Detection Of Electrohydrodynamic Ejection Based On Scattered Light Methods

Electrohydrodynamic(EHD)micro-droplet ejection technology is different from the traditional inkjet printing technology in the way of "pushing".It uses electric force to produce micro-droplets in the way of "pulling",which not only improves the printing resolution,but also greatly reduces the risk of nozzle blockage.In recent years,this technology has attracted extensive attention in the fields of biomedicine,flexible circuit printing,functional material preparation and so on.However,as a continuous ejection process,the stability and consistency of EHD ejection are affected by many factors,including voltage and liquid flow rate.Detecting EHD ejection state is necessary for precision printing applications.High speed camera and image processing are the conventional methods to detect the ejection state.However,the high-speed camera equipment is expensive and the image processing process is time-consuming,so it is not practical for industrial applications.In this Master thesis,elecytric current measurement and scattered light measurement are used to moniter the EHD ejection state.An experimental setup for EHD micro-droplet ejection was built,including microdroplet ejection system and a monitering system for ejection state.The liquid flow is supplied by a syringe pump.The monitering system is composed of current measurement module and scattered light collection and detection module.Specifically,when the liquid meniscus at the nozzle vibrates axially,a current pulse will be generated;when a micro-droplet passes through the irradiated laser beam under the nozzle,a pulse signal will be output in the scattered light collection and detection module.Two sets of detection modules are used to moniter the micro-droplet ejection process in combination.For a droplet with radius r,the scattering light intensity I versus scattering angle θis obtained with the aid of a Mie scattering calculator;The scattered light power P collected by the detection module is calculated quantitatively;According to the sensitivity of the scattered light detector,the corresponding relationship between r and amplitude V of the output pulse signal is established;With the help of r-V relation,the droplet volume is calculated by analyzing the scattered light pulse signal within an unit time,that is,the ejection flow rate is obtained;The ejection flow rate is compared with the set flow rate of the liquid supply,and the scattered light detection module is calibrated by adjusting the irradiation laser intensity.For the cases the liquid supply of syringe pump is pulsate,the ejection state of micro-dripping mode was studied by taking electric current and scattered light measurement.It is found that the amplitudes of the scattered light pulses are modulated,and the modulation frequency is the same as the stepping frequency of the syringe pump.Although the amplitudes of the current pulses are quite stable,the time interval between adjacent pulses changes periodically,and in a frequency same as the stepping frequency of the syringe pump.According to the above experimental phenomena,a criterion for characterizing the ejection stability is also proposed.