Physics Photo of the Week
Itsy Bitsy Spider
exploring our garden I found this tiny spider web - only about 2 inches
(5 cm) across. The tiny spider web would not be noticed were it
not for the dew clinging to the tiny silks. In fact I didn't even
notice the spider until I had taken a few photos. The spider is
only about 3 mm across, and she is in the process of spinning the
sticky silks around the spokes of the web in order to create the snare
to catch insects.
Notice the dew drops. Why does
the dew form on the spider silk? Why do we see the larger dew
drops evenly spaced on the web spokes? The answers to these
questions are based on the physics of condensation, evaporation, or any
phase change from liquid to vapor or liquid to solid or the reverse of
these transitions. Whenever the temperature is such that water
vapor condenses to form water, it needs a nucleation site - a small
object to condense upon. The larger droplets are formed where the
foundation spiral silks were attached to the radial spokes - forming a
"double" nucleation site. The spider quickly spun the
widely-separated silks as a "foundation". After rapidly spinning
the widely-spaced "foundation" silks, the spider spins more
closely-spaced sticky silks beginning on the perimeter of the
web. This is what the spider is doing in the picture.
Without a small object to attract the droplets, the water vapor would
become supercooled - cooled below the dew point where water would
condense from the vapor. When a nucleation site finally appears
in a supercooled or supersaturated air, a large condensation suddenly
appears making the nucleation site much more visible. The same
principle of supersaturated air is used in a cloud chamber to make
tracks of sub-nuclear particles visible as they are emited from a
radioactive substance - see PPOW
for April 29, 2005.
Photo of the
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 email@example.com.
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