WALKIN’ ON THE WATER

There are animals that live in the water, and others that live inside the actual surface layer. And then there are the bugs who travel on top. Why is it that a pin-brained insect can rip right by my kayak and get across the pond in seconds––sans boat and paddle? 

Water striders create one of my favorite images in nature: a dent in the water. If you think that’s ho-hum, go make a dent in some water and then rejoin us. A dent is aesthetically beautiful––the slope, the shadow. It also is beautiful physics. It’s fascinating to dissect how something can stay on top of the water: the weight of the beast pushes down through its feet; the determination of water molecules to stick together creates pressure upwards from the surface; and the insect's feet have a built-in tendency to repel water. A drop of oil is hydrophobic, and so are water strider feet.

Just standing there on the pond is a complex situation, but these fellows also move and pretty darn fast at that. Movement adds all kinds of new physics issues. Suppose you were lucky enough to have feet that repelled water, and you found yourself standing out there in my pond. Now what? If you start to move, I bet you sink. Not so the strider. His little feet create these whorls or vortices; the water is thrust backwards, and he is propelled forward.

This is all plenty cool enough for me, but it gets even cooler. You may remember looking at liquid in a test tube and noticing that it doesn’t meet the glass at a right angle; it slopes upwards a bit at the edge. The word meniscus is burbling up through your cerebral cortex right now, I bet. When the strider gets to the edge of the water, he faces a climb up a watery hill, but that’s a story for another day.

I am amazed at what it takes for a bug of negligible weight to stroll on my pond. Imagine my surprise when I learned about the Basilisk or Jesus Lizard who runs on the water. Or Pygmy Geckos. Check out the links below to see these beasts defy gravity.

By way of contrast, one lovely afternoon my husband and my kayak did a barrel roll in the pond, providing an excellent demonstration of the limits of surface tension.

To see the Basilisk––and believe me, you want to see this––check out this video:

http://video.search.yahoo.com/search/video?p=basilisks+lizards+walk+on+water

To see Pygmy Geckos:
http://www.youtube.com/watch?v=pimZMS-B4dQ

To see human guys trying to walk on water:

http://videogoneviral.com/2010/05/guys-who-walk-on-water/