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Underground > Laboratory >

The Physics of Lather

19:50 / 25.12.02

So, I was just in the shower. I squeezed some shampoo gel onto my hands and rubbed them quickly together, making lather. After I rinsed, I squeezed some conditioner onto my hands and rubbed them quickly together, and no lather appeared. Hm.

I put some water in my hands, rubbed quickly--a bubble or two, which burst immediately, but no real lather. Hair gel looks like the shampoo gel, but it doesn't lather. Hand soap lathers; hand lotion doesn't. Tap water doesn't lather; seawater does. People sweat doesn't lather; horse sweat does (or so I'm told).

What's going on here? What is lather, anyway? If I were building a machine that needed lubrication, would this question be at the forefront of my mind?

05:31 / 27.12.02

Lather is a bunch of (long-lived) bubbles. See also: foam.

Bubbles are usually formed by either soaps and detergents, or by proteins - sea foam, horse sweat, egg whites or cream beaten to form peaks... It's a function of how the surface tension of water gets altered by the addition of a protein or detergent.... and, as any cook will tell you, the addition of an oil will inhibit the formation of lots of bubbles - any egg yolk at all will prevent getting a good meringue. (Don't ask me why something with the fat content of cream will froth like it does - I haven't a clue.)

Aethelwine Jedi

13:21 / 27.12.02

I thought that friction would figure in to it somewhere, given that soap only lathers when stirred up by whatever means.
But anyhoo, here's an explanation of soap bubbles that I could understand.

15:27 / 27.12.02

Here, if you want to know about the science and art of frothy milk.

One of the key terms seems to be "colloid" or "colloidal" - referring to a substance made of two materials mixed together, in this case, a liquid and a gas. It's a related term to "emulsion" which I *think* refers to a similar mixture of two kinds of liquid, but may also have more to do with the shape of the particles being mixed (with a colloid being made up of little round bits suspended in a big mass - fog is a colloid, and so is "colloidal silver," a mineral supplement used by the alternative medicine crowd).

Milk gets frothy only when it is steamed, which "denatures" the molecules, lowering the surface tension (which is why it tastes different than cool milk). If it boils, the proteins not only decrease the surface tension, they all clump at the top of the pot, ruining the foam.

Here, on the other hand, is a Straight Dope column on foam in beer. In particular, on whether or not chicken hearts are used as a foam stabilizer. Here's a similar one on bubbles in Guinness (and why some of them sink rather than rise). Cecil also has things to say about soap, too. (Follow the link within that article, and brace yourself.)

This page on surfactants may help answer some of your questions about why shampoo will lather but conditioner won't.

But if you'd like some more scientific stuff on how bubbles are actually formed, I think this page here, Soap Films Made Easy by Dr. Maarten A. Rutgers might be your best bet.

20:26 / 27.12.02

Hey, it's fun killing detergent lather with real soap!! Don't know how this works, would be interested to learn.

Less searchable M0rd4nt

12:26 / 28.12.02

As I understand it, the soap reduces the surface tension of the water so the bubbles can no longer maintain their integrity-- they get weaker and pop.