Mixing fluids

  • Thread starter Mk
  • Start date
  • #1
Mk
1,984
3
If I stir cream into my coffee, what prevents me from stirring counter-clockwise, and the cream seperating again?
 

Answers and Replies

  • #2
Tide
Science Advisor
Homework Helper
3,076
0
There are several more or less sophisticated answers one can give to such a question but basically it's the same reason why you cannot ungrate cheese by reversing the motion of your hand on the grater.
 
  • #3
Danger
Gold Member
9,607
249
I saw a right freaky demonstration a few decades ago. There were two very specific chemical combinations that did separate when the stirring motion was reversed. I have no idea where I saw it, or what the compounds were. At the time, I thought that it was a serious experiment, but I suppose that someone might have been reversing the film. Anyone out there familiar with it?
 
  • #4
Mk
1,984
3
Tide said:
There are several more or less sophisticated answers one can give to such a question but basically it's the same reason why you cannot ungrate cheese by reversing the motion of your hand on the grater.
If I have a pool of water, make a wave, then put opposite wave on it, the pool will be tranquil again.
 
  • #5
Bystander
Science Advisor
Homework Helper
Gold Member
5,203
1,226
Danger said:
I saw a right freaky demonstration a few decades ago. There were two very specific chemical combinations that did separate when the stirring motion was reversed. I have no idea where I saw it, or what the compounds were. At the time, I thought that it was a serious experiment, but I suppose that someone might have been reversing the film. Anyone out there familiar with it?

Yes. How desperately did you want a reference? And, strictly speaking, it wasn't/isn't perfectly reversible.
 
  • #6
Tide
Science Advisor
Homework Helper
3,076
0
Mk said:
If I have a pool of water, make a wave, then put opposite wave on it, the pool will be tranquil again.

I don't think so. Once the wave is generated and propagating away from the source your "opposite wave" can't even catch up let alone cancel it out.
 
  • #7
Mk
1,984
3
Mhmm, you're right. My plans for world domination have been foiled again! Curse you!!
 
  • #8
32
0
The cream has basically diffused into the coffee (it ain't black no more.) You can't reverse time by reversing your hand movement so it won't return to its original state of being seperate.
 
  • #9
Danger
Gold Member
9,607
249
Thanks, Bystander. I don't actually need a reference; I just wanted to make sure that I hadn't either imagined it or been taken in by a bogus demonstration. I was just a wee innocent lad at the time. Knowing that someone else is familiar with it is sufficient. :approve:
 
  • #10
Integral
Staff Emeritus
Science Advisor
Gold Member
7,201
56
Danger said:
I saw a right freaky demonstration a few decades ago. There were two very specific chemical combinations that did separate when the stirring motion was reversed. I have no idea where I saw it, or what the compounds were. At the time, I thought that it was a serious experiment, but I suppose that someone might have been reversing the film. Anyone out there familiar with it?
I believe that the demo you are referring uses glycerin to fill the region between a pair of concentric cylinders. A streak of dye is injected into the glycerin then the interior cylinder is slowly rotated, apparently mixing the dye into the glycerin. Reversing the motion of the inner cylinder "unmixed" the dye. It returns nearly perfectly to its original steak like condition.

This is actually a demo of laminar flow of a fluid, the dye is not really mixed, just stretched out. Over time the dye will diffuse into the glycerin, truly mixing it.
 
  • #11
Danger
Gold Member
9,607
249
Aha! Indeed, Integral, that is exactly the thing that I was thinking of. Most of the details had leaked out of memory. Thanks.
 
  • #12
Mk
1,984
3
Integral said:
This is actually a demo of laminar flow of a fluid, the dye is not really mixed, just stretched out. Over time the dye will diffuse into the glycerin, truly mixing it.
I saw it on television, then they explained how it realted to quantum mechanics. How?
 

Related Threads on Mixing fluids

Replies
1
Views
6K
Replies
2
Views
4K
  • Last Post
Replies
6
Views
2K
  • Last Post
  • Optics
Replies
2
Views
2K
  • Last Post
Replies
2
Views
2K
Replies
10
Views
4K
  • Last Post
  • Optics
Replies
12
Views
5K
Replies
2
Views
1K
  • Last Post
Replies
3
Views
3K
Replies
6
Views
8K
Top