Off topic but

[Dave]

As you can make this yourself, I thought I would come and ask the experts on all walks of life :-)

My wife is a Beaver Scout leader and has come back from Australia with a very simply mixture that defies logic.

If you mix a cup of corn flour with half a cup of water, you end up with a complex compound.

If you are aggressive with it, it is a hard substance. If you are gentle with it, it stays fluid.

Frinstance.

Grab a handful and it will solidify. Let your grip go loose and it will flow off your hand.

Now to my question. What name do I give to this metamorphose and why does it happen?

[geoff]

In message , Dave writes

I trust this isn't a euphemism

It's thixotropic

like paints tend to be nowadays

There was a good demonstration of it in a pseudo science program on BBC4 a while back where they filled a swimming pool with custard (made in cement mixers) and had someone running across it. When he stood still he sank and they had a devil of a job getting him out

[Lee]

As was demonstrated in the Sky1 show "Braniac", when the Tickle walked on custard :) :)

Lee

[mrcheerful]

This property is called it's "DILATANCY"

(A bit like "thixotropic")

mrcheerful

[Lee]

Isn't Thixotropic a definition of a particular Non-Newtonian fluid behaviour, not the general definition? Been a while since I did Physics... :)

That was the one I saw (on Sky), still a fun demo though ;)

Lee

[geoff]

A while ?

25 years in my case

The answer is that I don't know, but it'll prolly be enough for the OP's wife

Yes you're right it was Sky one - where they blew up some caravans and tested whether real or silicone were more buoyant - crap, but fun

[geoff]

In message , mrcheerful . writes

Dilatancy (googled) - Tendency of rocks to expand along minute fractures immediately prior to failure; stress may be from earth movements or from controlled laboratory experiments

err ...

[mrcheerful]

NO. Have a look at

dilatancy: from :dilatant: increasing in viscosity and setting to a solid as a result of deformation by expansion, pressure, or agitation

mrcheerful

[geoff]

In message , mrcheerful . writes

Bur agitation de"thixotropicises" so that definition can't be right

[mrcheerful]

mrcheerful

have a read here:

for the second instance of the word cornflour. this seems a good explanation of what it does.

mrcheerful

[The Natural Philosopher]

Its a dilatant mixture. That expresses its physicalproperties. The oppostie of thixotropic, which goes thin under pressure, but gels by itself - a useful property in paint.

The chemistry is just cornflour and water, mixed.

[The Natural Philosopher]

It IS Dilatancy (sp?) Google has it wrong. Its NOT thixotropic - thats teh other way around.

[RichardS]

"geoff" wrote

but cornflour & water doesn't exhibit that property. It's a wierd substance...

you can move a spoon through it slowly with little resistance - like a perfectly normal liquid.

try and stir it rapidly, however, and it behaves as if it were solid.

Thixotropic stuff is the opposite, such as tomato ketchup. Initially highly viscous, almost gel-like, but once it starts running it's viscosity decreases dramatically and it flows easily (the reason you always end up with rather more than you expected coming out of the bottle).

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[mrcheerful]

Cornflour/water has the bizarre property of becoming hard under sudden pressure. Dilitantic.

Tomato sauce is thixotropic: gel till agitated then liquid.

I should not have mentioned thixotropic earlier, I was just pointing out a similarity (in a reverse way) to dilatancy.

mrcheerful

[Nick Brooks]

I believe it works with any starch and water mixed. Cornflour works well because it has a high starch content which is why it's used to thicken sauces

Nick Brooks

[RichardS]

precisely - I was agreeing with you! Of course, I managed to miss out the word Dilitantic!!!

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[Nick Brooks]

Sounds like a dilatant mixture could be used to solve the problem of speeding yobs (see other thread) Drive slowly over a ramp and it collapses slowly, drive fast over ramp and it stays up

Nick Brooks

[Ian Middleton]

Used in nursery schools by the trayfull for kids to play with. Just called Gloop.

[Elessar]

I know you can make great "slime" from PVA and borax. On googling for the exact recipe it seems another term for the opposite of Thixotropic is "Rheopectic".

-- LSR

[Rod Hewitt]

"Elessar" wrote in news: [e-mail]:

I suppose this is where I pull an earlier thread into this one - custard filled speed bumps. See:

(My word - haven't looked at this for ages - and it's still getting posts! And I have always loved the term non-Newtonian fluids.)

Also - New Scientist this week (maybe last?) had an article on the *lack* of understanding of cornflour and water. If I find it I shall post a link.