# Surface tension in food

Based on a related question, some of us are curious about surface tension in liquids commonly used in food and drink. There's a table on Wikipedia containing a tantalizing amount of information, including:

• The surface tension of water decreases from 76 mN/m to 59 mN/m as temperature increases from 0C to 100C. It's 72 mN/m at warm room temperature, 25C.

• 10% acetic acid (very strong vinegar) has a substantially reduced surface tension (55 mN/m at 30C)

• Alcohol can strongly reduce surface tension, to 46 mN/m at 11% and 30 mN/m at 40%.

• A concentrated sucrose syrup (55%) has somewhat higher surface tension than water, 76 mN/m at 20C.

• Very salty water (6M, compared to seawater at .6M) has higher surface tension, 83 mN/m at 20C.

Of interest would be:

• How does surface tension typically depend on temperature? (Does it always decrease with increasing temperature?)

• How do various everyday solutes (e.g. sugar) and mixture components (e.g. alcohol, acetic acid) affect surface tension of water? Actual data on measured surface tension of liquids would be wonderful - for example, what is the surface tension of milk, tea, vinegar, syrup, various alcoholic beverages, or anything else we commonly cook with or drink? What determines whether something increases or decreases the surface tension of water?

• Are there any more exotic (but edible!) solutes or mixture components with dramatic effects on surface tension? Especially interesting would be ones without flavor, which could be used to tweak existing liquids.

Note: I posted a related question on the physics stackexchange.

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+1 for the nice question. For dramatic effects, try applying pressure to a starch slurry (for a slightly wide definition of edible). – rumtscho Sep 9 '11 at 20:19
I think this question is fairly irrelevant in the realm of cooking. Textural perceptions of a given liquid are going to be dominated it's viscosity. – Nick T Nov 7 '11 at 20:01
@NickT: Sure, if you're drinking it - though still, I think that we notice things that seem slippery. But what if you want to turn it into a foam? What if you don't want it to foam? People do crazy things these days, and surface tension affects some of them. – Jefromi Nov 7 '11 at 22:06
@Jefromi 'slippery' would be a function of interface properties, not just one thing in isolation – Nick T Nov 7 '11 at 22:08
@NickT: Sorry for the imprecision there. I'm aware that material properties are a complex thing. My point was that there were situations in which it could have a noticeable effect. – Jefromi Nov 7 '11 at 22:48

This article suggests that starch gives a significant increase, and oil emulsions a significant decrease.

The same principles should apply to any small nonpolar molecule (reduces surface tension) and large molecule (increases surface tension). Note that this generally correlates strongly with viscosity. Forces at the surface forces are generally related to forces in the interior of the liquid.

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What about sugar and alcohol? Do they follow a generalization about polarity or molecular size? (Sorry for the very late question!) – Jefromi Mar 23 '13 at 12:14
@Jefromi: Alcohol is small, but has a non-trivial polarity. Like water, it contains an oxygen atom with a significant negative charge. But it's less polar than water due to the carbon atoms. Acetic acid is a lot more polar than alcohol, with two oxygen atoms. Sugar molecules are much, much larger than alcohol, but much, much smaller than starch. They're somewhat smaller than fat molecules, but far more hydrophilic. They've got oxygen atoms all over the place. Those allow the formation of many hydrogen bonds, even though individually those bonds are weak. – MSalters Mar 23 '13 at 12:28
Hm, so do polar molecules also reduce surface tension then? And sugar is large enough to increase it? – Jefromi Mar 23 '13 at 12:36
Tricky, I initially overlooked that vinegar has a pH of just 2.4. That's not making things easier. As for ethanol, it's less polar than water. For sugar, I expect that the effect depends a lot on sugar-sugar interactions. Since it depends on the proximity of two molecules in solution, that means you'd expect the surface tension change to be a second-order function of the sugar concentration. – MSalters Mar 23 '13 at 12:48