Physics Photo of the Week

September 11, 2009

Itsy Bitsy Spider
In 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





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 dcollins@warren-wilson.edu.

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