Liquid soap pulsing as it's drawn into a syringe

[puc]

TL;DR

I asked NIK282K who watermarked the following one minute video if I could post it here to see if anyone might be able to say exactly what is happening and explain it. Yes was his answer.

So here it is. I took a guess but I couldn't really answer, hopefully you can.

Slow motion video of pulsations occurring while sucking liquid soap into a syringe.

 

[puc]

I'm not even close to a physicist, so if this needs to be and a different forum to get the best response feel free to clue me or move it where it needs to be. Thanks.

 

[A.T.]

Summary:: I asked NIK282K who watermarked the following one minute video if I could post it here to see if anyone might be able to say exactly what is happening and explain it. Yes was his answer.

So here it is. I took a guess but I couldn't really answer, hopefully you can.

Slow motion video of pulsations occurring while sucking liquid soap into a syringe.

Things like this happen if you have a combination of resistance and elasticity. Soap has long molecules and can build up some tension when stretched, so when the connections on one side are torn it is pulled back by those on the other side.

 

[Sabertooth]

There is a vortex forming just behind where the soap is drawn into the syringe, along the metal.
When the liquid in front of the "syringe-hole" is pulled into the syringe it causes the soap behind the syringe hole to be pulled forwards.

Seems the phenomena would also be present in other liquids, but a lot harder to observer with a lack of bubbles.

 

[sophiecentaur]

Things like this happen if you have a combination of resistance and elasticity. Soap has long molecules and can build up some tension when stretched, so when the connections on one side are torn it is pulled back by those on the other side.

There seem to be a lot of bubbles in the solution. These would give the bulk a lower modulus than a bubble free solution. It seems very much like the effect of vibrations in in the tube of a wind instrument only with, of course, different values for modulus and density (plus viscosity).

If the bubbles are in there to improve the visibility of the liquid flow then perhaps they could be replaced with small solid particles of neutral density. Also the number and size of bubbles could be varied.

Alternatively, it could be something to do with the driving mechanism not being decoupled properly or a springy tube.

 

[256bits]

Alternatively, it could be something to do with the driving mechanism not being decoupled properly or a springy tube.

The tube certainly moves around a lot, which makes the fluid do the same.

 

[sophiecentaur]

So we could be looking at an oscillator involving the mass of the syringe and the modulus of what is holding it OR the behaviour of a fairly compressible viscous liquid. Seems we need quite a bit more information about the setup before we could suggest anything useful.

 

[puc]

Several hours ago I wrote the guy and included what has been offered here and suggested that more information would be needed. When I get a response I'll post again.

Thanks for all the replies. There will be updates.

 

[puc]

From NIK282K

"Interesting replies, I'll have to do some more work and probably make a better setup.

I had thought of the long-ness of the soap molecules, this soap in particular can make threads ~0.5-1mm wide and up to 5mm long that slide around on a smooth soap surface.
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Hadn't thought about vortexes!
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the bubbles are there to help visibility BUT I did try again with completely clear soap and saw a similar effect in the index of refraction (the same tugging and jittering). I'll have to try again with some mica powder.

The syringe is fully retracted and held in place but I could probably do a better job at that.
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I do have shaky hands but the frequency of the soap jitters is a lot faster.
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Just off the top of my head. Lots to think about."

 

[sophiecentaur]

Can you drive the piston with something stronger than by hand? If you use water with a much smaller hole, can you produce oscillations? I'm wondering if the syringe is the cause of the oscillations. Try a metal (more rigid) syringe?
So many things to eliminate - something to really get your teeth into.

Another idea. If you have an oscillation / resonance there will be a modulus and mass / density. Helmholtz Resonance? Measure the (change in) volume of the syringe under various loads to find the modulus.

 

[A.T.]

I'm wondering if the syringe is the cause of the oscillations.

I have definitely seen this: The before mentioned combination of resistance (friction at the syringe piston seal) and elasticity (plastic/rubber deformation) makes pulling a syringe jittery.

 

[sophiecentaur]

(plastic/rubber deformation) makes pulling a syringe jittery.

It wouldn't happen with a Gas Syringe, I'm sure. A gas syringe has strong glass sides and a cylindrical glass piston with a deep ground glass seal.

 

[puc]

I've relayed your responses

 

[puc]

And his reply.

"The piston is being drawn up its full length until there is a void between it and the soap entering at the bottom. At that point the force is dictated by the ambient air pressure forcing the soap into the tip. Until the volume is nearly full I'm pretty sure that the force is constant. Its one of the reasons they use vacuum cylinders on workout equipment in space (lighter than springs and its a constant force, unlike springs)."

 

[puc]

Oops. Apparently he didn't read your relayed responses before writing the above.

 

[puc]

More video with no bubbles in both high speed and real time. The jittery motion is still visible as changed in the index of refraction of the soap. Also a wide view of the dish and syringe and a macro.

 

[sophiecentaur]

I'm afraid there is a vital missing piece of evidence and that is the mechanical setup to drive the piston and to hold the syringe. It looks as though it's being held by someone because the needle jiggles about. That has to be sorted out. The syringe has to be clamped firmly and the force on the piston must be uniform and well defined. Without that, we have no idea where the oscillations are coming from.

 

[puc]

I passed your response on to the guy