Droplet Formation And Stability Analysis

Inkjet technology serves as an essential tool in various fields including 3D printing,electronic technology and bioengineering due to its high resolution and wide applicability.Although scholars have implemented massive research on its model development,printing material development,and performance control,the internal mechanism of the generation of droplets has not been well explained through theoretical analysis,experimental observation and numerical calculation.In addition,there are no unified evaluation criteria and effective optimization means to achieve a comprehensive improvement of the printing process.Therefore,with the focus on the droplet formation process,this study utilizes a piezoelectric drop-on-demand inkjet system and level set interface tracking method dominated numerical models to observe the droplets' behaviors.The in-depth investigation on the internal mechanism and performance control is of great significance for both theoretical and practical development of the inkjet technology.The key parameters influencing droplet formation and stability are driving parameter,fluid properties,nozzle structure and external thermal field temperature.Among the driving parameters,pulse amplitude and pulse width have leading impacts on droplet velocity,jet length,breakup time and process stability.Fluids with low density,low viscosity and high surface tension are beneficial for improving printing stability.In the design of nozzle structure,the larger nozzle diameter increases the breakup time dramatically,decreases the droplet velocity and jet length,and enhances the process stability.The droplet volume can be effectively reduced by 31% when adopting curvilinear triangle shaped nozzle,and the droplet velocity can be largely promoted up to 240% when adopting Laval shaped nozzle.With the rise of the external thermal field temperature,the droplet velocity is greatly increased and the overall stability is damaged.However,the variation of the droplet volume is hard to predict due to the impacts from both fluid flow and heat transfer.Based on the above fundamental research,the range of droplet stable ejection can be extended to over ten times by adjusting the driving parameters.To investigate the effect of the operating parameters on the droplet stability more completely,a new non-dimensional parameter,Pj,is proposed as a stability criterion.Coupled with the traditional stability criterion Z,a comprehensive stability judgement for stable droplet ejection is realized.Then,by tracking the oscillation process of droplet which is directly related to the printing resolution,it is found that the maximum deformation extent of droplet first increases and then decreases with the increment of driving force.The relationship between the highest point and the turning point located in this curve and the surface tension is a good reference for the accuracy control of the printing process.Finally,the optimal parameter combinations are presented using orthogonal test method to improve single desired aspect of printing performance,and a new parameter Sc combining droplet velocity,droplet volume and process stability is proposed to evaluate the overall delivery efficiency.