What is a gel that turns to liquid with minor agitation?

May 30, 2009 2:48 AM

What do you call a gel that turns into a liquid with very minor agitation/pressure?

I have an alcohol hand cleanser which, when you push the pump, comes out like a gel (like thick shampoo or the like), but when you start rubbing your hands together, turns into a regular fluid, like straight alcohol. I remember seeing something on a science show on TV which talked about this type of gel, which had a special name. What was that name?

(Note: telling you the name of the hand cleanser probably wouldn't help, as it's Japanese)

posted by Bugbread to Science & Nature (10 answers total)

Best answer: Thixotropic. Honey is a good example.
posted by shelleycat at 2:50 AM on May 30, 2009

Actually, reading that wikipedia article further, your gel may be pseudoplastic. It's a bit hard to tell without playing with it directly.
posted by shelleycat at 2:53 AM on May 30, 2009

Could also be hygroscopy, e.g. the cotton candy effect.
posted by knave at 3:52 AM on May 30, 2009

It is hygroscopy. Hand cleansers are made with a high concentration of ethanol, which is hygroscopic, so they absorb moisture from your skin. Note that if you squeeze a dollop of hand cleanser onto your hand, it is a relatively viscous gel when it leaves the container, but quickly turns into a runny liquid just by sitting on your hand. Try it - you don't need to rub your hands together to produce the effect you describe.
posted by ssg at 6:17 AM on May 30, 2009 [1 favorite]

Response by poster: Thanks for the answers.

Reading through the descriptions of the various suggestions on wikipedia, it can't be hygroscopy, because the TV show showed another example, which was a soy sauce gel which could be converted to liquid soy sauce by stirring it with chopsticks, which wouldn't have enough moisture for the hygroscopic effect. And it just seems radically different from the examples given for pseudoplastics. The explanation of thixotropism seems to match up pretty well.
posted by Bugbread at 6:36 AM on May 30, 2009

Many paints are made to be thixotropic, for the same reason as your hand cleanser. The paint stays on the brush and goes on the wall without dripping, but it becomes more liquid as you brush it out into an even coat.
posted by weapons-grade pandemonium at 11:17 AM on May 30, 2009

If you read the wikipedia stuff it points out that paint is pseudoplastic even though people always think it's thixotropic. I think the main difference is that thixotropic stuff stays liquid once it's had enough pressure whereas pseudoplastic becomes increasingly liquid with increasing pressure, then goes back again when pressure is removed, but I'm not entirely sure if the part about staying/not staying liquid is correct. The time vs amount of pressure thing is right though, and thixotropic makes more sense for hand gel because then it will always become liquid if you rub long enough even if you can't rub very hard.

And the gel probably is hygroscopic as well just because of what it's made of but that doesn't explain the effect that bugbread is interested in.
posted by shelleycat at 3:22 PM on May 30, 2009 [1 favorite]

We always called it rheopectic, when I was working on gels that would liquify under shear.
posted by atchafalaya at 11:40 AM on May 31, 2009

From your description, we can't tell which kind of shear-induced, viscosity-reducing behaviour the gel has. It could be thixotropic or pseudoplastic. The difference is subtle: thixotropic gels get more liquid over time under a (constant) shear pressure; shear thinning (pseudoplastic) gel get more liquid as more shear stress, more pressure, is applied. Under constant stress, psudoplastics will remain at the same viscosity, thixotropics will continue to get less viscous.

Pseudoplastics are much more common that true thixotropics. Seeing as how your hand gel is probably based on glycerin and/or gelatin, it's almost certainly pseudoplastic.

Rheopectic is the opposite of thixotropic; those are gels that get more viscous over time under shear pressure. They're quite uncommon. Dilatants are the opposite of shear-thinning liquids, they get thicker, more viscous under increasing pressure. Cornstarch in water is the canonical example, silly-putty is another.
posted by bonehead at 9:00 AM on June 1, 2009

Response by poster: The gel I am thinking of (and the soy sauce) were very, very unusual, so while pseudoplastics are probably quite common, that almost rules them out (because this was so very different than the usual stuff). Shelleycat's comment, that thixotropic liquids stay liquid, combined with bonehead's comment that thixotropics continue to get less viscous, makes it most likely that it's thixotropic.

Thanks, all!
posted by Bugbread at 8:43 PM on July 1, 2009