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How would the world be different if water didn’t display a meniscus or surface tension?

Ron Dippold
San Diego, California, US

In its most basic form, surface tension is the tendency of liquids to bead up rather than spread out because they are more attracted to themselves (cohesion) than they are attracted to other things, for example the air or a surface (adhesion). So, the liquids act like they have a thin, elastic membrane on them. In the absence of other forces, every drop of liquid would tend to be a sphere.

I will only consider liquid-to-air interfaces here, since if liquid-to-liquid surface tensions were to disappear, cell membranes couldn’t form, and there could be no life as we know it.

To start with, there would be various minor but noticeable differences. Water wouldn’t bead up, it would run right down your window or off leaves, depriving plants of some water. The thin liquid film protecting the surface of your eyeballs relies on surface tension, so your eyes would be unprotected and dry.

The biggest catastrophe would be the loss of capillary action. If a tube is narrow enough, and the water adheres to the tube more than it coheres to itself, you get a convex meniscus, where the liquid spreads a bit along the tube to adhere to the surface. Then more of the liquid is attracted to the liquid that is further along. This repeats, so the liquid moves further along the tube until, if the tube is vertical, the weight of the liquid stops it. You can see this by sticking a transparent straw in a cup of water. The liquid in the straw will be slightly higher than the liquid in the cup. The liquid metal mercury has surface repulsion, so it would do the opposite.

Capillary action gives you “free” liquid movement along a tube, even against gravity. Without it, a staggering number of physical and biological functions would be compromised. The tiny air sacs in your lungs where oxygen and carbon dioxide are exchanged with your blood would be crippled. Expelling mucus from your lungs would be much harder.

In the world of technology, inkjet printing relies on capillary action to pull tiny bits of ink through a tube. There are industrial uses too, like heat pipes and microfluidic devices, and medical uses like pregnancy tests and covid-19 tests. All of these would no longer work.

So this apparently tiny change would have more consequences than anyone could possibly enumerate, but it would certainly be bad for us. If it had always been like this, then life, if it existed, would be totally different. Needing windscreen wipers seems like a small price to pay.

Chris Daniel
Colwyn Bay, Conwy, UK

In a body of water, cohesive forces between H₂O molecules act in all directions, resulting in a net force of zero, but at the surface, the molecules bond only with others below them and to the sides. This imbalance of forces causes them to be pulled more tightly together, resulting in skinning or surface tension. When water meets another surface with a lower surface energy, such as the side of a glass container, a meniscus is formed that rises until it is balanced by the force of gravity. When unconstrained, water will try to minimise its surface energy and therefore its surface area by forming spheres such as those found as droplets of mist, fog and rain.

Some insects, such as pond skaters, make use of surface tension by skimming on the surface of ponds, their feet indenting but not breaking the surface of the water. Light objects like leaves and even sewing needles can be supported in the same way.

If there were no surface tension in water, then there would be no cohesive forces anywhere in the water and it wouldn’t be able to exist in a liquid state. Life on Earth would then be impossible.

While surface tension cannot be eliminated, it can be greatly reduced with the use of surfactants, organic molecules that have water-loving, or hydrophilic, heads and water-hating, or hydrophobic, tails. The heads of the molecules (which may or may not be charged) intersperse themselves among the water molecules at the surface, reducing the net force or surface tension between them. This, in turn, reduces the angle of the meniscus, allowing the water to spread more easily onto other surfaces.

Surfactants in detergents cause water to penetrate fabrics or wet the surface of dishes, the hydrophobic tails attaching themselves to dirt and grease that can then be washed away. Similarly, surfactants in emulsifying agents allow oil-water mixtures such as mayonnaise or paint to become homogeneous by preventing the oil molecules from coalescing and separating from the water.

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