I slip into a body of water–a tub, hot tub, creek, lake, or ocean–and the water supports me, buoys me, and makes intimate, surrounding contact. It releases my tension.
But that’s not the surface tension I’m talking about.
The bathwater’s contact isn’t too intimate, the way a bath in rubbing alcohol or gasoline would be (with both having surface tensions of ~ 22 mN/m). They would feel like they penetrated my vary pores. A bath in mercury (surface tension of ~ 480 mN/m) would probably feel more like taking a bath in a vat of beads—it wouldn’t have intimate contact with my contours. Of course, don’t even consider taking a bath in alcohol, gasoline, or mercury because of the dangers! But speaking materials only here …
Water eases the soul. And its surface tension ( ~ 59 mM/m at body temperature) has a lot to do with it.*
A water strider walks across the top of a pond, its hydrophobic (water-repelling) feet not breaking through, but the surface of the water visibly depressed at the points of contact. A spider has appeared in a saucepan soaking in the sink, and it is walking along the top. I squirt a bit of dish detergent in the water and the spider sinks. Why? The detergent made the tension of the water decline.
What’s surface tension, and who cares?
Surface tension is one of the factors that makes that bath so comforting, that swim so intimate, and detergent so effective at sinking spiders and getting fats off frying pans.
I think of the molecules in a liquid as holding hands with each other. At the interface of liquid and air, the molecules in the liquid want to hold hands with each other more than they do with the air. If they have an extremely strong preference for each other, they will form a ball (which has the lowest surface area per volume), and if I dribble them onto a piece of wax paper or a shiny leaf, they will tend to make a ball or at least a tidy mound, with a sharp angle of contact at the base.
In water, the hand-shake results from hydrogen bonds. Water molecules are shaped like a person at the beginning of a toe-touch—the butt is an oxygen with a negative charge, and the hands and feet the oxygens with their positive charges. The butt plus the two extremities makes a neutral molecule, but the butt is the negative end and the hands/feet are positive so in a liquid, the hands/feet of one molecule have a tendency to grope toward the butt of another molecule, and on, making these weak “hydrogen bonds.” They’re weak compared to other types of bonds, but they do give water its higher surface tension than something like the alcohols or other molecules that have rings of carbon and that share electrons within the molecule in a more balanced way.
To repeat, this relatively higher affinity for itself than for air, or in this example, wax, explains the shape of water droplets on wax paper. In contrast, put alcohol or gasoline on wax paper and it will spread out.
Even rounder yet would be a droplet of mercury, because the bonds between adjacent mercury molecules are very strong compared to the hydrogen bonds in water.
And what about paper towels versus waxed paper? In contrast to on waxed paper, if you put a water drop onto a paper towel, the droplet spreads out because the water has some affinity for the cellulose in the paper towel.
Playing with surface tension
We have lots of examples of surface tension when we’re cooking. I remember playing with the Campbell’s minestrone soup or chicken noodle soup mom used to serve us. Did you ever notice the oil droplets crawling up the edge of the plastic bowl? The oil molecules had more affinity for the bowl surface than for the water around them. And in the minestrone, the oil had red pigment in it, presumably from the tomatoes, making it more visible.
How about playing with water in lupine or lotus leaves, where the water balls up? Studies have shown that lotus leaves are extremely hydrophobic (making it so water has a hard time wetting the surface like a paper towel—it stays balled up instead). they achieve that through the waxes on their surface, and also by studding their surface with tiny hairs (~ 10-20 microns high and in diameter) that hold the water up the way a bed of nails might hold up a sleeping person—and like the sleeping person, the liquid being held up can’t curve into the spaces between hairs, and thus stays up. In these photos, it looks like the water barely contacts the leaf. The adaptive values are thought to be that the water droplets clean dust and pathogens off the leaf surface. That concept of making the surface so hydrophobic with nano-engineering is the basis of an industry that is designing paints and coatings for things like fabrics and glass, and they call the process “the lotus effect.”
And did you stick a fork tine into the flattish oil blobs on the surface and coax them into coalescing? Remember how when two oil blobs touched they suddenly became one and the entire shape sprung from two O’s to a figure eight and then a larger, ghosty-shaped-almost O? That is because the oil had an affinity to keep touching other oil, and not to have the water/oil interface, and a disk minimized the exposure of the oil to water.
You might have played the same game with gasoline or oil slicks in puddles in the driveway. I did. You might have fought the same processes trying to keep your oil-and-vinegar dressing mixed, or trying to get the bath oils you added to your bath tub down in the water instead of floating on the surface and getting your neck oily. And then before the next bath, you might have used detergent in water to make a liquid with a surface tension less repulsive (more attractive) to the oil. The oil bubbles could then be smaller (as they became “emulsified”) and then washed off.
And if you ate frosting or butter or meat, your body did some of the same as the detergent so you could make the fat bubbles smaller and digest them. In fact, do you know Gas-X (simethicone), which is used to treat flatulence? It acts by changing the surface tension of gas bubbles that form during digestion in the stomach and intestines. It changes the surface tension so larger bubbles can be formed from the smaller ones, and presumably it’s easier on us to “eliminate” the bigger bubbles. In a few big puffs, I guess.
This knowledge of surface tensions is key in many industries and scientific processes. For example, the development of advanced wood composites (like cross-laminated timber, laminated veneer lumber, oriented strand board, particle board, wood-plastic composites, and even plywood) has depended greatly on the development of adhesives that have the proper surface tension to penetrate the geometries they will encounter.
But I want to talk about an application I often use. I need to know the volume of small chunks of wood on a regular basis, for many different studies. How can I get an accurate value for an odd-shaped sliver or chunk of wood, say the size of a key or a roll of Scotch tape?
The first method uses the Archimedes principle. I put a beaker of liquid on a balance and hit the tare button so it reads 0.0000 grams. I skewer my odd-shaped lump, then submerge it in the liquid. The weight on the balance goes up. That’s because I’m exerting force into the liquid (even though I’m not touching the beaker)–in this case, enough force to keep the block of wood from floating back up. People defined units long ago so that 1 cm3 of water weighs 1 gram (at standard temperature and pressure). If the balance reads 53.0000 grams, that means the sample is 53.0000 cm3. Yay.
I write down the number. Then I look up and the balance says 52.4442 grams. I erase my number, but now it is below 52.0000 grams. The water is getting into the wood, and so the apparent amount of water in the beaker is going down.
- Be really quick: write down the number as soon as the balance stabilizes. But that’s not something I can standardize.
- Spray the sample with PAM so water doesn’t get in it so fast. But that messes the sample up for future work.
- Dip it in paraffin. Same problem, plus the paraffin changes the volume.
- How about using a liquid that won’t penetrate the wood pores as much? And how about if that liquid is also a lot denser than water, so that it will read much greater differences in grams between samples that are, say 53.0 and 53.1 cm3 in volume? That liquid would be mercury. Again, it has a surface tension (480 vs 59 mM/m) so it has a greater affinity for holding its own hands than water does, making itself into balls and not penetrating the pores, and it is much denser that water. Rather than weighing 1 gram for a cm3, it weighs ~13.6 grams.
- But it’s not that great a solution, either. The darn liquid has such a high surface tension that it doesn’t close in well enough around the samples. It leaves little air gaps. I can even see that as I submerge the sample—a sort of dimple, or even tunnel forms before the mercury snaps shut on top of my sample.
- And there’s the mercury toxicity. The risk with elemental mercury is less with ingestion–I understand it is not very reactive—than with the long-term exposure to little balls of it and evaporating from the lab floor, giving chronic exposure to the more reactive fumes. And on the edges of wood samples, in fact, little balls of wood do cling. In my lab, we stopped using mercury for small rough samples after making that observation, and we have used it rarely since.
So what is the solution (no pun intended)? It’s simple, and a bit clever. It’s called “the maximum moisture content” method.
Dry wood has two compartments: cell wall material (C) and gas (G). Thoroughly soaked wood has two compartments: cell wall material (C) and water (H20). We know the density of cell wall material (1.53 grams per cubic centimeter) and the density of water (1.00 gram per cubic centimeter). We use those two values, and voila, get the volume.
Oven dry the sample above the boiling point of water until all the water is driven out, as shown by a constant weight between weighings. Write down that number and call it A, the number of grams of pure cell wall material.
Now drive out all the gas by putting the sample under water and then pulling a vacuum on it. (That is similar to what you do when you put one of those specially designed corks on your wine bottle and pump a couple of times). You make it so the air above the water is pulling on anything in the sample to expand if it can. The gas bubbles expand and come out. For samples the diameter of pencils, it may take three to seven days, with exchanging the water every day and drawing a fresh vacuum, until all the gas is drawn out. You dab off the excess water with a towel, then quickly weigh it, put it back in the vacuum, and continue, until it has constant weight.
At this point, record the weight, and call it B, the weight of all the water in the sample plus the weight of all the cell wall in the sample.
Now some calculations. We want the volume of the sample. That is the volume of the cell wall material (C) plus the volume of the gas (G). The gas has been replaced by water. We know the volume of water (it is the wet weight minus the dry weight, B – A), a certain number of grams, which we can convert to volume because water has that known conversion of 1 gram being the weight of 1 cm3.
And we can calculate the volume of cell wall material, too. We have its weight already (A, the dry weight of the sample). Because pure cell wall material is known to have a density very close to 1.53 grams for 1 cm3, we can then calculate the volume of C.
Then the total volume of the sample is the volume of C plus the volume of A.
Surface tension: it’s not only important to water striders and lotus plants. Take a bath, take a swim, use bath oils, eat frosting, use wood glue play with lupines, and measure volumes of wood slivers. But give a nod to surface tension.
*Water is amazing—with its density, thermal conductivity, properties in compression and tension, latent heat of evaporation, properties with phase changes in the temperature range in which we live. Wow, there is so much that is cool about water.
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