Physics Photo of the Week

November 21, 2014

Beaded water - Photo by Rebecca Hettrick

Rebecca noticed the strong beading effects of a few water drops on a tablecloth in a local restaurant.  The tablecloth was woven of a strong hydrophobic ("water fearing") fabric.  The threads in the fabric are either synthetic, plastic coated, or the tablecloth may have been treated with a water repelling material.  The result is that the water molecules bind to themselves much more than they bind to the fabric.  It was fun to nudge the droplets around the table - they would roll smoothly, combine with other drops, but not wet the tablecloth.

The key to understanding the attraction or repulsion of different substances lies in the structure of the molecules of the different substances.  Water molecules have more electrons on one end than the other.  See the drawing at right.  There is a cloud of electrons represented as a green "cap" on the oxygen atom.  The two hydrogen atoms are angled away from the central oxygen atom.  The large blue sphere is the basic oxygen atom.  The small yellow spheres are the two hydrogen atoms.  In forming the molecule, the two electrons from the hydrogen atoms tend to form at the other end of the oxygen - represented by the green "cap".  As a result the water molecule is polar.  The top end is more negatively charged (due to the electrons) than the bottom end (due to fewer electrons.  Even though the water molecule is electrically neutral, it is still polar.  Because the water molecule is polar.  Because the water molecules are polarized tge water molecules are strongly attracted to each other - the negative ends of one molecule are attracted to the positive ends of another polar molecule.  The other molecules can be other things besides water: cotton, salts, any thing that is made of polar molecules.  Thus water easily wets cotton, and it requires a relatively large amount of energy to dry cotton.

However, if a water molecule is brought close to molecules that are non-polar such as many organic molecules, oils, paraffin, plastics - there is no attraction because the non-polar molecules have no charged ends.  Thus the table cloth threads in Rebecca's photograph are probably coated with a non-polar coating - not attracting the polar water molecules.

Notice also the magnifying glass effects of the spheroidal water drops.  Because the drop is thicker in the center than the edges - due to surface tension forces in the surface of the liquid water - each bead of water up acts as a miniature magnifying lens.


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 dcollins@warren-wilson.edu.

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