Monet's Water Lilies

When my wife and I were in Paris several years ago we visited the Musée de l’Orangerie, where Claude Monet’s beautiful water lily murals are displayed. Monet (1840–1926) is the famous impressionist painter who, during the last decades of his life, painted lilies floating on the surface of the pond at his home in Giverny. I remember sitting in one of the oval rooms staring at these giant paintings. It was so quiet and peaceful.

Water lilies take advantage of some interesting physics. First, their stalks and leaves contain air pockets, reducing their average density and making them buoyant. Russ Hobbie and I compare the effect of buoyancy in terrestrial and aquatic animals. I quote this comparison below, but I have replaced the word “animals” by “plants”.

Plants are made up primarily of water, so their density is approximately 10³ kg m⁻³. The buoyant force depends on the plant’s environment. Terrestrial plants live in air, which has a density of 1.2 kg m⁻³. The buoyant force on terrestrial plants is very small compared to their weight. Aquatic plants live in water, and their density is almost the same as the surrounding fluid. The buoyant force almost cancels the weight, so the plant is essentially “weightless.” Gravity plays a major role in the life of terrestrial plants, but only a minor role for aquatic plants. Denny (1993) explores the differences between terrestrial and aquatic plants in more detail.

Another piece of physics important to water lilies is surface tension, a topic only briefly mentioned in the fifth edition of Intermediate Physics for Medicine and Biology, but which (spoiler alert!) may play a larger role in the sixth edition. The lily’s leaf is waxy, which repels water and enhances its ability to remain on the water-air surface. In addition, small cilia increase the surface area.

A last bit of physics has to do with the surface-to-volume ratio. Usually surface tension can’t support a large object, because its weight increases with the cube of its linear size, whereas the effect of surface tension increases with the object’s perimeter. Therefore, the impact of gravity increases with size more dramatically than does the impact of surface tension, so a large object sinks like a rock. The water lily’s leaf, however, is thin, and making the leaf larger increases its surface area but not its thickness. The weight only increases as the square of its linear size, not as the cube. If the leaf is large enough, gravity will still win out, but the leaf can be larger than you might expect and still float on the water surface.

Monet donated his water lily murals to France at the end of World War I, to create a place where people could reflect on those who gave their life for the nation. When visiting them, you can also contemplate the role of physics in medicine and biology.

Happy Veterans Day.