The Study On Prediction And Control Method Of Pneumatic Micro-droplet Ejection State

Micro-droplet ejection is a technology for depositing tiny amount of liquid sample.It has wide application prospects in many fields including microelectronics and biomedicine.This dissertation presents studies of pneumatic micro-droplet generation.Specifically,the pneumatic ejection is actuated by setting a solenoid valve to"conduction"state for a briefly period of time?t.The high pressure gas of P₀ enters the liquid reservoir through the solenoid valve and then releases through a venting pipe.The process produces an oscillating pressure pulse P(t)in the reservoir,which forces the liquid out through the tiny nozzle to form a micro-droplet.Every time the solenoid valve is actuated,the P(t)is obtained by a high-speed pressure sensor,and the ejection state is studied by high-speed photography and image processing.Some considerations for the prediction of the ejection state are discussed.Based on the method of electro-acoustic analogy,a calculation model is presented,where P(t)can be calculated from?t,P₀,as well as the geometric sizes of the venting pipe and the reservoir.It is suggested that the acoustic resistance of venting pipe is mainly caused by viscous effect.Compared with setting a constant acoustic resistance empirically in our previous publication,a formula of acoustic resistance is given here.We speculate that the acoustic resistance changes with time during the whole ejection process.Based on this assumption,the calculated P(t)is more consistent with the actual measurement.Limitations of the model are also presented.In practice,the inconsistency of the actual conduction time of the solenoid valve leads to significant fluctuation of P(t),which deteriorate of the consistency of the ejection.In addition,to optimizing the venting tube length L,the ejection process for a set of L are studied experimentally.With both P₀ and?t set,the peak value P_MAX1 and duration of the first positive pressure period increase with L,and more droplets are ejected from single actuation.The more reasonable way is to set different P₀ for different L,so that their P_MAX1 are adjusted to the same and appropriate value,so as to ensure that a single micro-droplet is ejected during the first positive pressure period.However,with the increase of L,the peak value of the second positive pressure period P_MAX2 increases.Therefore,there is possibility that a second droplet is ejected.It is shown that the increase of L can reduce gas consumption,but may cause the multiple ejection.This is a practical issue that should be considered in the design of pneumatic ejection system.The pneumatic micro-droplet ejection technology has been widely used for fluids of high viscosity and temperature.Recently,the droplet ejection of particulate suspension attracts a lot of attentions.Here,the micro-droplet ejection of suspension with different solid volume fraction(?)is studied by using our lab-build pneumatic ejection system,where the optimized venting pipe length is taken.By setting appropriate P₀ and?t,single droplet can be ejected for suspensions with?up to 33%.For?less than about 18%,the required pressure amplitude increases roughly linearly with?.With?increased above 18%,the demand for pressure amplitude is significantly higher than the linearly increasing trend.Especially as?is increased above 24%,the liquid at the nozzle tends to stretch to form much longer liquid band,while the break-up is delayed drastically.Some experimental results can be attributed to increase of effective viscosity of suspension versus?.In practical applications,the precipitation of solid particles in suspension is a common phenomenon.The precipitation results in the increase of effective viscosity of the suspension and clogging of the nozzle.These effects cause the inconsistency of the droplet ejection states.Experimentally,a monitoring method based on machine vision and image processing and an automatic control method for P₀ by electric proportional valve are realized.It can keep the ejection state more stable and consistent when the solid particles precipitate.