Mechanism And Experimental Study Of A Piezoelectric Print Head For Microdroplet Jetting

Droplet generators have been applied to many occasions such as the printing of biological droplet arrays,the printing of flexible electronic circuits,the production of biological tissues and organs,the fabrication microelectromechanical devices,the preparation of drugs,the rapid prototyping,and so on.The droplet dispensing system based on the tubular piezoelectric print-head is widely used because of its high precision,fast speed and easy operation.Therefore,the study on the operation mechanism and the droplet dispensing characteristics for a tubular piezoelectric print-head is very important.Also,the study on the related mathematical model of the break of the liquid cylinder and the research on the optimization of the excitation waveform for a tubular piezoelectric print-head are of great value.These studies are thought to have great significances on the design of a piezoelectric droplet dispensing system or on the guide of the droplet generation tests.Based on the inverse piezoelectric theory and the deformation theory of the thick walled circular tube,along with the displacement and stress conditions between the piezoelectric tube and the glass tube,the displacement field,the stress field and the potential distribution for the tubular piezoelectric print-head are obtained.The generated fluid pressure for the internal liquid is obtained either.Furthermore,it is further discussed the effects of the wall thickness and the liquid bulk elastic modulus on the overall deformation field and the driving process of the tubular piezoelectric print-head.These results are beneficial to the understanding of the deformation process of the print-head and are helpful in the designing of a new piezoelectric print-head.Based on the acoustic propagation theory of the viscous liquid in a tubule,it is found that the liquid viscosity has a retardant effect for the acoustic propagation speed and a damping effect for the harmonic amplitude.A mathematical model is established to study the acoustic propagation process and the acoustic reflection process.Then,the pressure response at the print-head nozzle is obtained.A droplet ejection experiment and a meniscus vibration experiment are designed to test the excitation waveforms with different dwell times to validate the established model.The states of the ejected droplets and the vibrating meniscus can be imaged and obtained by a droplet imaging system.It is found that the pressure response at the nozzle is the combined action of the acoustic pressures produced during the rise time and the fall time and after their propagations and reflections.A dwell time which is too small or too large is not able to generate droplets unless the voltage is elevated to an abnormal high level.From the aspect of the ejection kinetic energy,the optimal dwell time is between l/c(the glass length divided by the static sound speed)and 2l/c and is related with the liquid viscosity and excitation frequency.The Young-Laplace law is used to build the relationship between the local curvature of the axisymmetric smooth surface and the local normal stress during the evolution process of liquid cylinder.Then,some important boundary conditions are obtained.Combined with the continuity equation and the momentum equation,the mathematical model for the breakup of the liquid cylinder is established.The spatial derivatives of the partial differential equations are made to be difference by staggered mesh and finite difference method.Then,the profile and the velocity distribution of the liquid cylinder are obtained by the solver ODE15 S in Matlab.For solutions such as pure water and ink,the liquid cylinder breakup processes are modeled and simulated.The effect of the liquid viscosity and surface tension on the liquid cylinder breakup process is also discussed.For the droplet breakup process of the tubular piezoelectric print-head,the three-dimensional calculation model is established according to its structural parameters.The suitable boundary conditions are applied according to its driving characteristics.The calculation principles of the free surface flow problem in fluid simulation software are introduced.Taking ethylene glycol aqueous solution as an example,the droplet dispensing processes of the tubular piezoelectric print-head are simulated.The droplet breakup pictures are obtained by a droplet imaging system,which validate the correctness of the calculation model and the numerical algorithm.The droplet dispensing process under different input displacements or for the liquids with different viscosities and different surface tensions are simulated.The computing results show that the liquid dispensing capacity for the print-head is closely related with the input displacement.It is very important to seek an appropriate incentive condition for a liquid.The excitation that is too small cannot generate any droplets and the excitation that is too large will produce droplet with one or several satellites.Increasing the viscosity will retard the droplet formation process while increasing the surface tension will fasten the droplet formation process.A real-time droplet monitoring system is designed to image the droplets in real time.The droplet position curve,length curve and speed curve can be plotted in real time either.The droplet breakup status,the generation process and the flying speed can be monitored or judged by these characteristic curves.Therefore,the impending recombination between the main droplet and the satellite droplet can be anticipated based on these characteristic curves.The amplitude and the shape of the e xcitation waveform are optimized by the droplet monitoring system.The optimization principle is that when the droplet strikes the substrate,there are no redundant satellite droplets,and at the same time the dispensing efficiency is high.The two prerequisites for the high quality droplet dispensing are proposed: an appropriate excitation waveform and suitable liquid properties.To adjust the droplet size,a fuzzy controller is designed.Experiments validate that it is able to adjust the droplet size in a certain range.