f@*g surface tension... how does that work?
August 3, 2010 4:36 PM   Subscribe

Which substance is more efficient at destroying water surface tension: detergent or oil? This would be for a hypothetical trap for pest insects.
posted by crapmatic to Science & Nature (14 answers total)
Oil doesn't mix with water and will float on the surface of the water. A surfactant lowers surface tension because it has both hydrophobic and hydrophilic properties.
posted by Cool Papa Bell at 4:40 PM on August 3, 2010 [1 favorite]

Slightly off-topic:

For fruit flies I had great luck by making a paper cone, cutting a hole just a little larger than the fly, and placing it narrow side down in a jar. The jar had an inch of beer in it before sealing the cone all around with tape.

I didn't have to bother with surface tension. I woke up 1 day later with more than 70 dead in each trap.
posted by just.good.enough at 4:41 PM on August 3, 2010 [1 favorite]

I don't know firsthand either way, but Cook's Illustrated tested homemade fly traps using cider vinegar and Dawn detergent back in May, as I recall. They claimed it works well.
posted by ifjuly at 4:47 PM on August 3, 2010

not sure if you're looking for this either, but for my fruit fly prob, I put an inch of apple cider in a wide-mouth wine glass and added a tiny bit of dish soap to it and swirled it around. Decent dead population a few hours later.
posted by bunny hugger at 4:47 PM on August 3, 2010 [1 favorite]

We use cider vinegar and oil on the top, rather than soap. Works very well.
posted by RedEmma at 4:55 PM on August 3, 2010

Cook's Illustrated tested homemade fly traps using cider vinegar and Dawn detergent back in May, as I recall. They claimed it works well.

Just did this. Killed all the flies within hours.
posted by miss tea at 4:57 PM on August 3, 2010

Response by poster: (This is not for fruit flies, but thanks)
posted by crapmatic at 5:02 PM on August 3, 2010

Strictly speaking, I think oil is a surfactant as it adsorbs to the surface of water, though many surfactants have chemical properties similar to detergents.

Oils are, however, entirely hydrophobic (does not like to dissolve in water), whereas a detergent surfactant is polar, having a hydrophobic (water-fearing) and hydrophilic (water-loving) end.

The hydrophilic ends of detergent molecules adsorb to a layer of polar water molecules, leaving the hydrophobic ends sticking out into the air.

In a way, you have an "oil"-like layer on top of water when using detergents.

Detergents work to reduce surface tension by interfering with hydrogen bonding between water molecules.

Hydrogen bonding between water molecules creates a "net" of sorts, providing a repulsive tensile force that allows some insects to walk on the water surface.

Since oil will form a layer on top of water and doen't disrupt the hydrogen bonding of the water layer underneath, one question is whether the downward force from a walking insect will overcome the repulsive force of pushing hydrophobic oil into water.

Further, oil is wholly hydrophobic, lacking any polar property to disrupt water's hydrogen bonding, so it lacks the disruptive capability of detergents.

I'm not positive that you could answer this definitively without some experimentation, but I would guess that detergents have greater disruptive power than oils, in general, because of its polar end.
posted by Blazecock Pileon at 5:15 PM on August 3, 2010

Blazecock Pileon's answer is great, and as a soap maker, I'd like to know what you're getting at. Most soap is quite high in PH, being made from fats. Detergent is also made from fats but it's got additives. In fact, the fatty acids binding to the sodium hydroxide base are what makes soap and detergents are a further byproduct of soap, eh?

I'd think the detergent or soap would be better, because in my humble experiences, soap is more sticky than oil, but I could be wrong.
posted by Marie Mon Dieu at 5:52 PM on August 3, 2010

Ethanol (alcohol) reduces surface tension quite well too. Hence the reason why it is so easy to spill a pint of beer crossing a crowded bar...
posted by SueDenim at 6:35 PM on August 3, 2010

Simple answer: soaps/detergents mix with the water, creating a solution that has lower surface tension. While oils merely float on the water. In effect, they depend on the surface tension rather than changing it.

However, oil on water may well disrupt the doodads on the insects' feet and render them less able to walk on the water, or disrupt their ability to escape from the water once they are in it.
posted by gjc at 6:36 PM on August 3, 2010

Soapy water is a pretty common way to trap and kill pest insects. It's at the heart of my favorite DIY flea detector (pie pan full of soapy water beneath a gooseneck lamp left on all night long). Soapy water also does a great job killing ant hills and ground-dwelling bees' nests, just pour in a big ol bucket of soapy water and yer done.

You may have heard of oil on water as an insecticide for certain insects. It works a treat for mosquito infestations, for example. The larvae are aquatic, so if you pour oil in a pond, the layer of oil smothers the larvae. I imagine this will work for other aquatic insects as well.

Oil can also be used on its own to trap and smother insects. I have chickens, and many chicken authorities recommend painting the undersides of perches with vegetable or used motor oil, in order to foil chicken mites.

But oil on water is probably not going to be useful as an insect trap.
posted by ErikaB at 6:45 PM on August 3, 2010 [1 favorite]

While oils merely float on the water. In effect, they depend on the surface tension rather than changing it.

Uh, no. Oil floats on top of water because of it is less dense. Surface tension allows denser than water objects (e.g. insects) to do the Jesus thing.
posted by randomstriker at 8:03 PM on August 3, 2010

To rid the yard of grasshoppers, we used to do gallon jars half-filled with sugar water with a cone-shaped piece of metal screen slipped into the lid. They'd hop in via the chute, but couldn't get back out.

I doubt the sugar altered the surface tension.

When we had a cat, we'd brush him occasionally with a flea comb and deposit the little beasties in a bowl of soapy water. The soap changed the surface tension enough to allow them to sink.
posted by notyou at 7:14 AM on August 4, 2010

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