Today, we look at the edge of a water droplet. The
University of Houston's College of Engineering
presents this series about the machines that make
our civilization run, and the people whose
ingenuity created them.
Imagine releasing water in
the space station and letting it float before you.
It forms a spherical liquid drop. The sphere is
tremulous. Touch it and it vibrates. Waves ripple
around it.
It's as though the liquid were held in a gossamer
rubber bag, for a skin forms outside the liquid.
That skin is called a surface layer, it's
made of pure water, and it exerts a tension.
The water surface pulls upon itself with the same
intensity everywhere, regardless of the size of the
sphere. There's no retreating from the force. Make
the sphere smaller and the surface pulls just as
hard.
The surface layer is formed by strange balancing
act. If you could make yourself as small as a
molecule and get inside the layer, it would look
like a vast region of change -- maybe a hundred
molecules thick. The liquid molecules at the bottom
would form a closely packed thicket. They'd thin
out as you rose through the layer and be far apart
where they formed a gas, above.
Each molecule pulls on all the others, most
strongly on the ones nearest. They take up a
precise vertical and horizontal spacing in the
layer that lets each be in perfect balance with all
the others. After they've spaced themselves out,
they leave large net forces acting along the layer,
on either side of the molecule.
It takes energy to create that surface layer. Just
as a spring stores energy, so too does the surface
of a drop. Suppose I inserted a water-filled
hypodermic needle into that liquid sphere on the
space station. I'd have to press the water
into it if I wanted it to be larger. I'd be adding
the energy needed to increase the surface. Or the
sphere might drive water into the hypodermic,
powered by the energy that the shrinking surface
gives up.
So much human ingenuity has been invested in the
problem of making water into small droplets. Next
time you use a pump atomizer, notice how you have
to work to break liquid into a spray. It takes an
effort to create all those new surfaces.
The more I study water surfaces, so deceptively
simple, the deeper I'm led into surprise and the
beauty of nature at its most elementary level. Try
an experiment: Find a long wide tube, maybe three
feet long. Glue aluminum foil across the bottom,
with a clean eighth-of-an-inch hole in the center.
Fill the tube with water, and let it drain onto the
wide flat head of a roofing nail.
The liquid will flow off the nail head in a sheet
that thins until it's only thousandths of an inch
thick. Watch how surface tension finally brings the
sheet to an abrupt halt -- how it collapses into
tiny droplets. That's the same way liquid oxygen
and hydrogen are turned into droplets in a
liquid-fueled rocket.
I never cease to be delighted by the surface layers
all around me: rain on my car window, water leaving
a dripping faucet, or that space vehicle moving
into the ether -- far beyond rain, beyond dew, and
beyond Earth itself.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
(Theme music)
For some discussion of surface tension, the behavior
of the surface layer, and many surface-driven
phenomena, see: J. H. Lienhard IV and J. H. Lienhard V, A Heat Transfer Textbook. 3rd
ed., Cambridge, MA: Phlogiston Press, 2004, Click here for a free copy.
Chapter 9.
For a treatment of the so-called water bells caused
by the impact of a jet on a flat surface (like a
roofing-nail head) see, e.g., Huang, C. P., and
Lienhard, J. H., Influence of Gravity upon the
Shape of Water Bells, Journal of Applied
Mechanics, Vol. 33, No. 2, 1966, pff. 457.
Two jets, approximately 1/8th-inch in diameter with
the top jet slightly larger, collide and form a
very thin spreading sheet of water. Surface tension
pulls the sheet inward until it closes into a bell
shape.
(Photo by John Huang and John
Lienhard)
Acetone boils on a thin heated wire. The escaping
vapor is shaped by the forces of surface tension to
form a lovely periodic pattern of bubble removal.
(The true width of the image is about 1-3/4
inches)
(Photo by Bill Sun and John
Lienhard)
The Engines of Our Ingenuity is
Copyright © 1988-2000 by John H.
Lienhard.
