Who among us hasn’t been enchanted by dewdrops on a spider’s web or raindrops clinging to the tips of fir needles? A garden after rain is filled with pools of quicksilver as droplets merge together on leaves and glisten in the sunlight. It’s magical, and it draws the eye of many a naturalist and photographer. The perfectly round shapes, clearer than the finest crystal, reflecting the world in perfect (if upside-down) miniature…what isn’t there to love?
I’ve been thinking about raindrops recently, wondering why some are flattened blobs while others maintain their spherical figures. I was also wondering why they make such wonderful lenses, even if they reflect a topsy-turvy world. These are really very elementary questions, to which we all learned the answers back in high school physics, but sometimes these lessons are forgotten in the fog of time. Perhaps now, when rainy summer days provide us with ideal laboratories for study, is the time to revisit them.
First, why are some droplets round and others flat? Water is composed of molecules that have a positive charge on one end and a negative charge on the other. As we all know, opposites attract. This attraction means that the water molecules within a raindrop or a dewdrop cling tightly to each other. In the absence of any outside influences (gravity or a container), the droplet wants to assume the smallest possible shape with the least amount of surface tension, and that shape is a sphere. Spheres have the smallest amount of surface area for any given volume. A sphere is a conservative shape and the easiest shape to maintain.
Now, take your droplet, and put it on a flat surface, like a leaf or a tabletop. Depending on the texture and composition of the flat surface, your droplet is likely to lose its perfectly round shape (unless it is a very tiny droplet). Gravity is working against it, flattening it out, leaving something that resembles a dome more than a sphere. If your surface is something that produces great adhesion, like a paper towel, your droplet will disappear as gravity pulls it completely into the surface.
On the other hand, if your surface of choice is broken up (say, really bumpy or hairy), and if it has water repellant (hydrophobic) material on it (like waxes), your droplet will retain its shape. Lotus leaves are classic examples of perfectly waterproof surfaces; water droplets roll off them like so many ball bearings. This is because the surface of a lotus leaf, when seen beneath a powerful microscope, has more bumps than a sheet of sandpaper. These bumps are topped with waxes. The end result is that the water droplets are able to maintain their spherical shapes and just roll along the surface.
Water drops are extremely popular with photographers because they are tiny little lenses. Almost anyone with a camera has at one time or another taken a photograph of a droplet and noticed that inside the globe of water was a tiny upside-down world. This is a property of convex lenses, which is essentially what a droplet is. Whatever is behind it appears to be captured, upside-down within its sphere – almost like a snow globe. Some photographers stage their images, to capture a perfect rose within the droplet, or the surrounding landscape as though shot with a fisheye lens. It can be very dramatic. The trick, however, is to focus on the reflection, not on the droplet itself. This is sometimes more difficult to do than you would think.
The next time it rains, or we have a good and dewy morning, go outside as the sun rises and search for the perfect droplets. A wee crystal ball perched on a milkweed leaf; a spider web sparkling with watery beads. Knowing the science behind the magic will not diminish your experience; it should make it all the more marvelous. Now, if you see a perfect ball of water, you know to look closely at the leaf on which it is perched: is it hairy or lumpy? You might just need to use a hand lens to know for sure.