Microfluidics in pressures lower than one atmosphere

[cedric goossens]

Hi everyone

I am trying to build an experiment with microfluidics. I have a reservoir containing a bellow. This bellow is a compensation mechanism and makes sure that the liquid (in this case water) can be pushed out of the reservoir. Before storing the fluid in the reservoir, the reservoir is evacuated up to 10^-2bar and the fluid is degassed. Right after the reservoir I have a first valve followed by stainless steel tubing with a diameter of 0.35mm. THis tubing also contains a pump. After the tubing I have a second valve which connects the tubing to a controlled (pressure and temperature) and closed chamber. My experiment is as follows:
- reservoir valve is closed
- chamber valve is opened
- chamber and tubing are evacuated
- pressure is set in chamber and tubing
- reservoir valve is opened
- reservoir valve is closed when liquid reaches chamber
- chamber valve is closed
- chamber is evacuated
- pressure is reset (same value as before)
- chamber valve is opened
- pump is used to inject liquid

Now this works perfectly fine when the set pressure is 1bar, 400mbar,...
But when the pressure is below 200mbar the liquid keeps on flowing out of the tubing. I was wondering what the cause is of this. Is this due to the capillary effect or is it something else that I am missing?

 

[Bystander]

10^-2bar and the fluid is degassed.

Through a capillary? Of what length? Containing a pump? And what other impediments to flow? For how long? And what percentage of saturation is 10^-2 bar?

 

[cedric goossens]

Well the liquid lines consist of a few types of tubing. First of all there is a 1.6mm diameter tubing approximately 20cm in length, as a sidebranch of this tubing there is a volumetric plunger pump to push the liquid out. Behind the final valve there is a tube of 0.35mm diameter tubing of approximately 2cm. The pump is set to work at a speed of 10µl/s and needs to pump out volumes between 2 and 130µl.
The last question I don't quite get. Before filling the reservoir with degassed liquid, it is set to 10^-2 bar. After that the reservoir is filled.

 

[Bystander]

The last question I don't quite get.

What's vapor pressure of water?

 

[cedric goossens]

Considering it's at room temperature this should be about 31mbar

 

[cedric goossens]

What's vapor pressure of water?

Sorry to bother you but do you have any insights?