Physics Photo of the Week

September 26, 2008

Beaded water and pollen
After a brief rain shower in mid May and a recently re-stained deck, the rainwater beaded up (see  PPOW for September 1, 2006).  However, mid May is the time for white pine to produce prodigious amounts of pollen - it collects on all horizontal surfaces.  When the pollen-strewn deck was moistened with the rainwater, the pollen grains tend to coalesce and float to the tops of each beaded water droplet.  These collections of pollen show up as the light specks in the center of each droplet.  On the left end of the photograph the water drops have evaporated and left the coalesced pollen piles in a random array.  You can notice the same effect on practically any horizontal surface that has a layer of uniformly distributed dust, which has been sprinkled with rain - especially cars soon after washing! 

The physics of the effect of coalescing is quite complicated.  If you sprinkle pepper on your soup, the flakes of pepper are seen to attract each other.  This is a combination of surface tension of the water and self-adhesive forces in the dust, pollen, or pepper.

To show that these specks are actually pollen, I made two close-up photos of the patches, which are reproduced here on the right.  The left picture is one of the patches of pollen floating on top of a water droplet.  The bottom image is a patch of dry pollen grains after a water drop evaporated.  Note the interesting convolution and "folds" in the floating pollen patch.  The physics of this effect is a complete mystery to me.

These close-up photos were taken by the simple technique of holding a powerful magnifier up to the lens while the camera "looks" through the lens.  This is similar to using a magnifying lens in front of your eye.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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