Silly water structure question

Factual Questions
1

I’m sure I should know this, but I would appreciate the assistance. Water is fascinating, isn’t it? What I’m wondering about specifically is water being absorbed by fabric and then wrung back out. What are the mechanics of that? How does the water distribute itself throughout the fabric and then “reassemble” itself when the fabric is squeezed? If it’s all spread out in the fabric, what am I feeling that makes it feel “wet”?

OTOH, if I boil pasta in water, the water is absorbed for good and can’t be squeezed back out into the same water that went in. And some things don’t allow water to spread through them at all.
No, as a matter of fact I am not smarter than a fifth grader.

2

Fabric isn’t a solid thing. It’s composed of woven threads and those threads, in turn, are composed of fine fibers twisted together. Water simply fills the spaces between these fibers. When you wring out the fabric, you squeeze the fibers more tightly together, which reduces the space that is available to be filled with water, so the excess runs out. When you touch a piece of wet fabric, you feel the wetness because the water isn’t bound up inside anything, it’s free to flow around and between the fibers.

Pasta absorbs water on the molecular level–it fills the spaces between protein molecules (which are much, much smaller than the spaces between fabric fibers), and so it takes a lot more pressure to force the water back out. But, if you squeeze it enough, you can press the water back out again. You can tell the water isn’t permanently bound up inside the pasta because if you leave it out long enough, it will dry out again.

3

What makes the water want to combine itself back together in a solid stream when the fabric is squeezed? Like molecules are attracted to one another?

4

Sort of. Water (and most other liquids to one extent or another) have a property called cohesiveness, which is a fancy way of saying it wants to stick to itself. In the case of water, the molecule has a polarity; that is, one side of it is slightly positively charged, while the other is slightly negative. The positive side of one water molecule tends to try to stick to the negative side of another. Since all the molecules are trying to stick to each other in this fashion when you have a bunch of them in one place, you tend to get either continuous streams or discrete globules of water depending on the dynamics of the situation. When water boils, the kinetic energy imparted to the molecules is sufficient to overcome this cohesive attraction, and the individual molecules separate and spread out, becoming a vapor.

5

The water molecule is highly electrically polarized. THe molecule is sort of L shaped with the oxygen atom at the corner and the hydrogen molecules at the ends of the L.

The hydrogen’s electron tends to be over next to the sygen atom leaving the hydrogen nucleus, which is positive, sticking out at the ends of the L. The corner the L is composed of a bunch of the oxygen’s orbital electrons. So one end of the molecule is negative and the other end positive. This results in a strong tendency for the water molecules to stick together.

That also explains why the water soaks into the cloth. Water molecules flow into the interstices by capillary action and pull other water molecules along behind them by electrical attraction.

6

I guess if Q.E.D. and I both say the same thing it must be believable. Right? :smack:

7

Also, this property is what lets PTFE membranes (Gore-Tex) work. Because the water molecules stick together in liquid phase, they’re going to form droplets of a certain size or greater. If you make a membrane with holes that are smaller than that size, liquid water won’t go through them. However, water in gas phase can, because it doesn’t “clump up” in droplets. This is why you can buy a jacket that is “breathable” (lets your sweat evaporate through it), yet waterproof.

8

Isn’t it usually a liquid stream?

9

Sorry, a globby stream of liquid. :slight_smile:

Is this attraction dealio what keeps water in a dome above the rim of a glass if you overpour it very carefully?

10

Yep.

11

Actually, liquids and solids (I mean the ones that aren’t giant single crystals or crosslinked like a mass of epoxy) generally hang together because they all attract each other. The polarity of a molecule (like described for water above) influences how much it likes other molecule types, so for instance what liquids will mix with each other and what liquids separate like oil and water. But even pretty dissimilar materials want to cling together. Water doesn’t like wax much, but if you spritz water on a candle it doesn’t all just fall off.

The biggest reason most things stick together is the van der Waals force. This is what makes adhesive tape work, for instance. If it weren’t for this, there would be very few objects.

12

I thought that a general theory of adhesion was still under development.

13

Well, that’s what I was wondering about the water distibuting itself throughout the cloth. If the water is attracted to itself, why would it “break up” to flow between threads.

14

Because it’s also attracted to the threads, and the threads have a lot of surface area.

15

It’s a balancing act. Some of the polarized water molecules are attracted to polarized places on the surface of the threads and pulled to them. The water doesn’t actually “break up”, it’s just that in the nature of things there are always some water molecules having one of the electrical poles unoccupied and it is attracted to the surface of the threads. The resulting capillarity keeps pulling the molecules into the fabric because when one molecule is attracted to a spot a little further along the thread it pulls other water molecules along behind.

16

Then how does polymethacrylate work? This is the chemical that is inside baby diapers.

17

Just wondering…isn’t pasta a majority of carbohydrates, and very little (if any) protein? - Jinx

18

Pasta (from wheat or rice) has proteins which provide the structure that keeps the polysaccharides and simple sugars together; without this, pasta would just dissolve in warm water like cane sugar. Dry white flour pasta is approximately 12% proteins, primarily the glutens (gliadin and glutenin). These are not “complete proteins” (i.e. they do not provide all nine essential amino acids, in particular lysine) and thus are not considered credible sources of dietary protein, but they do contain proteins.

Stranger

19

Yes, but pasta is still mostly carbs. The carbs are very long chains (muuuuuuuuuuch longer than table sugar and non-linear); if you cooked them long enough (specially with the addition of a bit of acid) they’d dissolve, but it’s similar to the difference between trying to dissolve a spoonful of table salt or a chunk of table salt (in the case of pasta, the difference is bigger than in the case of salt). Proteins hydrolize and dissolve too.

I read a fascinating article in an ACS journal once, about “why pasta sticks”.

Picture that O is a ring of glucose (whole or missing a bit in order to link to another ring). Table sugar is OO. A molecule of water can break the bond between the two rings (hydrolisis), sticking an -OH to one ring and the +H to the other ring.

Food carbs (potatoes, pasta) are branched strings of OOOOOOOOOOOOOOOOOOOO… The most common bond between consecutive rings is the same as in table sugar; it can be hydrolized easily by regular water; more easily by acidic water; even more if certain enzymes that human bodies do have are present.
The bonds where a branch attaches to the “main one” are the same in chemical nature as the regular ones (polyeter, do not mistake with polyester) but they happen in a different position. A ring of glucose has 5 positions from which in theory it could bind to another. These bonds can still be broken by hydrolisis, acidic hydrolisis or by a different set of enzymes the human body has. Their regular hydrolisis is more difficult that for the straight lines, though, because they’re physically harder to reach (among other things, because as they happen at branch points, there’s three rings very close).

Fiber is mostly carbs, too, but carbs where the branching points are in a position for which the human body has no enzimes. Since we can’t absorb whole chunks of carbohydrate (they need to be broken down into small-enouogh chunks), we only can absorb the carb portions of “fiber” that have managed to break apart during the acidic digestion in the stomach. A straight line broken apart from the fiber is chemically and spatially identical to one broken from non-fiber carbs… it’s digestible.

OK, so you have your pasta. Cooked pasta. Part of the water in the pot (the part that got absorbed) has become part of the noodles themselves, by hydrolizing the carb (and aA) chains and incorporating itself into them.

If the pasta is left to rest warm, it sticks. How? By reverting the hydrolisis across noodles. A string of carbs from Noodle1 is in contact with a string of carbs from Noodle2, they get in the right position, have the necessary amount of energy (because the pasta is still warm), and they lose a molecule of water, creating a bond Noodle1-Noodle2. A single bond would be so weak that you wouldn’t even notice it, but if there is a large enough amount it becomes noticeable. This is also the reason why sometimes you think you’ve drained the noodles and after resting you can see more water: no, it’s not your imagination, there IS more water. The water that has (using the OP’s expression) squeezed itself back out of the noodles (some of it was also in the interstices between molecules, same as for the clothing).

The re-bonding happens also if you’re moving the noodles while they cool down, but it happens within each noodle, no noodle-crossing.