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In a piezoelectric drop generator used for generating a single droplet which is 0.5 mm in diameter, there is a piezo-actuated brass diaphragm to generate droplets (for example here or here). What is the main factor that pushes the water inside the generator in order to cause a droplet to be ejected from nozzle? (whether the main factor is diaphragm bending OR it is acoustic wave that piezo element may produce?)

Note: I use a piezo element which has a brass plate with 27 mm diameter and 0.3 mm thickness. However, when I installed in its place (Fig. 1) and fill the beneath reservoir with water (without any air bubble), I am not sure whether the brass diaphragm should bend rapidly downward to push a droplet out of the nozzle OR it should produce acoustic waves that push the water to eject a droplet. Which one is actually supposed to happen?)

Thanks

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Figure 1. Schematic of a piezoelectric drop generator

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Interesting question - I have to speculate about the answer.

First off, to generate an acoustic wave, you need to bend the diaphragm... so in that sense the question makes no sense. But I think you are really asking whether the situation should be thought of as static or dynamic.

I am pretty sure that the advantage of the piezo transducer is that it can produce a lot of force for a short time, so that you do get a pressure wave (in other words - the pulse is short compared to the transit time of the acoustic pulse / shock wave across the cavity from the transducer to the nozzle). To do that effectively, you would want the inner surface of the cavity to be a smoothly varying shape - like an old fashioned gramophone. The sharp edge is more likely to cause significant reflections, and suggests that either no thought went into the design, or that the intended use is with a slowly rising pressure (compared to the acoustic transit time).

I don't know how the drive electronics of this device is configured - but you should be able to measure the voltage. If the rise time of the pulse is short (microseconds), you have a "mostly acoustic" setup. If it's slow (milliseconds), then the setup is "static".

The speed of sound in water is about 1500 m/s, or 1.5 mm / us. I am eyeballing the transducer and thinking it's about 5 cm in size - or about 30 us. That's why I came up with the above rule of thumb.

But as I said - this is me speculating. I hope someone else has a better answer, and actual experience with this device.

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  • $\begingroup$ Dear Floris, 1- In the beginning of your answer,what do you mean by bending in "bending the diaphragm to generate acoustic waves?" 2- Based on your main paragraph, how are the pressure waves produced by piezoelectric diaphragm? (through bending or acoustic waves) 3- The first paper that I linked to in my question mentioned that each pulse that they sent to piezo had a 10 $\mu$s rise time and a 10 $\mu$s fall time and around 1500 $\mu$s pulse width. What do you think with this information regarding which one is the cause:"plate bending" or "acoustic waves"? Thanks for all your helpful comments $\endgroup$ – vorujak Nov 5 '14 at 1:12
  • $\begingroup$ What I mean is - a sound wave is generated because "something" moves. In this case, the piezoelectric transducer's surface - what I call the diaphragm. If the pulse duration is 1500 $\mu s$ then this is a "plate bending" situation - the sound / pressure wave had plenty of time to bounce around and come to equilibrium. That explains why they get away with the strange shape of the interior cavity... $\endgroup$ – Floris Nov 5 '14 at 1:28

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