# Using a Microcentrifuge.

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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.

Blender

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.

Bomb

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 Motor

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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## 21 Discussions

Hey i know this hasnt had a comment in a while so i just wanted to say Thanks helped me alot i actually have a instructablein the planning stages of making a microcentrifuge

not bad, i still have to go back and read it all but it looks like a good well done instructable. i wonder, maybe there is a way to make a centrifuge. most likely not with knex because the motors would just be to slow, even with gear ratios. but maybe with something like a drill. maybe.

5 replies

A friend in primary school built a LEGO gear train that geared up something like 10:1 or 20:0.. then chucked the end into an electric drill and span it up. Another, slightly less sensible friend decided to touch the end gear (rotating at 20 times electric drill speed >_<) with the tip of one finger. The lego broke apart, the cogs flew everywhere and the not-sensible friend had a small cauterised burn/cut in the end of his finger. A cautionary tale against using power tools and plastic construction sets together.

lol. man that must have been spinning fast. thats one advantage of K'Nex, they connect differently so they don't just fly apart. the one thing i would be worried about if i made one of these would be that by using gear ratios it would have not torque.

I believe Killerjackalope has a centrifuge ible in the planning stages, and one or two others have made noises along those lines. I'm looking forward to their work.

I know where there still may be a hand cranked centrifuge, in an oil field pump house. I need to remember and ask the owner of the land where it should be I can go ahead and swipe it.

Not yet. I believe a couple of members have it on their list of planned projects, but nobody has published yet.

Even if they do, I'd invoke Kiteman's Law anyway (search for notes on the zeroth law).

well done we used them at work to test lead and drug levels as well as other things ours were a bit bigger and when they closed the place they threw 2 away...I took the suction machines and a pump never saw a use for a Centrifuge...sighs and slaps myself in the head ...had a plethora of test tubes in different sizes and pipettes till a friend decided he would open a head shop and made pipes out of them ...I think I could make one with a few spare parts the balance would be the problem

"What? You're Latin's a bit rusty?"

You're English seems a little rusty as well! ;-)

Sorry - I couldn't help myself... :-D

Nice job! Larger centrifuges that are spun at super high speeds for hours on end have platinum (i think) innards, and are the size of dishwashers. If they are even slightly unbalanced (that's why upipetter skills are needed), they can vibrate with enough force to break through walls. If a centrifuge ever does begin to move, the safest way to stop it is to unplug the entire machine.