Monitoring Methods For Ejection States And Flow Rate In Electrohydrodynamic Printing

In recent years,droplet ejection technology has received more and more attention as a non-contact application technology.Traditional droplet ejection technologies include thermal bubble,piezoelectric,electrostatic,pneumatic and so on.However,the diameter of the droplets produced by these traditional techniques is usually larger than the diameter of the nozzle;if you want to get smaller droplets,reducing the nozzle diameter is the most direct way,but it is easy to block the nozzle.Therefore,this dissertation focuses on a new type of droplet ejection technology-electrohydrodynamics(EHD)droplet ejection method.Different from the traditional technology,this technology mainly "stretches" the droplets through electric field force.Therefore,this technology may eject droplets much smaller than the inner diameter of the nozzle,improve printing resolution,and alleviate the risk of nozzle clogging.In this dissertation,a set of EHD droplet ejection device is designed and built.The ejection device mainly includes five parts: liquid supply module,high voltage power supply,droplet receiving module,machine vision detection module,and induced current detection module.Among them,this dissertation focuses on two technical options of the liquid supply module,namely the liquid supply method based on the Utube liquid level difference and the liquid supply method based on the syringe pump.The uniformity of the liquid supply of these two liquid supply systems is analyzed by the machine vision detection module.And studied the EHD droplet ejection state under two liquid supply methods.The experiment found that once the liquid supply flow rate of the syringe pump is set,it has nothing to do with the electric field.In contrast,although the U-tube-based liquid supply method has a simple structure,the flow rate is obviously affected by voltage(electric field)factors.Specifically,when the liquid level difference of the Ushaped tube is large enough,the pressure difference of the hydrostatic pressure is enough to overcome the capillary effect at the nozzle,and the liquid can flow out continuously.When the height difference is small enough,the capillary effect exceeds the hydrostatic pressure,which will prevent the liquid from flowing out.At this time,applying a strong enough electric field can still "pull" the liquid out of the nozzle.The flow rate increases with the increase of the electric field intensity.Moreover,the flow rate of the fluid is not stable during a droplet ejection cycle.In our EHD droplet ejection experiment,droplet ejection has three common states:stable low-frequency ejection(state A),stable high-frequency ejection(state B),and the transition between the two(state C).State A should be the so-called dripping mode(or enhanced dripping mode in some literature).State B is closer to the so-called microdripping mode.This dissertation introduces a detection method that combines machine vision detection and induced current detection,which can reliably distinguish the above-mentioned droplet ejection states.Different from the usual methods,we do not deliberately track the breakage of the liquid band in the experiment,but focus on measuring the change in the liquid volume at the nozzle.This method can also accurately measure the droplet size,droplet ejection frequency and other information,and use it as an index to judge the droplet ejection stability.