When rain hits a cabbage leaf, water droplets explode off the leaf in all directions, leaving the leaf bone dry.
That’s because cabbage leaves are hydrophobic—they repel (or more accurately, fail to attract) water molecules.
It’s been raining a lot lately and I consider this hydrophobic ability, somewhat grimly, as I watch the rain soak through my old jacket. I decided to try an experiment. The hypothesis is simple: remove the hydrophobic stuff from the plant leaves and use it to re-waterproof my jacket.
The first part of my experiment involved observation. If I scrubbed one part of a leaf with my finger, it turned a much brighter green than the untouched part. Spritzing both areas with water revealed that droplets stuck to the scrubbed area and roll right off the untouched area. From this, I concluded that the hydrophobic quality of the leaves comes from a substance on their surface. When I rubbed the stuff that I scraped off between my fingers, it felt like wax.
The next step was to get as much of the waxy stuff off the leaves as possible and onto my jacket. I didn’t much like the idea of scrubbing leaves, so I tried to scrape off the wax with other tools, but that didn’t work. Then, I recalled that boiling cabbage leaves often makes them greener. If you’re really lazy and leave the pot on the stove, a cloudy white ring will appear where the water used to be.
I tried boiling a couple leaves to extract the wax. Sure enough, the leaves left a waxy ring behind. I felt optimistic. I figured, all I needed to do was to scrape the wax from the pot and smear it onto my jacket. But, to my dismay, the amount of wax I harvested was tiny—about the size of the tip on a ballpoint pen. Not nearly enough to experiment with. So, I boiled up a whole cabbage, in the hope that the waxy ring would get bigger. It didn’t.
I noticed that the water in the pot, when boiling the leaves, gains an oily sheen. Perhaps the wax was in the water? I thought that separating wax from water would be easy since there are a bunch of differences between the two substances:
- Water molecules are much smaller than the oils that make up wax. But passing the water through coffee filters didn’t work.
- Water has a lower boiling point than wax. But boiling away the water didn’t really leave any wax behind.
- Water is less dense than wax. But spinning tubes, containing the oily water, in a centrifuge really fast didn’t provide enough force to bring the wax to the bottom of the tubes.
In short, I was stumped. Each time that I would jump up from my chair, certain that I’d cracked the problem, I would end up slumping back into it, after trying the experiment. It was then that I decided to admit that this experiment would not prove my hypothesis.
I was pretty bummed for about ten minutes. Everything I had tried to do to get the wax off the leaves ended in frustrating failure. Still, trying to get the wax off those cabbage leaves left me with a new appreciation for plants.
My failed experiment reminded me of something that one of my heroes, Richard Feynman, said while arguing about the beauty of flowers with an artist friend. The artist claimed that flowers become dull when scientists try to pull them apart to understand them. Feynman said, “…science knowledge only adds to the excitement, the mystery and the awe of a flower. It only adds. I don't understand how it subtracts.”
I feel the same about my cabbage leaves. I discovered for myself that plant leaves are covered with wax. I saw through a microscope the little hexagonal cells that cover the surface of each leaf. I found out that the amount of wax which protects these leaves from water is tiny. All of these things are amazing to me. Discovering this led me to search the literature to find out how so little wax can have such a big hydrophobic effect. If you want to find out for yourself, click here. But that’s another story; for another time.
There are some pretty great experiments that you can do at home with a cabbage. Why not try your own Cabbage Chemistry experiment?