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 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 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.