If you have or know middle school student you have likely heard this infuriating – and to some, nonsensical, question. This question is today’s fidget spinner, annoying teachers and parents alike and providing endless entertainment for kids.
It started with this video in which a few guys debate whether a fish is wet when it is under the water. One claims that you can only be wet or dry and the other says there is a third state that is surrounded by water, but not wet and not dry. What do you think?
But fish aren’t people, they have scales instead of skin and are covered with slime. So you could say that the fish’s slime coat is wet but not the fish itself. This is like saying your skin is wet but not YOU. So you need to decide if the slime coat is part of the fish or separate from it. The slime coat can be removed but then the fish will leak electrolytes out of its skin and die.
If we take the accepted definition of “covered or saturated with water” then the fish in the water is indeed wet. You (or a fish) can be only be wet in the water but you can be wet or dry when you are out in the air, such as when you get out of the shower or bath.
Cohesion and Surface Tension
Water is wet for the very same reason – it sticks to itself extremely well! Water sticks to itself so much that it forms a type of skin across the outside of a droplet or the top of a glass of water. Scientists call this stickiness of water cohesion. Where water meets the air water is particularly sticky and has very high surface tension. That is, water has a strong skin on its outside surface. You can do some fun experiments with surface tension here, and use surface tension to sent a boat sailing across your bathtub here. For more details on why water is wet you should read this interview with chemist Richard Saykally.
And then consider the case of the cabbage leaf. If you pour water on a cabbage leaf, the water beads up and rolls off without sticking to the leaf. Compare this to lettuce or most other leaves where, like your skin, the water spreads out and covers the leaf. Is the cabbage leaf wet? How about the lettuce leaf?
Other objects become wet when there is more adhesion than cohesion. If cohesion is water sticking to itself, then adhesion is water sticking to other things. Put a drop of water on a cabbage leaf and the water will stick to itself more than the leaf and stay in a round drop. But put a drop of water on a paper towel and it instantly spreads out and soaks into the towel. The adhesive force of the water to the paper towel is more than the cohesive force to itself – the paper towel is wet but the cabbage leaf is not!
Why doesn’t water stick to the cabbage leaf? Scientists call the phenomena the Lotus Effect after lotus leaves who are, like cabbage, superhydrophobic. Hydrophobic means they repel water so as you can imagine, superhydrophobic means they REALLY repel water. It turns out that there are tiny waxy nano-hairs that cover the surface of these leaves. The hairs are so much smaller than the water droplets that the droplets sit up on top of the surface – with the added bonus of picking up any dirt or dust on the leaf surface too. Butterfly and dragonfly wings are coated in a similar way so that water does not weigh them down but still cleans off the dirt. Chemists have replicated this property in paints, fabric, roof tiles and other surface to make them water repellent and self cleaning.
So grab some leaves and pour a little water on each to test if they can indeed get wet or are superhydrophobic instead. Try lotus, nasturtium, broccoli, Brussels sprout, cabbage, collard greens, kale, taro (elephant’s ear), tulip, turnip greens or water lily. Some other substances like wax paper or styrofoam are simply hydrophobic. You can test all sorts of materials to see how easily the get wet.
For another activity on hydrophobic and hydrophilic surfaces check out this experiment on keeping the bathroom mirror from fogging up.
One of the cases in the “is it wet?” argument is a fish in the water. What if we replace the fish with a bubble? Would it be an antibubble? If a bubble is a sphere of soapy water in the air and filled with air, then an anitbubble is a sphere of air in soapy water and filled with soapy water. With a little patience you can actually create antibubbles at home.
Resources for learning more
- Water Drops: Cohesion and Adhesion
- Nasturtium Leaves Activity from PBS Kids Dragonfly TV
- Superhydrophobicity – The Lotus Effect from TeachEngineering
- Wetting and Contact Angle from TeachEngineering
Many thanks to the teachers in the Kesler Science PLN group on Facebook for discussions that helped improve this post!