Problem Solving Strategies

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Ashish Ahuja

10:27 AM

@JohnRennie hi

John Rennie

@AshishAhuja Hi :-)

Ashish Ahuja

actually sorry, I realised the answer to my question right now as I was typing it out.. my bad.

John Rennie

OK :-)

Ashish Ahuja

11:01 AM

@JohnRennie hi, in most surface tension demonstrations, soap solution is used instead of water. For example, consider dipping a loop of wire in (a) soap solution and (b) water. From the demonstrations we know that (a) will form a nice film while in (b) we won't get a film (with ease). (1) Soap solution has a lower surface tension, which means that the force it exerts on the loop of wire is lower (and by newton's 3rd law force on the film is also lower), which seems to imply that a soap solution will have a smaller tendency to form a film with the loop than water.

John Rennie

Hi :-)

The formation of a soap film is not due to surface tension.

Suppose you have a thin film of water. Films like this have an instability called a Rayleigh instability that means the thickness starts to vary. Some parts of the film will decrease in thickness while other parts will increase in thickness.

Where the thickness decreases the two sides of the film will approach each other, make contact and fuse, and hole develops in the film. Once the hole has been formed surface tension quickly enlarges the hole and the film of water collapses.

OK so far?

Ashish Ahuja

I didn't understand what you meant by "Where the thickness decreases the two sides of the film will approach each other, make contact and fuse, and hole develops in the film."

John Rennie

Let me draw a diagram ...

On the left we have a uniform film of water. The film is shown edge on.

The Rayleigh instability causes the thickness to start to vary as in the middle diagram.

Ashish Ahuja

John Rennie

If the two surfaces of the film touch in the middle they will fuse together and we get a hole in the film as shown on the right.

Does this make it clearer what happens?

Ashish Ahuja

11:17 AM

ah, got it. So the reason why soap films are more stable is because they are less prone to the rayleigh instability?

John Rennie

No.

It's because the soap molecules adsorb at the air/water interface, and because they are negatively charged they give the two surfaces a negative charge.

That means the surfaces repel each other.

Ashish Ahuja

by "negatively charged" you are talking about the hydrophobic ends just inside the water surface, not the lyophilic ends sticking out, correct?

John Rennie

A soap molecule is the salt of a carboxylic acid.

Ashish Ahuja

yes, and the carbon chain sticks out while the carboxylic acid part stays in the solution (talking about the surface).

John Rennie

At the air water surface the alkyl chain sticks out into the air and the COO⁻ group sticks into the water.

@AshishAhuja Yes

Ashish Ahuja

11:21 AM

so you are essentially saying that the COO- repel each other, right?

John Rennie

This means it is hard to desorb the ions from the surface because you would have to pull the alkyl chain back into the water, where it doesn't want to go. So you end up with the interfaces having a pretty stable negative charge density.

@AshishAhuja and yes, it is simply that the two negatively charged surfaces repel each other and therefore they will not fuse.

Ashish Ahuja

the alkyl chain is called the lyophilic end (or hydrophobic), and the carboxylate is called the lyophobic end (or hydrophilic), or am I getting my terms mixed up?

John Rennie

I can't remember what lyophilic and lyophobic mean. They are old terms not used these days. Colloid scientists just say hydrophobic and hydrophilic.

Ashish Ahuja

I get it now, thanks. This was a neat explanation.

John Rennie

Apparently lyophobic = hydrophobic and lyophilic = hydrophilic

Ashish Ahuja

11:26 AM

hmm yeah, I'll check that out once again then..

John Rennie

The stability of a bubble is actually only a kinetic stability. Bubbles are thermodynamically unstable and exist only due to the ions forming a bariier to the edges of the film merging.

You've probably found that if you touch a soap bubble it bursts.

That's because your finger makes a hole in the surface and once a hole is created the film immediately collapses.

Ashish Ahuja

yes, and I've even had peculiar experiences where with some soap bubbles, when you put a finger in, and you take it out nothing happens to the bubble.. although these were not standalone bubbles, but bubbles formed on the palm.

John Rennie

That will happen if your finger is coated in the soap solution.

Ashish Ahuja

ah yes probably. It was while washing hands.

John Rennie

Typically your finger only makes a hole in the film if your finger is dry.

Ashish Ahuja

11:30 AM

does surface tension also not help keeping the bubble stable? Because it counters the excess pressure inside the bubble?

John Rennie

Well the bubble expands or contracts until the pressure inside balances out the surface tension.

Ashish Ahuja