Ferrofluid is a suspension of tiny iron nanoparticles in mineral oil that behaves like a moving liquid magnet when placed in a magnetic field. It can now be inexpensively purchased at magnet supply stores like Apex magnets. It has some fascinating and beautiful behavior. You can use it make a liquid piston with no moving parts, unless you count the liquid moving. Since this could be made very small, researchers have been interested in trying to make small pumps with Ferrofluid for the purpose of manipulating tiny samples for chemical analysis, perhaps by making a “lab on a chip”.
In part due to interest from a young student, Huy, I started experimenting with ferrofluid. This is the story of a miserable failure; as a Public Inventor, it is my ethos to publish failures as well as successes.
Although Huy was most interested in build a soft actuator, I developed the idea that this could be accomplished with ferrofluid most basically by building a ferrofluid pump — that is, a pump which had no moving parts except ferrofluid manipulated with electromagnets. If we could pump ferrofluid inside a bladder or membrane, we would have the beginning of a “hydrostat”, an object that could change shape base on internal pressure changes. In theory we might be able to make a model of the human tongue or a snail’s body or an octopus arm.
But before we did that, I insisted on trying to build a pump, and the basis of a pump is a piston and valves. In particular, “check valves”, or one-way valves. Valves that allow air (or water) to flow in one direction only. With two check valves and piston you an make an oscillating pump, like a bicycle pump for example.
If you put a blob of ferrofluid inside a tube and then apply a strong magnetic field to the blob, it “seals” the tube. That is, it forms a somewhat airtight seal. If the air pressure on one side becomes high enough, it will explosively spurt away for the magnet and thus break the seal. But until you reach that much pressure it is airtight. You can easily move the blob in the tube by moving the magnet. I used powerful 1/2" diameter wide by 1" long neodymium magnets, but any magnet will do — the stronger the better, up to the point of staying safe. You can easily observe that this moving blob acts as a piston, pushing air around inside the tube.
A standard way to play with ferrofluid is to put it in a covered petridish and then apply a magnet beneath it. Then slowly remove the petridish cover. Why slowly? Because ferrofluid stains like ink and tends to splash. It is best to wear goggles and use gloves and old clothes you don’t mind being stained when playing with it. It is very messy.
I was doing this and playing around, more or less without purpose, and decided to inject air into the ferrofluid with a hypodermic syringe. The result was interesting — the bubbles that were produced always came out in the direction of lowest magnetic gradient. That is, the bubbles fled the magnetic field where it was strongest.
This of course makes sense. In the presence of a magnetic field, the ferrofluid becomes “solid” or semi-solid. Perhaps “solid” is not the right word — but ferrofluid in a magnetic field resists deformation of its shape. In some sense, the ferrofluid in a stronger field has more internal pressure.
The affect with a syringe was very clear: bubbles travel in the opposite direction of the gradient of magnetic flux.
This led me to an idea: this asymmetric behavior should allow us to create a one-way valve. If we could place ferrofluid into an magnetic field that has a strong gradient, we can imagine making a blob that lets air pass in one direction but doesn’t let air pass in the other direction.
Another way of saying this is that if you put air pressure on the surface of a blob in direction of increasing gradient, you move the whole blob. If you put air inside a blob, it forms a bubble that flows outward.
So, I reasoned (erroneously) that I could easily build a geometry that would let me inject a bubble into a point of high gradient and it would flow to a point of low gradient (that is, air would pass easily in this direction.) But air in the other direction would push the whole blob, and thus be sealed and not flow. So, I build six distinct apparatuses to test this theory, pictured above.
The first was with hypdermic needles. It seemed to work, but didn’t. Over about 48 hours the ferrofluid tends to get sticky and clog the needle.
So I build another similar apparatus made purely out of acrylic tubing. That is, a small acrylic tube took the place of the steel needle. This made it clear that the idea does work — with a catch. Once there is air (not ferrofluid) in the tube so that the air really does get into the point of decreasing gradient, then it works. I could accomplish this by moving my blob by hand over the acrylic inject tube (which started with air in it.) However, I could not “blow” air into the blob. Once ferrofluid was in the needle, it took high pressure to clear it. That is, this defeats the idea of allowing air to pass at low pressure.
I then decided the problem was that I had to overcome this increasing gradient before I got to the decreasing gradient. That is, if I injected the air from the same direction, without going through the center, high-flux part of the blob maybe it would work.
Perhaps, gentle reader, you now see my mistake, but let me continue. At considerable trouble, I laser cut some sheets of acrylic to make an easy-to-see two-dimensional version. Then, at the cost of even more trouble, I cemented them together and made them airtight, sealing them to urethane tubes with hot-melt glue. As usually happens, I had to do each of these operations twice to get it to work.
But it didn’t work. I just couldn’t understand it. I tried an even simpler experiment, of using an iron yoke (actually, mild steel) to divert magnetic flux from the end of the single hollow cylindrical magnet. This definitely produces a magnetic gradient of weak (where the yoke is) to strong (where there is not yoke.) At least, if the tube is full of air.
But it didn’t work. Using $40 gauge that measures air pressure in column inches of water and carefully sealed urethane tubes, it was clear that the blob sealed well and then broke down at approximately 14 column-inches of water on both sides.
Confusion is to be cherished because it precedes enlightenment.
Once my experiments dragged me kicking and screaming into the light, I finally get it: The surface of any blob of ferrofluid tends to have the same magnetic flux at any point on the surface, and the surface will be lower flux than points deeper in the interior. It is not, therefore, possible to blow air into a blob of ferrofluid, not matter how you shape the injection needle or where you put it, without first moving fluid in a low-flux position to a higher-flux position. As we have seen, this will not a allow a small bubble to pass, the whole blob will move.
Yes, you can inject air into a ferrofluid blob and have the air flow out easily — once you have used enough pressure to move the fluid in the needle out of the way.
In other words, I conjecture that it is not possible to make a one-way passive ferrofluid valve based on any geometry that involves simple air interfaces.
This does not mean that we can’t “smuggle” air into a blob some other way, but any active measure ceases to be a passive valve.
So here are some challenges to mathematically/physically inclined:
- Prove or disprove that the surface of a ferrofluid blob in the presence of a magnetic field (discounting gravity) is a surface of equal flux density per unit area.
- Prove or disprove that the magnetic flux gradient at a blob surface always points inward to the blob.
- Prove or disprove that an blob physically contained within a tube, having two air interfaces (at either end of the blob) will have equal flux density at each side.
- Prove or disprove that is not possible to build a passive ferrofluid geometry, no matter what magnets you use or what physical shape you use, which presents an air surface of decreasing flux gradient, and therefore it is not possible to build a simple passive one-way valve with ferrofluid as I attempted.
I claim that all of these assertions are true, but do not claim to have produced compelling proofs of these statements.
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