Solving the Problem of Capillary Action in Tiny Peristaltic Pumps

Hello,

I designed a tiny peristaltic pump which works really fine. I am using a very thin tubing of 0.5mm diameter, which makes me struggle with capillary action, unfortunately.

When my pump is not working, sometimes liquid still flows into the tubing because of capillary action. Do you have an idea to prevent it? I am trying to design a "pusher" that "sandwitches" the tubing at the exit of the pump, that pushes enough on the tubing to avoid capillary action but still allows liquid to flow when my pump is working. So far I couldn't find the right pressure to make it possible. I was wondering whether there existed some kind of valve that prevents capillary action?

cheers,
sylph

Welcome to PF.

Did you use the traditional layout of a Peristaltic Pump, where there is always at least one roller pinching off the tubing even if the pump is not spinning?


https://control.com/technical-articles/understanding-peristaltic-pumps/

Welcome @sylph !

As mentioned above, at least one roller should be fully blocking any back-flow by reducing the cross-section area to zero.
Could you explain that capillary action a little further?

If that is the case, and you still experience some of it, no check valve will solve the problem, unless is spring loaded or relative high output pressure; therefore, introducing a higher load on the pump during normal operation.

Thank you for your answers, berkeman and lnewqban :D!

Yup I am using a typical layout for my pump, I have 6 roller bearings and 3 of them constantly pinch the tube shut while at rest.

The liquid doesn't go through the pump "at first", but after some time (probably because my tubing gets wet and creates some air bubbles) capillary action makes the liquid flow, in some pumps only, it's quite random...

My pumps are tiny, so reducing their diameter again (to push very hard on the tubing) might prevent the pump from working properly... and also damage the tubing rather quickly.
I also read that too much pressure on the tubing might actually make the capillary action worse, by increasing tension. Any idea?

cheers!

sylph said:

The liquid doesn't go through the pump "at first", but after some time (probably because my tubing gets wet and creates some air bubbles) capillary action makes the liquid flow, in some pumps only, it's quite random...

Capillarity action through the material conforming the walls of the tube?
Do you have a way to equalize upstream and downstream pressures while the pump is not working?

Lnewqban said:

Capillarity action through the material conforming the walls of the tube?
Do you have a way to equalize upstream and downstream pressures while the pump is not working?

Yes, capillary action in the tubing: the wall is very thin, so is the OD (5mm). And no I have no way to equalize the stream pressures, but making the tubing horizontal helped! (it used to be vertical which would worsen the capillary action, logically!)

sylph said:

TL;DR Summary: How can I prevent liquid flowing in my tubing when my peristaltic pump is not working?

I am using a very thin tubing of 0.5mm diameter, which makes me struggle with capillary action, unfortunately.

sylph said:

Yes, capillary action in the tubing: the wall is very thin, so is the OD (5mm).

I am now really confused. Maybe you can let us know the ID, OD and wall thickness of the tube you are using. We can check it is consistent before proceeding.

Welome to the insane world of microfluidics. The real killer in the capillarity is that it can be very sensitive to small (seemingly inconsequential) changes (pH, impurities, dilution, polarity,temp ) in the target fluid as well as surface features of the pump interior. This produces a matrix of possible interactions that rapidly becomes daunting.
How "standardized" is the fluid?
What needs to be controlled: flow ? pressure?
This can be a subtle art.

sylph said:

TL;DR Summary: How can I prevent liquid flowing in my tubing when my peristaltic pump is not working?

I was wondering whether there existed some kind of valve that prevents capillary action?

How would a small section of wider tube affect the unwanted flow? Placed at the output of the pump, wouldn't it break the circuit? It seems to me to be an obvious solution so I'd be surprised if it's not already been tried and rejected for some reason.

Also is there any reason not to use a wider bore and slower rotation of the pump rollers. Perhaps the device needs to be really tiny.

The 'Gordian' solution is a shut-off valve. If you need 'passive,' a low-cracking check valve might work - the trick is having a high enough spring force to positively seal while having it low enough that the pump can overcome it.

You don't say (or I missed) what your 0.5mm OD tubing wall thickness is. It is likely a significantly larger fraction of the OD than for larger tubing - that is going to make the 'squeeze' force required completely close off the tubing large (relatively) and the required precision of the pump/roller gap high.