Frost on Edges
On a cold, clear morning in early December last year, a heavy frost formed on some outdoor hardware. I was intrigued by the concentration of frost crystals on the edges of the hardware as opposed to the general flat areas. The corners and edges of the bolt and the inside edge of the hole in the steel strap support a large concentration of frost crystals.
The reason for this concentration around the sharp
edges and points is the fact that frost requires a nucleation site in
order to form. In order for water vapor to condense
into a liquid or solid, it requires a
small object, or a "sharp" edge of a larger object in
order to form a nucleation site. This is a
complicated surface effect in which the electric fields at
the surfaces become more pronounced in regions of pointed
objects that help guide the bipolar water molecules into
the solid state. In the formation of frost, the
water vapor condenses directly from the vapor state into a
solid - bypassing the liquid state. This is called sublimation - a
change of phase from vapor to solid or vice versa -
skipping the liquid state. Ordinary snow is
sublimated water vapor.
The picture at the right shows another close-up of
another piece of hardware photographed on the same
day. The frost has formed predominantly on the edges
of the square cross-sectioned steel plant hanger.
Physics Photo of the Week is 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.