The Physics of the Familiar
How paint dries, the way flags flutter, how Nature discovered origami, and other marvels of the physical world
by Jonathan Shaw
Photograph by Jim Harrison
Lakshminarayanan Mahadevan
“Just because something is familiar doesn’t mean you understand it. That is the common fallacy that all adults make—and no child ever does,” says Lakshminarayanan Mahadevan, England de Valpine professor of applied mathematics. Mahadevan enjoys explaining mathematically the phenomena of everyday life: practicing the old-fashioned method of scientific inquiry called natural philosophy, where one wonders about everything.
He has compared instability in nanotubes to the wrinkling of an elephant’s trunk, and explained, together with his students and postdocs, how the leaves of the Venus flytrap can buckle shut fast enough to trap a fly. [*See “Leaves That Lunch,” May-June 2005, page 14. The work was done together with his student Jan Skotheim (now a postdoctoral fellow at Rockefeller University) and postdocs Yoel Forterre (now an assistant professor of physics at Marseilles) and Jacques Dumais (now an assistant professor of biology at Harvard).] With former postdoctoral fellow Enrique Cerda, now a physics professor in Chile, he has devised equations to describe how fabric drapes on a clothed figure or from a table. Inspiration is everywhere. “Look around the room,” he tells his students. “Look out of the window.” Mahadevan sees trees lifting water from roots to crown, or perhaps a drop of paint drying in the sun, and wants to understand in a deep sense what is happening right before his eyes.
What will happen as the drop of paint dries? Why should we care?
“In the same way, you may ask, ‘Why should you climb a mountain?’” Mahadevan says. “Some people do it for the challenge, but there is another aspect to it, and that is that you can actually see around you. You can see where you are.” But the path straight up the mountain may be too hard. “The difficulty could be technical, or experimental, or it could be mathematical—it doesn’t really matter.” But then, he continues, you see a little hill, and think, “Maybe I’ll climb up the hill. I will still be able to see around me—maybe not as far as from a mountain, but perhaps, once I get up there, I will find a path from that hill to a larger hill, and so on, from which I will be able to see the range, to understand the lay of the land, and know how to approach questions of a similar kind.”
So with the paint drop. As it dries, a skin will form. That is the natural consequence of liquid solvent leaving the surface fastest. “Now you have a skin, which is covering a drop, which is sitting on a surface, and you want to remove more liquid from inside. That is like drying a raisin,” says Mahadevan. When you remove some of the volume from beneath a fixed amount of surface skin, “the only thing the skin can do is wrinkle.”
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