of air entrapment at the bottom of droplets under the action of electric field is clarified.Three contact modes are defined based on the evolution of the air film under electric fields: ring contact,multiple contact,and central contact.A theory of the electric field enhancement at the droplet bottom is established based on the dipole charge mirror mode.The scaling law of the critical electric field intensity required to eliminate air entrapment is obtained by analyzing the competition between the electric stress on the droplet surface and the lubrication pressure of the air film.(3)The scaling law for the critical electric field intensity required to eliminate air entrapment in droplets impacting onto solid surface with arbitrary properties is unified.The size effect and net charge effect during microdroplet impact are explored,respectively.A scaling law of droplet bouncing on the air cushion between the critical velocity and droplet size is established,and the contributions of electrostatic field and net charge to the space electric field strength are decoupled.A unified theoretical of the scaling law for the critical electric field required to achieve wetting mode without air entrapment is presented,which is verified through phase diagrams of microdroplet impact modes.This study provides theoretical support for eliminating air bubble defects during droplet impact,including the size effect and net charge effect of the droplet’s physical properties.