Is it possible to increase the surface tension of water? My sci-fi writer friend claims it can be done, but is he just a bit wet?
鈥� You can view surface tension as a measure of how much a substance like water prefers to bond with itself (cohesion) rather than other things (adhesion), and how much effort is needed to stretch the boundary or break the bond. It鈥檚 very easy to demonstrate that you can reduce surface tension in water 鈥� carefully rest a toothpick or paper clip on top of some tap water, add a surface active agent (or surfactant) such as soap and watch the object drop.
Water has a relatively strong surface tension to start with, but you can add an inorganic salt to achieve about a 10 per cent increase. Take two identical glasses and fill them almost full with tap water. Stir a couple tablespoons of table salt into one and then top them both up so they are almost overflowing. Carefully slip coins into the glasses 鈥� you should be able to add more to the salty water before the tension breaks and it spills.
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鈥淵ou can add an inorganic salt to water to achieve a 10 per cent increase in surface tension鈥�
If you insist on pure water, there鈥檚 an easier way: cool it down. Surface tension rises almost linearly as temperature decreases, until, of course, it freezes.
Ron Dippold, San Diego, California, US
鈥� Though water molecules are electrically neutral, they are dipolar, with a slight positive and negative charge at either end. This means they tend to stick together, with each molecule attracted to the oppositely charged end of neighbours. These van der Waals forces are responsible for water鈥檚 viscosity and surface tension.
Surface tension occurs because water molecules have a greater electrical affinity for each other than for the air molecules above. This electrostatic attraction leads to a net inward force, which is why water behaves as if coated in an elastic membrane, allowing insects such as pond skaters to walk on it. Molecules of water at the surface have higher energy than those completely surrounded by other molecules. Minimising the surface area minimises this energy 鈥� a sphere achieves this. These electrical forces explain why water beads on a freshly waxed car, which offers a hydrophobic, or water-repellent, surface. Once this wax coat wears off, rainwater spreads out on the hydrophilic bodywork.
Thermal vibrations disrupt electrostatic attraction between molecules, reducing surface tension by about 25 per cent if temperature is increased from freezing to boiling point. Another method is to dissolve salt in water, causing positive sodium ions to attract negative chlorine ions and thus augmenting the polar attraction between water molecules.
Mike Follows, Sutton Coldfield, West Midlands, UK
This article appeared in print under the headline 鈥淭ense situation鈥�