Watching an eider duck float on the water, so gracefully bobbing up and down on the water, makes me envious. Despite their chunky bodies, they are perfectly designed to float. First, they have air sacs inside their bodies much like a fish has a swim bladder. They can store air here for long periods of time. Their feathers also trap tiny pockets of air. Each feather attaches to the one next to it by sets of tiny hairs that line up and stick together like Velcro, sealing air in and water out. In addition, the oils in those feathers also help to keep water out. Groups of eiders that float together are even called rafts because our attempts to create a something that floats mimic what they do naturally.
None of these elements of duck design would work, however, without the bizarre properties of water that help support them. During the recent rain, my daughter was mesmerized as she watched the droplets of water stick to our window, then join together into little rivulets, and then pull apart again into glassy clear beads. Because it was evening, the reflection of the light on the droplets made them look like sparks. In fact, that isn’t so far off since the reason that water droplets stick together is electricity. The simple adage “opposites attract” applies. The negative charge of the oxygen atom is attracted to the positive charge of the Hydrogen atom. That’s how one water molecule “sticks” to another. The way that one drop of water pulls another along is also called capillary action – that’s how a straw works and also how liquids like sap are moved through a tree.
The bonds between water molecules are not fixed, however, but are instead able to pull back apart into their individual molecules. That can happen either from a physical force like sticking to something else like glass. Or, it can happen from the addition of energy like when heat turns liquid water into a gas. That connects back to last week’s column on clouds and the recycling of water from one liquid to gas to solid.
The connection to the floating ducks is surface tension. That’s when water molecules at the surface stick extra close together and essentially squeeze other things out – including ducks. You can float all kinds of lightweight objects on top of water because of this. Or, if you design a heavier object just right, it will also float. That design depends on an object’s buoyancy (like how much air it traps) as well as how much water it displaces, or pushes away. If you’ve ever seen the shadow of a water strider, you might have noticed that it looks like it has giant feet. If you see it out of the water, however, you’d notice that its feet don’t look nearly so big. The big shadows are the little pools of water it pushes down under each foot. It displaces more water by spreading out its weight so much that it doesn’t break through those watery bonds. This is the same principle that snowshoes use. They turn your otherwise skinny foot into a wide platform that can spread your weight out over the surface of the snow, keeping you from sinking.
In addition to watching ducks, water striders and droplets of rain, you can also watch water’s charades on the sand as the water runs up the beach and back, pulling itself along in streams. At the heart of it, it is amazing that the forces that connect droplets of rain also connect clouds – and also the ocean water that helps a duck float.
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