Some considerations for designing a pneumatic micro-droplet generator

Micro-droplet ejection is a liquid dispensing technology that has potential applications in many fields. Specifically, pneumatic ejection is actuated by a solenoid valve, which is set to 'conduction' state for a brief period of time Δt. High pressure gas of P ₀ enters the liquid reservoir, then releases through a venting tube, creating a oscillating pressure waveform P(t), forcing the liquid out through a tiny nozzle to form a micro-droplet. For each actuation, P(t) is acquired by a high-speed pressure sensor, and the ejection state is obtained by high-speed photography and image processing methods. Some issues for the design of pneumatic micro-droplet ejector are discussed. For simulation of P(t), it is proposed within an electro-acoustic analogy picture that the acoustic resistance of the venting tube is mainly due to viscous effect and may vary with time during the whole ejection process. Based on this assumption, the calculated P(t) is more consistent with the actual measurement. Experimentally, the droplet ejection process for different length of venting tube is studied. With P ₀ and Δt set, by increasing the venting tube length L, both the peak value P _MAX1 and duration of the first positive pressure period increase, and more droplets are ejected from a single actuation. By setting different P ₀, P _MAX1 for different L is tuned to an identical and appropriate value, so that single droplet is ejected due to the first positive pressure period. However, with the increase of L, the peak value of the second positive pressure period P _MAX2 increases. There is a certain probability that another droplet is ejected. It is realized that the increase of L can reduce gas consumption, but the multiple ejection is a drawback that should be considered in the design of pneumatic ejection system. (paper)