DR. JOHN BRACKENBURY / SCIENCE PHOTO LIBRARY DR. JOHN BRACKENBURY / SCIENCE PHOTO LIBRARY
Surface tension. Water droplets supported inside goat's beard seeds by surface tension. This effect is caused by the attraction between the molecules of water. This attraction is random inside a body of water, but at the surface the attraction is not random; the surface water molecules are attracted more strongly to other water molecules than to the air molecules. The surface water molecules align themselves to form an ordered layer that acts as a containing 'skin' for the droplet, preventing the water flowing out between the spokes of the seeds. The tension of this surface layer can only support a certain weight, and the droplets would collapse if they were too heavy.
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