Low pressure, pulsating, series of water droplets

Question

I want to create a stream of water that emits only a droplet of water, waits a few milliseconds, and then continues. The important thing is that I need to create a visible gap between drops.

Considering the desire to have droplets created with a consistent size and shape (asked in this question), how would one go about creating a stream analogous to a morose-code-stream of droplets?

[Edit]

I'm considering having a solenoid valve connected to a tube pointed downward, where capillary action holds the water in place. The solenoid releases the amount equal to one droplet of water. What I'm having trouble with is making the drops look semi-uniform as they fall.

Here is an artistic rendering of what I'm trying to accomplish:

Answer 1

What I would do is similar to the answer of @Igor Ivanov, but a bit more specific and tuned.

In microfluidics the problem of creating uniformly sized droplets (for all sorts of biological and chemical process) is an active field of study. The sound to make droplets break according to a dominant wavelength is indeed one option (see e.g. the technique used in here).

What I think you would like is a branching, tree-like structure that guarantees that your outlet droplets are all close to the same size. For that to be possible you will probably find the technique in this paper interesting. What they do is, breakup droplets in two equally sized smaller droplets in a sequence which keeps on expanding to give a large amount of uniformly sized droplets.

In your case you would need something like this device before your 'screen' and route the droplet exits into your screen. Note that you can only use this technique for channels with a size of about 2 mm or less, but of course the remaining droplet length can be such that the droplets are bigger.

Answer 2

I guess you can do it with "brute force", like pushing water through a narrow nozzle by applying very short, hard and precise hits on a piston behind the nozzle. But that seems to be a rather technologically challenging device for a "do-it-yourself" lab.

A simpler solution you could try is just to have a steady water flow through a nozzle. A cylindrical column of water is intrinsically unstable due to capillary forces and starts breaking into droplets when set free. This breaking begins by generation of ripples of a certain wavelength (proportional to the radius of the column) which corresponds to the most unstable mode. So, if you let this instability develop by shooting the water jet at some distance, then you'll have at the end a more or less periodic sequence of droplets of roughly the same size.

However this instability needs some time to develop, and a big difficulty here is to suppress other instabilities which unavoidably begin to develop in parallel. You could perhaps do this by applying sound with a well calculated frequency to the tip the nozzle. The sound should selectively excite your preferred oscillation mode so that the desired instability sets off immediately after the water leaves the nozzle, while other instabilities don't have enough time to develop. However there is no guarantee that it will really work :)