Centrifuges are common, useful tools in the modern laboratory, especially in the biology lab.
I happened to acquire a small version (a microcentrifuge, or microfuge), and this is how I use it.
Caveat: I'm not saying this is the correct way to use a microfuge, since I have never had any actual training in same, there does not seem to be much in the way of advice online, and manufacturers I have contacted have decided not to respond to my requests for advice (after all, who is going to be in the position of acquiring a piece of delicate laboratory equipment without acquiring the appropriate skills? Apart from me, I mean.)
Step 1: What Is a Centrifuge?
Centrifuges are, at heart, simple devices - samples are put in and spun. Fast.
Through centripetal forces, this subjects the sample to artificially-high "gravity" (actually acceleration, but it amounts to the same thing), often thousands of times the gravity acting on you as you read these words.
It is exactly the same effect you exploit when you spin a bucket of water over your head - if the water wasn't subjected to slightly over one gravity of acceleration ("1g"), then it would pour out of the bucket and all over you.
Sinking and Floating
As you already think you know, heavy things sink and light things float. To be more accurate, denser substances tend to sink in less dense substances. If the difference is great enough, and the particles large enough, the sinking happens at visible speeds, say stones in water.
However, if the particles are very small, or the difference in densities is very slight, or the liquid very viscous, then the sinking can be infinitesimally slow, or even non-existent. The perpetual, random motion of the particles of liquid can constantly re-mix the solid into the liquid, or the liquid's viscosity can simply trap particles and hold them in suspension.
Increase the gravitational forces, though, and even tiny differences in density can be exploited to separate mixtures into layers. That is what a centrifuge does - it uses centripetal forces to increase the apparent gravity acting on the sample, which makes things sink or float more quickly.
Centripetal versus Centrifugal.
We often talk about centrifugal force that pushes things outwards as they spin. This is an intuitive concept (after all, we can feel the force pushing us sideways when we corner in a car), but it is wrong.
But let us remind ourselves of Newton's First Law of Motion:
Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
What? Your Latin's a bit rusty? OK:
Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.
That is - nothing changes speed or direction unless there's a force acting on it.
So, for our high-speed sample to curve away from it's straight line into the circle of the centrifuge, there has to be a push from the outside or a pull from the middle. Since there is nothing outside the circle, it must be the pull.
The forces involved
There is a simple calculation for the g-forces generated by a centrifuge:
RCF = 0.0001118rN2
RCF = Relative Centripetal Force (the "g" forces exerted)
r = the radius of the centrifuge in centimetres
N = the rotational speed (revolutions per minute)
Step 2: Safety.
Given that centrifuges spin at tens of thousands times per minute, they can be exceedingly dangerous.
Even though the rotor has no cutting edges, sticking your finger in it at speed would be equivalent to sticking it directly into an industrial blender.
To prevent this happening, my microfuge has no off-switch.
Instead, its lid is held on by three separate bolts, all of which need to be screwed down before the microfuge will work, and as soon as they are slackened the power is cut to the rotor. The length of the bolts mean that, by the time you have gotten them all unscrewed, the rotor will have slowed to a safe speed.
Under the extreme forces involved, centrifuges don't just break, they catastrophically fail. They come apart with the energy (and reportedly the sound) of a small explosion, complete with shrapnel.
For this reason, my microfuge has tough walls, which appear to contain fibrous reinforcement.
Larger centrifuges presumably have tougher reinforcement.
Pager motors vibrate because they spin an un-balanced weight.
If you do not balance the load on the rotor, the whole centrifuge will vibrate alarmingly, dancing about the place like a thing possessed. If allowed to carry on, there is the risk of bending the rotor, leading to catastrophic failure.
To prevent this, my centrifuge seems to have a rotor that is much more massive than the tubes, and so is harder to put off-balance. However, you should always use paired microfuge tubes, each filled with the same load (that is, the same amount of the same stuff), to reduce the risk. There are enough spaces in the rotor to take up to eight tubes - when I use more than two tubes, I use four or eight at once.
On top of all this, remember that the centrifuge is an electrical device, and most of the samples I plan on testing will be water-based. Vulnerable points are where the power lead plugs into the microfuge, and the wall-wart itself.
Step 3: Consumables.
Although you will probably only ever need to buy one centrifuge (certainly, if this one ever dies I will not be buying a new one), there is an on-going expense in centrifuging.
Most obviously, there are the tubes that contain the samples.
In larger centrifuges, for testing larger samples, these tubes can be glass, and relatively easy to clean.
Microfuge tubes, however, are always made of plastic (typically polypropylene). This makes them resistant to almost everything you are likely to put in them, but cheap enough to be disposable. They can be purchased sterile, and they are so small and awkward to clean (they only hold around 2ml) that it is considered easier and cheaper to throw them out instead of cleaning them.
Because of the company's market dominance, the tubes are also known as Eppendorf tubes (in the same way that vacuum cleaners are called Hoovers in the UK).
You will also need to transfer liquids and from the tubes.
The cheapest option is to use dropper pipettes like the one pictured. They are easy to use, fairly easy to clean of soluble samples, and cheap enough to throw away after using biological or toxic samples (such as blood).
You may be able to scrounge both tubes and pipettes from a local hospital, university or high school, but they are usually available on sites like ebay. Pipettes may also be available from hobby and craft stores, as they are used by people who mix their own paints and perfumed oils.
Samples will need stored, and the way you store them depends on what it is and how much there is, but small samples of almost any liquid or powder can be stored in a 35mm film cannister, especially the translucent white variety with the tight-fitting lids. Ask at any photo-developing shop and they will give you bags-full for free.
Step 4: Using the Centrifuge.
Here we will be centrifuging a sample of melted margerine. A mixture of water and fats, it is a potentially interesting subject.
First, we need two microfuge tubes (remember - safe microfuges are balanced).
Into each tube, we squirt the same amount of margerine (again, balance).
The centripetal force exerted by this microfuge is determined by the input voltage. Checking the side of the device, we see that an input of 12V and 1A gives us 13,000rpm and 8,500g. Sounds fun - that should be enough to encourage the solids to separate out of the liquid. Five minutes should give us visible results.
The tubes go into opposite side of the rotor (balance!), and we bolt the lid down firmly.
Check the wall-wart is at 12V, set the timer and switch on.
Five minutes later, switch off the power, open the lid, and have a look...
Step 5: The Results.
The margarine gave a clear set of layers - presumably the denser layers (those closest to the bottom of the tube) are water-based (the tubs' ingredients include "buttermilk"), and those higher up are the fats and oils respectively. The top layer remained liquid even after the sample cooled down.
Not all samples give such clear results, and not all samples survive high-g so well or so quickly. I messed up a sample of lamb's blood by spinning it at 8,500g for five minutes instead of 3,500g for half an hour.
Fortunately, if you are lucky enough to get access to a microcentrifuge, you can work with tiy amounts at a time, so even the last few drops of sauce from your plate can provide enough materials to keep an interested mind busy for the rest of the evening.
My first results
Blood - the wrecked samples
My Science Clubbers' results
Have you got access to a centrifuge, micro or otherwise? Spin something up, and let us see what you get.
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