Physics Photo of the Week
"Dichromate", the mascot cat that lives in the Warren
Wilson Chemistry Department is admiring the tiles on the patio in front
of the Hamil Science Center at Warren Wilson College.
Notice that the edges of the tiles near the gaps are wet, whereas the
main parts of the tiles are dry. Having rained the night before
the tiles hadn't yet completely dried. Why are the tiles consistently
wetter along the edges while drier in the centers?
The tiles are laid on
top of a solid concrete surface. Water is somewhat slow to
drain out from underneath the tiles, thus the bases of the tiles are
immersed in about a centimeter of water. Capillary action on the
surface of the concrete tiles acts as a wick to conduct the water at
the base up the edges of the concrete. The water then
dissipates along the
tops constantly replenishing the water on the surface that
evaporates. See the drawing below.
The roughness of the concrete forms the capillary channels for the
"wicking" of the water. The capillary effect is not caused by the
gap between the slabs. Eventually, the whole tile dries out in
the absence of additional rain. The water on the edges of the
tiles has less sunlight to cause evaporation and the water at the bases
of the tiles acts as a reservoir.
Capillary action - the adhesion and flowing of water through small
orifices - is very important in nature. Water adhering on the
rough surfaces of rocks contributes to the erosion, support and
nourishment for plant life, and the eventual production of soil.
Many times we have seen trees growing out of cracks in the rocks, which
not only provide a pathway for roots, but also a reservoir for water
that trees require. Capillary action is partly enables trees to
draw water from the ground to the treetops, but that full explanation
is another story...
Many chemistry students have watched capillary action in thin layer
chromatography where a solvent migrates along a paper or silica gel
surface and separates components of a mixture deposited on the surface
of the gel.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature 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 firstname.lastname@example.org.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
here to see the Physics Photo
the Week Archive.
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digital photos to: