Physics Photo of the Week

Sept. 1, 2006

Beaded water

The picture of water beading up on a finished wooden deck features very prominently in advertising of deck products.  The deck had just been re-finished with an oil-based stain the day before a light rain.  The linseed oil in the stain polymerized and bonded to the cellulose fibers in the wood.  The molecules in the polymerized linseed oil bond together as a massive covalent bond leaving essentially no polar molecules on the surface.  Water molecules are highly polar, however.  The two hydrogen atoms are bonded on the same side of the oxygen atom.  As a result, there is a cloud of electrons on the oxygen side and a lack of electrons on the hydrogen side.  The water molecule is said to be polar (even though the hydrogen-oxygen bonds are covalent).  The chemical rule is that polar molecules attract other polar molecules ("likes attract likes").  Likewise non-polar molecules (oil, waxes, and paint) attract other non-polar molecules.  Polar and non-polar molecules do not attract each other.  Thus the non-polar molecules from the oil stain repel water molecules, and the water molecules bond to each other forming myriads of droplets that are repelled from the wood.  Thus giving credence to the deck treatment advertizer (or car wax advertizer) to repel water and protect the inner beauty of the surface that is being protected.

It is interesting to count the number of droplets as a function of size.  I have estimated that the number of droplets doubles as the size is reduced by half.  Thus there are many more smaller droplets than large drops.  The same phenomenon occurs in the formation of star clusters.  Star clusters such as the Pleiades have formed from the gravitational condensation of gas and dust.  When the density of a region of the primordial stellar cloud becomes large enough, the hydrogen in the cloud becomes pressurized enough by the self gravity to start the process of nuclear fusion of hydrogen into helium.  On the rain washed painted deck, there is no nuclear fusion, and the self-attraction by gravity is minimal.  The water attracts itself due to the polar nature of the water molecules and the surface tension on the edges of the droplets.  The star cluster is similar to the beading of the water on the deck in that there are many more small stars in the cluster than large, bright stars. 

More physics in the water droplets can be seen.  The curvature of the droplets causes each droplet to act asa simple magnifier.  Notice that the wood grain appears a bit more magnified when seen through the convex drops.  Notice also that each drop casts a shadow - even when the drop is transparent.  (The picture was taken early in the morning when the sun was very low to the horizon.)  The convex nature of the drops converges the light from the sun from going straight through the drops, thus leaving a shadow.  Finally, on each drop you can see the specular reflection of the sun. 

All photos and discussion by Donald F. Collins.
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.


Observers are invited to submit digital photos to: