Electrohydrodynamic Micro-Droplet Ejection And Its Visual Inspection

Micro-droplet ejection technology has attracted more and more attention as a small amount of liquid sample application technology,and its application fields have expanded from the traditional inkjet printing to the field of science and technology,such as micro/nanostructure fabrication,microelectronic packaging and genetic engineering,3D printing,combinatorial chemistry,bioengineering,biomedicine,etc.Conventional droplet ejection techniques include thermal bubble,piezoelectric,electrostatic,pneumatic,etc.However,the diameter of the droplets produced by these conventional techniques is usually larger than the diameter of the nozzle.If it is desired to obtain smaller droplets,it is the most direct way to reduce the diameter of the nozzle,but reducing the diameter of the nozzle makes it difficult to eject the liquid,and the nozzle is easily clogged.In recent years,a new type of micro-droplet ejection technology.-Electrohydrodynamic(EHD)droplet ejection technology has attracted the attention of researchers.Unlike conventional techniques,this technique primarily involves the application of high pressure between the nozzle and the collecting electrode,which uses the electric field force to "stretch" the liquid to break,forming droplets.This technology is capable of ejecting droplets that are much smaller than the diameter of the nozzle,enabling high resolution printing and less clogging of the nozzle.Therefore,this paper will focus on this technology for in-depth research.We designed and built an EHD droplet ejection system.And then,we proposed two kinds of machine vision-based detection schemes to detect droplet ejection state : one is based on high-speed camera to real-time monitor the micro-droplet ejection state,but the method has high experimental cost;The other is based on induced current(inductive charge versus time rate)of collecting electrode to trigger a low-speed camera photographing.This method not only reduces the cost of the experiment,but also effectively determines the stability of the droplet ejection.Experiments based on the above two machine vision detection methods show that there are three common states in the micro-droplet ejection process: low-frequency stable state,transient state,high-frequency steady state.The experiment also found that the critical moment of the meniscus fracture at the nozzle corresponds to the maximum value of the induced current.Based on this result,we proposed a method to roughly estimate the initial velocity of droplets and the droplet size.We used them as indicators to judge the stability of droplet ejection.In this paper,the self-built micro-droplet ejection system is applied to cell printing and isotope drug application.In cell printing experiments,the cell survival rate is as high as 96%.EHD droplet ejection technology can also accurately apply radioactive materials to prepare radioactive "seed sources" for radiotherapy.Experiments show that EHD droplet ejection technology is a promising micro-sample application technology in biomedical field.