This afternoon, my three-year-old grandson and I were playing with Play-Doh. A couple of questions occurred to me.
[ol]
[li]Is Play-Doh a solid or a liquid?[/li][li]What property allows Play-Doh to be separated into two or more pieces, then mashed back together with no evidence that it was separated?[/li][/ol]
(No, my grandson did not come up with those questions. We were busy with “Which ball is big and which is small?”
It’s a solid, in that it has a fixed volume and shape. You can mold a chunk into an elephant, and it will stay an elephant for hours or days. It doesn’t flow into the shape of its container – if you put the elephant into a coffee cup, it doesn’t change into the shape of the inside of the cup – which is a property of liquids.
I don’t know the name of the property of a substance that is divisible and re-combinable. But a cup of power has that property. You can measure it into two half-cups, then combine it again, and there’s no way to tell it was ever separated. Regular bread dough has this property, and chewed chewing gum, and so on.
I would have thought that it is a *plastic *solid.
Latin *plasticus *of moulding, from Greek plastikos, from *plassein *to mould, form
I was going to wager “non-Newtonian fluid”, though I admit I’m not 100% qualified on the exact properties that makes something that. However, I found a good answer for that on Physics stack exchange that vindicates me:
It appears specifically it’s likely a “shear thickening fluid” like corn starch in water (oobleck as that post calls it).
To add my own take on 2, I would guess that since it’s essentially “a suspension of solid particles in a viscous fluid” the recombining property is just applying enough force to move the solid particles around and allow the viscous fluid to combine like any fluid would.
Play-Doh[SUP]tm[/SUP] is a gel, which is a class of amorphous solid, i.e. it maintains form under normal conditions and a limited amount of stress, but will flow like a viscous liquid under high shear inlcuding shear from tensile or compressive loads.
Although it has no long range structure, the gluten-based startch in it will create short range intermolecular bonds (hence, while it picks up stuff) and cross-linking of the starchy molecules that allow it to reform, just like bread dough. It has various plasticizers, humectants, and stabilizers that keep it plastic and moist.
Stranger
Might be worth pointing out that Play-Do isn’t a completely super-magical thing. After all, a nice solid chunk of iron or granite will act similarly plastic if you get it hot enough (strong enough to hold its shape under normal gravity, but soft enough to be sculpted, able to be pulled apart and seamlessly put back together). Blacksmiths and welders do it every day (for iron anyway. For granite, I think there are a couple geology labs in the world that do it regularly).
We like to think of the world as fitting into nice clean distinct categories, but it isn’t the case. Not all animals are carnivores or herbivores. Not all politicians are liberals or conservatives. Not all planets are Earthlike or Jupiterlike. And not all substances are solids, liquids, or gases. Play-Doh has properties intermediate between those we associate with solids and liquids: Depending on circumstances and context, it might be more like either or neither. If this is a problem, it’s not a problem with Play-Doh itself, but with our classification system.
Those conditions are a little different, though. Steel and most other metals havd a granular structure consisting of metallic lattices (essentially a crystal) essentially up to the point that they become liquid, and if you seperate it below that temperature at ambient pressure the metallic structure will not fully reform, resulting in inclusions and voids, which also happen in welds when “full penetration”, i.e. fully melting the joint does not occur. Although they will soften at high temperatures below the liquid phase, they still demonstrate characteristics of solids, e.g. permanent deformation and residual stresses at equilibrium due to shear. Granite is similarly formed as a holocrystalline matrix of different minerals with different melting temperatures and so will plastically deform under stress at high temperatures but is not really amorphous.
Polymer and glassy materials, on the other hand, don’t generally have a long range structure despite local bonds, and wil flow under pressure or shear like a viscous liquid. There is a temperature called the glass transition temperature, T[SUB]g[/SUB], at which such materials behave in a pseudo-fluid fashion, but it is typically a fairly wide range of temperatures with a shallow change in modulus rather than the distinct phase transitions that crystalline solids display.
Stranger
Play-Doh® = Play - dough!!!
Wow! Some of my ignorance was fought (and defeated) today!
I know. Every time I make a cake in front of a 3 to 4 year old, they look at me and say “Play dough?” I guess many preschools are making their own play dough out of flour and water. Still, I always explain, “No little one. Flour and water make play dough, we don’t put sugar and eggs in play dough, I’m making cake.” Cue Rita Rudner.
Innit cool? And they add something – isn’t it kerosene? – to keep it from rotting. Also makes it inedible.
I don’t think that’s right.
I certainly ate a fair bit of Play-doh as a child, but I survived.
There are thousands of pounds of radioactive Play Dough buried in Narragansett Bay here in Rhode Island. Hasbro was attempting to make a glow in the dark version of the stuff without realizing they were using a radioactive coloring agent. Hasbro denies it, the US government denies it, but ask any Rhode Islander and they’ll tell you it’s true.
This, yet you never made the connection that Play-Doh == play dough?
William Poundstone gives the recipe for Play-Doh in his first Big Secrets book. It wasn’t hard to find – it’s in the patent (#3167440, issued 1965 – although they wer selling it earlier). As I read in an article a while back, Play Doh derived from a compound made for cleaning wallpaper. Its use as a tpy was realized later * See here, for instance:
http://www.uspto.gov/about-us/news-updates/patent-play-doh-issued-january-26-1965
Play-Doh does, indeed, contain other stuff – things to keep mold and bascteria from growing in it. Salt to make it unpalatable to kids. deodorized kerosene (!) to keep it pliable. And God Knows What to provide (as Poundstone puts it ) “that Proustian aroma”. But its mostly simply flour from wheat mixed with water.
*Just as the fact that people kept playing with a lump of boric acid mixed with silicone oil resulted in its being marketed as Silly Putty – “Silly” not just because of its humorous connotations, but also for “silicone”. Too many people claim to have invented this one. Silly Putty - Wikipedia
Salt isn’t just to make it unpalatable-- It’s necessary for the consistency. It’s the salt that makes the difference between paste and dough.
Isn’t there creme of tartar in it, too? I’m not sure what that does for it, but I know Mom’s recipe for homemade includes it.
Yes, but apparently there’s more in there than would be needed simply for that reason. When I make dough I put salt ion, too, but not enough that you can taste it.
Not listed in Poundstone or, I think, the patent.
The patent, interestingly, is surprisingly assertive about what kind of wheat you should make the flour from. Just any old wheat won’t do it. Or at least give you primo Play Doh.
Hey, everybody, what favorite ingredient would you like to see added to Play-Doh?
I would like to add some ketchup and mustard, or alternatively, some sweet-sour sauce. I’m sure many would appreciate some cayenne pepper. What would you like?
Well, THAT’LL be more effective than salt at keeping kids from eating it.