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Water Between Two Plates

Water Between Two Plates

Water Between Two Plates

(OP)
Hello,

I was wondering if anyone knew the equation to find the necessary clamping force to expel a saline solution out from between two parallel plates. I would like the most general form of the equation if possible, but will provide more details if necessary.

Thanks!

:)

Helen

RE: Water Between Two Plates

It would depend on how the saline is managing to stay between the plates now.  Normally I would expect it to run out from between them, so why/how is it being trapped there now.

We are more connected to everyone in the world than we've ever been before, except the person sitting next to us.  Lisa Gansky

RE: Water Between Two Plates

(OP)
Right now the fluid is being held to the plates by capillary force. Unfortunately, we have to use metal plates that have high surface energy and so the adhesion forces are relatively strong. We can get most of the liquid to run out from between them, but can't seem to remove all of it without extremely high forces. It would be great to characterize that force.

Thanks,

Helen

RE: Water Between Two Plates

Capiliary is characterized by surface tension, I think, which can be extremely high in relation to the forces that can be brought to bear on the very small areas that it seems to work in (the cross section of a vein in a tree trunk, for example).  I'm sure that between your plates, there are enough molecular gaps that prevent your plates from actually making perfect contact and are just the right size for capiliary action to take over.  It always seems to get (relatively speaking) more powerfull in the force matrix actually making contact as you scale down to smaller and smaller areas.  Good luck with that one.  

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

RE: Water Between Two Plates



I agree with BigInch, and it will also depend on how clean/dry you wishes the plates, purpose and surroundings.

Better to think outside the box?

What is the connection/gapform to outside?

Possible to use a combination of somwhat higher temperature (drying/steaming) and some vacuunm to suck the water out as water and/or steam? By wrapping the plates in a plastic sack and set vacuum to this?

RE: Water Between Two Plates

(OP)
Thank you guys for your input. We have several engineering solutions to this problem, but we were hoping for an equation to show our clients that it doesn't make sense to simply squeeze. Another example of brute force versus engineering!

RE: Water Between Two Plates

"...to show our clients that it doesn't make sense to simply squeeze. "

Make them a tiny model and bring it into their office. Common sense has very different meanings for an engineer, and everyone else. Seeing is believing ;)

RE: Water Between Two Plates

Bring in a toothpaste tube and let them get the last bit out.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

RE: Water Between Two Plates

I didn't know we were allowed to think out of the box.  I agree that the vacuum approach would probably give the best results.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

RE: Water Between Two Plates

(OP)
Unfortunately, we can't put it under vacuum or heat it up. There are several engineering solutions we can use, but it would be very helpful to have the equation for the force required to expel all the fluid from between the plates as well.

RE: Water Between Two Plates

centrifugal force by spinning the two plate in contact about their centers, may be ? probably will need very high RPM.

RE: Water Between Two Plates



... Please define 'all'.

And if you press the plates together, and the plates are smooth enough to press out 'all' water, do you have any inlet to let air (fluid?) in afterwards, to counterwork the outer (atmospheric environment or other fluid) pressure making the plates sticking together after all air and fluid is removed between the plates.

...And counterwise: removing 'all' the fluid will at least require the force counteracting the pressure from surrounding (atmosphere/fluid) acting on the plates from outside, as long as there is an open connection the same force will work on the inside of the plates.

How much over this force you need to counteract the capillary force, is still the question.

Or am I mistaken in the logic?  

RE: Water Between Two Plates

I don't know of any magic equation to do that, as there is essentially no resistance provided by the fluid to lateral flow at that small scale.  I'd say you could integrate the shear forces, but surface tension holding it there would be relatively much greater.  In other words, its not coming out that way.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

RE: Water Between Two Plates

What BigInch said.  Not sure I can think of an equation either.  But a simple thought experiment (or direct experiment if you want) goes like this:

The most perfectly flat and smooth surfaces you might encounter in daily engineering life are those of gage blocks (Jo blocks, Go blocks).  Wring two (clean, dry) gage blocks together and they stick.  There have been measurements made that show the gage blocks stick due to Van Der Waals forces, and the "stickiness" varies in strength relative to the inverse of the gap between the blocks (well made blocks can have tensile strength of the wrung joint exceeding that of the tool steel itself, i.e. the block will fail in the parent metal away from the wrung joint in tensile tests).  Ok, blah blah - but now wet the blocks with oil or water (not my blocks, thank you) and try to wring them together.  You will struggle to get them to stick if the surface is wet, no matter how hard you press or how well you wring them.  The film of fluid just can't be removed that way, likely due to the same effect of Van Der Waal's forces, only acting in the opposite direction (the thin film wants to stay on the metal block surface, with tenacity increasing as the film gets thinner).  The only way to get the film off the surface is to wipe it with an absorbent material...or by physically changing the liquid's state (drying water).

RE: Water Between Two Plates

(OP)
BigInch- You're right, the surface tension is definitely difficult to overcome!

btrueblood- That is very interesting. I have thought about gauge blocks in the sense of not being able to pull them apart (when clean and dry) and that the diameter of a tube is inversely related to the height of the fluid and thought they must have a relationship. You're thought (I'm not going to wet mine either!) experiment nicely illustrates the phenomenon, but I feel there must be a definable relationship between the surface tension, area, distance between the plates, etc. I'm currently looking into things like vacuum pumps (with fluid tight ceramic plates) and the lubrication pressure of journal bearings to try and get a grasp on similar relationships. In reality, I'll only need the force required to prevent fluid flow between two plates, not the force required to actually remove all the fluid.

RE: Water Between Two Plates

Btrue .. good point on the Van der Waals forces.  Forgot about those.  At those thicknesses of fluid, definitely important and probably undefeatable.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

RE: Water Between Two Plates

(OP)
Hey guys. So someone on another forum sent me the link to his thesis which addresses this problem. Basically the force is inversely proportional to the square of the distance between the plates. So, we now have mathematical proof of what we've been discussing here - it's impossible to squeeze all the fluid out! Thank you to everyone who contributed to this discussion. If you would like to review the thesis yourself, it can be found here:

http://opus.kobv.de/tuberlin/volltexte/2006/1179/pdf/richter_frank.pdf

The relevant info is in table 1.

Thanks again,

Helen

RE: Water Between Two Plates

Ya.  That's exactly what I was trying to say. smile

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

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