Monday, 5 October 2015


I'm tempted to put this whole blog entry in yellow, but it'd be illegible so I won't bother.

This is another home ed staple - oobleck.  Once again I'll start off by describing the practicalities and then move on to what it's all about.

Oobleck is a mixture of corn starch and water often referred to as custard.  However, it isn't really good quality custard in terms of food but an illustration of the weird things liquids can do sometimes.  The recipe is one part of water to one part of cornstarch, thoroughly but slowly mixed.  You end up with a liquid which will flow if poured, but if you hit it hard it will feel firm.  If you squeeze it in your hand, it will form a ball but that ball will then gradually trickle between your fingers and drip off it.  If you plunge a fist into a jug of it slowly, you can then lift the jug with your hand.  It also does entertaining things on speaker cones:

If you fill a swimming pool with it and stomp your way across it, it will support your weight:

Oobleck or "custard" and its physical properties also led to a bit of an obsession by various people on the Halfbakery, such as custard-filled speed bumps, custard-filled trousers, custard guns, custard running tracks and eventually the overuser of custard destroyer and the like.  There's also Ustard.  Oobleck occupies a strange position in my mindmap where two of the major things I've done overlap.

Custard, as Halfbakers call it, is one of many "non-Newtonian fluids".  This needs to be explained.  In the case of custard, the property is shear thickening.  I should probably explain what a Newtonian fluid is first.  A Newtonian fluid is one whose deformation is linear with respect to stress.  In other words, the harder you splash it, the further it goes.  What with the world being an imperfect place, where for example lightning doesn't strike in a straight line and trees are not brown cylinders with green spheres on top, strictly speaking most liquids are not Newtonian.  For instance, pond skaters can do this:

"Amenbo 06f5520sx" by Cory. Licensed under CC BY-SA 2.1 jp via Commons -
and it's possible to float needles and even razor blades on water, meaning that there can be quite a lot of stress applied to water before it starts to behave in a Newtonian manner.  Air and other gases are also fluids, and are closer to being Newtonian than water.  Water is in various ways a very unusual substance, one of which is its surface tension, which is stronger than most other liquids, an exception being liquid selenium.

Other examples of very non-Newtonian fluids are quicksand, wet cement, toothpaste, ketchup, silly putty and non-drip gloss paint.  From a home ed point of view, lots of these are easily available. 

 Ketchup is notoriously good at getting stuck in a bottle and then suddenly splurting out when you bang it too hard on the bottom while gradually flowing out under the influence of gravity, although recent redesigns of ketchup bottles have made this less problematic.  This is the opposite to custard, shear-thinning, and is due to the presence of xanthan gum.  That is, the stress put on ketchup makes it flow more than expected, or rather, than if it was water or even a much thicker liquid like mercury.

Non-drip gloss paint is an interesting one because it's easy to spread on a surface but won't flow down it because its viscosity (thickness) depends on time more than force.

"Toothpasteonbrush" by Thegreenj - Own work. Licensed under CC BY-SA 3.0 via Commons -

Another kind of non-Newtonian fluid is toothpaste, which is like water as described above only more so.  It can hold a peak because it requires a certain amount of force before it'll move at all, but after that point behaves quite normally.  (Incidentally, toothpaste is easy to make from chalk powder or dried clay with hopefully vegetarian glycerine.)

The quicksand problem is an example of the effect of shear-thinning.  Quicksand itself is about twice as dense as a human body so it should be impossible to sink entirely into it.  The thing to do to escape is to move slowly.  However, it can become so sticky that it would take a car to pull someone out and it also means things can happen like the tide coming in before you get out or a Boa constrictor coming up and eating you.  There was a point in the 1960s when 3% of all newly released films had a scene where someone got stuck in quicksand.

Silly putty is a particularly nice example of a non-Newtonian fluid.  I won't link to it because that would be advertising, but for people who haven't experienced it, it's a substance which is like modelling clay when you move it slowly but if you throw it, it will bounce, and if you drop it off a high building it will actually shatter.  Left to itself, it will gradually form a pool, which however can have started off as any shape.  Technically, silly putty is mainly polydimethylsiloxane, which is a polymer (chain of molecules) made of silicon, oxygen, carbon and hydrogen:

It's a fairly exciting substance because it behaves like the kind of substances which make up living things, such as latex, but is largely silicon-based, raising the as yet unanswered question of whether there's life out there in the Universe based on silicon rather than carbon.  I made a video about that once which was quite popular.  Anyway, the reason it behaves as it does is that its chains are very flexible, so it can hold its shape well over short periods but over long periods tends to droop under the influence of mechanical forces such as gravity.  It can also be used in hair conditioner.

At very low temperatures, helium becomes a very special kind of fluid called a superfluid.  This is particularly interesting because it flows uphill and leak through solid objects.  It has no viscosity at all, so it can be used to produce a fountain which never stops flowing:

One thing which interests me about superfluid liquid helium is what would happen if you tried to make custard with it.  That is, if you took a fine powder of frozen hydrogen and suspended it in liquid helium, what would happen?

Thursday, 1 October 2015

Diet Coke And Mentos

Like ooblek, this is another home ed classic, and of course it's out there in the ether with a load of other things like vinegar and bicarb.

What it comes down to basically is that you get a two litre bottle of Diet Coke, make a paper funnel and post five sugar-free Mentos down into the neck of the bottle.  Then you run away very fast unless you want to get covered in Diet Coke fizz, and you might I suppose, depending on your age.

It used to shoot ten metres in the air.  When we did it at John's Lee Wood home ed camp back in the 'noughties, it did exactly that.  However, for some reason Coca Cola decided they would make it less spectacular, so nowadays it only shoots up a couple of metres, but it's still quite impressive.

The experience of going into a shop and buying sugar-free Mentos and Diet Coke is quite a daunting one for me and this is the only reason I would ever do that.  Here are the ingredients for Diet Coke:

and here are the ingredients for sugar-free Mentoes:

Before I go into those lists and their ethical and other implications, I just want to describe the uses of the activity for educational purposes and a few other things about the process.  You should try not to shake the bottle before you try it, you can time the fountain and measure its maximum height, and you can also use different combinations of drinks and mints to see if there's a difference in these. I've used fizzy water, generic cheap sugar-free drinks, sugary drinks and Diet Coke itself in sequence.

You should also do it outside.

Another thing you can do is to make a soft drink of your own gradually by adding the ingredients one by one and seeing what happens.  What surprised me about this most was that the ingredient which made the biggest difference was actually caffeine!  I'll come back to that.

What's happening then?  Well, it's not a chemical reaction. If you look at a glass with something fizzy in it, you will often notice that there are strings of bubbles rising to the surface from a few points on the glass:

"Drinking glass 00118" by © Nevit Dilmen. Licensed under CC BY-SA 3.0 via Commons -
(I hope that's a moving picture).  This is because a fizzy drink is a solution of carbon dioxide dissolved in water and when something comes out of solution it often "crystallises" around an irregularity of some kind.  This is similar to the process whereby raindrops form around dust particles in the air.

Mentos, particularly sugar-free ones, work because they have a kind of "crazy-paving" surface like this:

"Desiccation-cracks hg" by Hannes Grobe 08:01, 27 October 2007 (UTC) - Own work. Licensed under CC BY-SA 2.5 via Commons -
When you put a mento into Diet Coke, most of the carbon dioxide in solution forms into bubbles on the corners of these cracks and comes out in one go.  Hence the fountain.

The energy of the fountain also illustrates the difference between potential and kinetic energy.  Just as a battery stores charge which can be used for all sorts of purposes, the energy used to get the gas into the drink in the first place is stored as potential energy in the coke bottle.  This also applies to shaking, bumping or dropping a bottle of pop, which will then go everywhere if you open it soon afterwards.  I realise this is obvious, but the point is that you can think of it as a way of demonstrating the storage of energy and the difference between the two types.  Interestingly, possibly just to me, if you leave a bottle which has been bumped for a bit and open it later, it won't overflow even if it hasn't lost any gas, and I think the reason for this is that it somehow loses the extra energy through entropy, although what actually happens isn't clear for me.

Although I don't wish to spoil anything, the spectacular fountain effect is less spectacular if you have sugar in either the Mentos or the Diet Coke.  My hypothesis to explain this is that dissolved sugar thickens the liquid and makes it heavier, so the energy from the bubbles comes out more slowly and can't lift the fountain as high.  Possible ways of testing this would be to weigh a bottle of ordinary Coke and a bottle of Diet Coke or to see if the lower fountain from the sugary Coke lasts longer than the higher fountain from the Diet Coke.  That's an opportunity to draw a line graph of course.

I have had a go at trying to make the "perfect" mix for this process, although missing out the aspartame.  This involved powdered calcium carbonate, citric acid crystals and gum tragacanth.  The last ingredient is supposed to be the "Mentos", which are largely made of gum Arabic.  I didn't use gum Arabic at the time because I was boycotting it for ethical reasons (at the time most gum Arabic sales were used to fund "terrorist" activity).  The idea was to mix the calcium carbonate and citric acid together before dumping them in water with the powdered gum tragacanth.  I was doing this at the request of someone else with whom I then lost touch, and the purpose was not to achieve the fountain effect, so I didn't go any further with the project.  I have also made my own cola, incidentally.

That brings me back to the ingredients lists:

To me, the Diet Coke ingredient list is at the "so bad, it's good" level because every single ingredient in it, including sometimes even the water, is a bad thing to consume!  It's actually genuinely impressive how bad they've managed to make it.  Phosphoric acid interests me because it's an inorganic acid:

This has the formula H3PO4, so it contains neither carbon nor chains or rings of atoms, which is unusual for an acid used in food or drink.  This is the ingredient famous for dissolving teeth left overnight in Coke, and if you use a strip of litmus paper on Coke it will show you that it's unusually acidic considering that it doesn't taste sour.  In my home made cola, I used lime juice, being the richest easily available natural source of citric acid, or I could've used citric acid itself.

Caramel is burnt sugar, whose safety has also been called into question.  In my cola, I replace it with muscovado sugar, which produces a paler-looking but still brown drink.

The sweeteners are usually where the arguments begin.  Acesulfame K is just uncontroversially harmful.  Aspartame is also very harmful but the allegation that it's harmful often sparks resistance.  Like many other artificial sweeteners and colours, aspartame is also used in medicines.  This is the reason the list says "Contains a source of phenylalanine" at the end, because there is a genetic condition called PKU or phenylketonuria where the consumption of significant quantities of the amino acid phenylalanine causes brain damage.  This by itself doesn't mean aspartame is any more dangerous than peanuts would be to someone who is not sensitive in that particular way.  There's a condition where fructose causes brain damage as well, so if the same thought was applied there it would mean everyone should avoid fruit.  That's not how things work.

Having said that, I would repeat that there is no doubt in my mind that aspartame is extremely harmful.

I will now explain why I don't think it's anyone's fault that it's turned out to be harmful!

Here's a model of an aspartame molecule:

Aspartame is made from two amino acids joined together by a peptide bond.  Amino acids are small molecules which are joined together to make up proteins, which are the main substances living things are made of along with water and carbohydrates.  When you eat protein, say in the form of tofu, egg white or meat, your digestive system has to deal with long chains of amino acids of all sorts which it has enzymes, which are also proteins, uncoupling and separating, breaking the same peptide bonds which are joining together the two amino acids from which aspartame is made, namely aspartic acid and phenylalanine.  Since there are all sorts of amino acids joined together in protein which we all eat every day, there is absolutely no reason to suppose that aspartame should be dangerous on that basis.  It is utterly, completely, 100% forgiveable that Monsanto, when they started to market aspartame as a sweetener, regardless of the opinion that they are evil (an opinion which I share by the way), thought it was safe.  The theory behind how the digestive system and biochemistry work would also support the idea that it is completely safe.  However, my empirical experience demonstrates that it isn't.

I'm going to quote three incidents.  I have many more in mind, but I have issues with patient confidentiality so I am only going to mention the ones which I have permission to quote.

A child has a constant wheeze and a relatively low peak flow.  There is a positive family history of asthma.  He never consumes aspartame and he has never had an asthma attack.  Unknown to anyone, he consumes a drink with added aspartame and experiences an asthma attack.

A middle-aged patient (given that this is a home ed blog I just want to point out that I am tempted to the point of torture to reveal their name but of course patient confidentiality is utterly sacrosanct, so I'm just going to drop this hint and leave it at that) suffers constant migraines with a cyclical pattern.  Their diet and lifestyle is utterly immaculate - I mean, they are utterly perfect, completely beyond any improvement - with one exception, which is that they drink a lot of Diet Coke.  Discussions about this fact do not persuade her to give it up, but interestingly there are all sorts of rationalisations about why it's okay to drink it.  Her migraines continue.  When she finally does give it up, her migraines stop too, immediately, and she hasn't had one since.

I am offered a cup of coffee sweetened with what I think is fructose.  Other than my gender dysphoria, I am an incredibly healthy person.  I don't know anyone who is as healthy as me.  Five minutes after drinking the coffee, I feel very dizzy and am unable to think clearly, then I almost lose consciousness.  I later find that the coffee was sweetened with aspartame.  I have never consumed aspartame on any other occasion.

These three incidents in my experience are enough to convince me that aspartame is unhealthy.  I'm aware of the research regarding ethanol and the idea that it causes brain tumours and also seizures.  I am not, however, basing my opinion on any of that.  Moreover, there is no reason in the world, so far as I can see, that its composition should make it dangerous to most people.  It's just two essential amino acids joined together in an equally harmless manner.  Nonetheless, it really does seem to be dangerous, and I don't know why.  However, it does occur to me that sweet substances in general are often harmful, for instance lead and beryllium compounds and antifreeze, so I would sometimes interpret a sweet taste as a danger signal.

The sweeteners in the Mentos, incidentally, are mannitol, sucralose and maltose, so far as I can see.  I'm not a massive fan of sucralose because I don't like organic compounds with chlorine groups, but apart from that the sweeteners concerned are utterly innocuous.

Right, that's it for now and I shall forthwith skip gaily down the shop for a packet of xylitol!