PDA

View Full Version : Corn Starch and Water



aboard_epsilon
12-17-2009, 03:47 PM
All sorts of tricks can be done with this stuff ..
there are various videos on you tube with speakers and vibration devices .

here's just one vid to show the properties of the stuff.

http://www.youtube.com:80/watch?v=amfjWWMg9c0&NR=1

but other than party tricks ..I'm sure we could put it to a more ingenious use ..


HE HE ...now you can have sleepless nights thinking about it :D

All the best.markj

Carld
12-17-2009, 03:51 PM
Corn starch, water and bouillon makes a gravy but I don't think I would try to walk across it.

Tony Ennis
12-17-2009, 04:21 PM
OMG! Add Carl's post to the Bachelor Food thread!


-=-=-

The video is freaky. I remember similar from grade school. No one knew if the stuff was a solid or a liquid.

Evan
12-17-2009, 04:37 PM
It is a thixotropic liquid, meaning it reacts in a non-linear fashion. So is ketchup but in the opposite way. Ketchup turns into a gel if not disturbed and thins if shaken. Corn starch and water becomes a liquid if not disturbed and gels when it is.

daveo
12-17-2009, 04:39 PM
Thats funny!

strokersix
12-17-2009, 04:43 PM
I think the term is rheopectic for the shear thickening effect.

Evan
12-17-2009, 04:49 PM
You are correct but so is thixotropic. It's a more general term.



Flow can induce reversible and irreversible structural changes in dispersions. The analysis of flow effects on microstructure and rheology remains one of the challenging problems in colloid science. The rheological manifestation of flow-induced structural changes is a variable viscosity. If the changes are reversible and time dependent, the effect is called thixotropy.

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5F-4TK92H1-4&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1139963538&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=2c68e153dcdf77eadb0d4a7d0ec18457

JCHannum
12-17-2009, 05:18 PM
Ketchup is thixotropic, it becomes more fluid when agitated. Cornstarch and water behaves in the opposite manner and is isotropic, it becomes firmer when agitated

Rich Carlstedt
12-17-2009, 06:16 PM
What you see is commonly refered to as a "non-newtonian fluid" characteristic
Liquid Plastic, like Polyethelene at 300 degrees becomes a non-newtonian fluid as well.
Say you have a half full pail of it, and pour some more on top (in the middle) it will not surge right to the bottom of the pail as water does ( or seen in a waterfall), but instead will flow out horizontally to the sides of the pail, and then down to the bottom.
You might say that the friction of one molecule against another is so great, that it is acutually easier to flow against a non plastic surface like air or the steel pail.
This makes mixing Plastics extremely difficult and gives the polymers memory of what they have been exposed to.
You might say they do not heal their wounds easily.

Rich

Evan
12-17-2009, 06:52 PM
Ketchup is thixotropic, it becomes more fluid when agitated. Cornstarch and water behaves in the opposite manner and is isotropic, it becomes firmer when agitated


Isotropic means the same in all directions, without a preferred direction. It has nothing to do with non newtonian fluids.

boslab
12-17-2009, 06:54 PM
Ketchup is thixotropic, it becomes more fluid when agitated. Cornstarch and water behaves in the opposite manner and is isotropic, it becomes firmer when agitated
We prefer custard to play with over here, tastes better!
you can al;so make some interesting body armour out of non-newty stuff
throw in some synthetic spider silk and itll stop a bus lol
mark

camdigger
12-17-2009, 07:01 PM
Shear thinning fluids are a fact of my life as a drilling engineer. Drilling fluids and cements are intentionally designed to be shear thinning and to develop high early gel strengths for special applications like "thief" zones for example.

FWIW, cements are also designed for setting/curing times. It's a bit tricky though, screw up the mix, miss-guess the temperature, or use an incompatible mix water and someone ends up chipping cement out of their equipment...:eek: Bin dere....:rolleyes:

JCHannum
12-17-2009, 08:14 PM
Isotropic means the same in all directions, without a preferred direction. It has nothing to do with non newtonian fluids.

Not so, this is just one of many explanations of the difference between isotropic fluids and thixotropic fluids. It just happens that cornstarch & water and ketchup are among the most common fluids displaying these characteristics.

http://fatlion.com/science/isotropy.html

Evan
12-17-2009, 08:24 PM
The website you linked to is incorrect in the use of the term. When applied to a fluid the term Isoptropy simply means that the fluid has the same measured characteristics in all directions.

JCHannum
12-17-2009, 08:50 PM
You will have to convince a lot of people of that. It is a commom term in that application. The ketchup and cornstarch & water demonstration is standard demonstration used to display the two different characteristics.

A quick search of isotropic fluid+cornstarch will turn up dozens of hits defining the terms.

Mike Nash
12-17-2009, 09:13 PM
You will have to convince a lot of people of that. It is a commom term in that application. The ketchup and cornstarch & water demonstration is standard demonstration used to display the two different characteristics.

A quick search of isotropic fluid+cornstarch will turn up dozens of hits defining the terms.
I'm breaking my own rule of not jumping in, but :):

A web search on "loose" or "then" will find so many examples of their misuse that web searches for proof by usage are pointless. "Then" being used in place of "than" and "loose" in lieu of "lose" has become the norm, especially in forums and blogs, but also in printed media. It's so bad that when I see the word "lose" used properly it looks wrong. If we can't even get the common words right, how can we hope to keep up with the technical terms?

I'm not meaning to pick on anyone in particular BTW.

Evan
12-17-2009, 10:07 PM
What Mike said. So many sites use other sites as their reference (especially wikipediea) that misuse and misinformation is an epidemic. Then sites that copy other sites are themselves copied with slightly different wording and spread the misinformation even further.

The first site you posted is copied word for word by another 6 unrelated sites for the phrase "Isotropy is the property of a fluid to become firm when agitated". However, those are the only hits for that phrase. If it were an accurate description I would expect to find many more hits for the phrase.

In fact, if I reduce the quote to just "Isotropy is the property of a fluid" I only get 8 hits which shows that Isotropy isn't considered to be a property of a fluid. If I reduce it to just "Isotropy is the property" I get 19,700 hits that indicate that it most certainly is a property of something, just not fluids.

JCHannum
12-17-2009, 10:26 PM
If you look for a phrase, you will likely find a phrase. If you look for a wordset, you will find that wordset.

Isotropic, dilatant and rheoplastic are all terms used to describe non-newtonian fluids that behave as the cornstarch & water mix. Thixotropic is the term used to describe fluids that behave in the opposite manner.

gmatov
12-17-2009, 10:33 PM
The rest of the thread aside,cornstarch, water and bouillon can NEVER be called "gravy". Pale imitation of it, the stuff you buy in a jar, and I wouldn't eat that.
If you are too lazy or uninformed as to how to brown some flour in the fats from the meat you are eating, flavour base if needed of that species of meat, water to make it of gravy consistency. Salt and pepper to taste. I like my gravy. My kids insist I make the gravy at holidays.

Cornstarch is strictly limited to thickening the juices in Chinese dishes, in my opinion. Pies, though I have never baked a pie, keeping the juices thick enough to fork up rather than have to eat pie with a spoon.

The scientolology argument between Evan and his major antagonists, above, I will stay out of.

WIKI, when we grow a branch of Exenologists who know everything, and can proof WIKI, and allow me to read more than "common knowledge", "everybody knows that", "babies come from under cabbage leaves", or the "stork" brings them.

Exenologist may be the wrong term. Who can remember a Sci-Fi novel about a scientist on a voyage who found an entire colony dead with pits and holes in their skeletons, and determined it was beryllium poisoning? Could the name have been "Polymath"? Poul Andersen comes to mind. About 40 years ago. May still have it.

Cheers,

George

Doozer
12-17-2009, 10:35 PM
Evan is right.
Just like wood has different strengths in different directions.
It is called anisotropic.

--Doozer

wierdscience
12-17-2009, 10:39 PM
Corn starch, water and bouillon makes a gravy but I don't think I would try to walk across it.

I have seen gravy thick enough to walk across ,infact I have also seen gravy thick enough to be worked dimensionaly with hand tools and made into a liveable structure.I've been to Waffle House:D

Evan
12-17-2009, 10:41 PM
Thixotropy defines both types of fluids. Ketchup exhibits positive thixotropy and cornstarch and water exhibit negative thixotropy.


From NIST:



Time-dependence includes those effects associated with transient flow conditions as well as those effects associated with irreversible changes that result from shear history.



negative thixotropy [anti-thixotropy]

A reversible time-dependent increase in viscosity at a particular shear rate. Shearing causes a gradual growth in structure over time.

http://ciks.cbt.nist.gov/~garbocz/SP946/node9.htm


Isotropy is not a listed characteristic.

Reoplastic is a different characteristic, not the opposite of thixotropy.



rheopexy

An effect by which a material recovers some of its pre-sheared viscosity at a faster rate when it is gently sheared compared to when it is allowed to stand. Not to be confused with negative thixotropy.

Ken_Shea
12-17-2009, 10:54 PM
but other than party tricks ..I'm sure we could put it to a more ingenious use ..


Sure can, that or fresh cow manure can be used for binder in charcoal, your choice :D

Doozer
12-17-2009, 10:55 PM
Ya don't ever order steak at Waffle House, fool!
Waffles and maybe grits, that's it.
--Doozer

http://photos-g.ak.fbcdn.net/photos-ak-snc1/v2626/115/85/1376627819/n1376627819_331652_3135703.jpg

tryp
12-18-2009, 07:05 AM
Doozer! There is a Waffle House in Buffalo? Or is that a picture from out of town?

aboard_epsilon
12-18-2009, 07:35 AM
but other than party tricks ..I'm sure we could put it to a more ingenious use ..

Then the autistic guys here start arguing what scientific name they can put on it ..so we see
Rainman -V - Rainman...and whose on first base..yadda yadda yadda...how many microbes you can fit on a pin head ....and i was there before Einstein. :D :rolleyes: :( :eek:

http://www.youtube.com/watch?v=hnzHtm1jhL4

all the best.markj

john hobdeclipe
12-18-2009, 08:28 AM
Regardless of what you call the stuff, is that demonstration seen on the vid from the OP actually possible?

Evan
12-18-2009, 09:50 AM
As usual I didn't start the argument.

Yes, the demostration is actually possible, I have done similar. When my children were growing up I used such demonstrations to teach them science. We regularly performed "experiments" that always involved a component of fun or wonder.

One particularly popular "experiment" was to demonstrate how the rays surrounding craters on the moon were formed. A cookie sheet is filled with a 1/4" layer of white flour which is then spinkled very lightly with cinnamon. Then small pebbles are dropped or even thrown at a sideways angle to make craters which splash out material just like lunar craters.

lazlo
12-18-2009, 09:54 AM
Then the autistic guys here start arguing what scientific name they can put on it ...

LOL! That made me spit coffee on my keyboard Mark! :D


I have seen gravy thick enough to walk across ,infact I have also seen gravy thick enough to be worked dimensionaly with hand tools and made into a liveable structure.I've been to Waffle House

When I first went to college, I decided to make pot roast and gravy. How hard can it be, right?

So I strain the beef juices into a pot, mix-up a little slurry of corn starch and cold water like I had seen my Mother do countless times growing up, and start adding it to the simmering beef juices. But nothing happend. So I figured I must not be using enough corn starch, so I mix up another, stronger batch, and do it again. Still runny.

After repeating this several times, I'm getting frustrated, and hungry. So I take the pot off the stove, and call my Mom to ask what I'm doing wrong. By the time she explained it to me, the mixture had cooled, and solidified harder than reinforced concrete. The spoon was sticking up in it like a gravy popsicle :p

pcarpenter
12-18-2009, 10:50 AM
Gotta love that delayed thickening effect from cornstarch....makes it a real guessing game. I get Chinese carry out from a place near work from time to time. Every once in a great while the stuff comes out like vegatables mixed in expoxy. I feel for them though...it's hotter then heck when it's still in the wok and you know what you got later. However, most of the Chinese cooking shows I saw had a cornstarch and water mix in a little pitcher so that the ratio was pre-determined-- presumably to prevent such accidents.

Another accident to avoid is adding corn starch or flour to anything hot without mixing it in cold water first. You get tiny dumplings :-)

Did any of you watch this other video that came up after watching the first one linked in the first post?

http://www.youtube.com/watch?v=NKxKVpHZe5Q

Forrest Addy
12-18-2009, 08:06 PM
We need some distionary work here

boslab
12-18-2009, 09:39 PM
i'm going too piss myself if anybody mentions Autistic arguments, hilarious, as for cornstarch, stuff that its custard you want man, stick it over your tacos for extra effect [ i'm biased my mum was a school cook and i keep seeing odd patterns even when i'm sober ]
Thank heavens for Scotch!
Oh the pain
[just had 6 teeth out, the British disease so dont try this at home]
mark