14 November 2006

A sidewalk puzzle


This is a picture of a sidewalk a few hours after a rain. It was mostly dry except for the spots covered by leaves. (The couple of leaves without wet spots under them had probably been blown on the sidewalk after it dried.)

So, here is my question: why were the wet spots under the leaves wider than the leaves themselves?

And, here is what I think is the answer: as the sidewalk was drying, the water prevented from evaporating by the leaves diffused out from under the leaves into the dry concrete creating a wet spot larger than the leaf area. The extent of the wet spot was probably determined by the amount of water that was originally under each leaf and the rate of evaporation opposed by the rate of diffusion of water.

If you can think of a better answer, please post it.


hartley said...

More important is the water that was prevented from draining by the leaves, due to surface tension.

As the water in the pavement dries, the small pores draw out water from under the leaf by the capilary effect, not just by diffusion.

Anonymous said...

I agree with Aydin and hartley.
I think the wet leaf has a much higher heat capacity than just the wet pavement by itself, so it creates an entended area of relative cold that inhibits evaporation. I'm sure an IR temperature probe would show an area of low temperature beyond the leaf itself. ultimately, the increased heat capacity is of the water held by surface tension by the leaf.