Vision based Studies of Electrohydrodynamic Ejection Process

Sample deposition based on micro-droplet ejection has broad application prospects in the field of biomedicine. Ejection of RPMI-1640 culture medium (without cells) is investigated experimentally using a home-build electrohydrodynamic (EHD) ejection system, consisting of a liquid supplier and a nozzle, a voltage source, a droplet collector, and a supplemented photoelectric monitoring module. Electric voltage is applied between the nozzle and the droplet collector. The liquid surface is electrically charged and the ejection takes place when electric force overcomes the surface tension. At appropriate voltages, uniform ejection states are established with ejection frequency ranging from a few to a few tens of Hertz. Monitoring of the ejection process is performed by two methods. The ejection process can be recorded by a high speed camera, and then analysed by image processing. Using output signal of a charge amplifying circuit as a reference, a single shot camera is controlled to capture images at certain delays, and the entire process of the droplet ejection can be recorded. Experiments have shown that when the liquid meniscus breaks up, the output signal of the charge amplifier reaches the maximum.