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11/02/1999
Bubbles
In past columns, I''ve established my appreciation of classical
music, as well as discussing turbulent and laminar flow in
connection with the knuckleball. Now a reader, Dan from
Hawaii, e-mails me to say that I showed no appreciation for
Bernoulli''s Principle and its key role in the flight of pitched
spheroids. An experienced pilot, Dan tells me that B''s Principle
also explains why an airplane can fly. In particular, the faster a
volume of air moves, the lower its pressure. The airplane wing is
designed so the curvature of the wing causes air to move faster
over the top of the wing than under the wing. Hence, the
pressure is greater under the wing and the airplane is pushed up.
I confess that I still don''t see how that fully loaded 747 gets off
the ground! Dan also applies this reasoning to Nolan Ryan''s
curve ball. I''ll let the reader to figure out which way the ball
breaks.
Dan''s message from Hawaii prompts me to confess that my
musical tastes also run in an opposite vein from classical. In
particular, I''m a sucker for Hawaiian music, particularly that Don
Ho standard "Tiny Bubbles". The coming millennium raises
questions not only about such serious matters as the Y2K
problem but also about holiday revelry and the availability of
champagne. Of course, a key difference between champagne and
wine, a more plebian libation, is tiny bubbles. Those bubbles
contain carbon dioxide. Why a little CO2 converts ordinary wine
into something worth over $100 a bottle (if you''re a Dom
Perignon devotee) is somewhat mystifying to me. The carbon
dioxide does lend an acidic touch to the concoction since it reacts
with water to give carbonic acid. This acid is one reason things
tend to corrode in humid environments whereas in desert areas
relics from thousands of years ago maintain a remarkable
faithfulness to their original condition.
One key to the proper enjoyment of champagne is the uniformity
of bubble formation and release, promoted by careful washing and
drying of the vessel in which it is served. Preservation of the
bubbly nature of the libation is also a factor for those who prefer
to savor the beverage as opposed to those who down it in one
gulp! Not too long ago, the custom was to serve champagne in a
wide, open glass and those not in the know (like myself) may still
employ these receptacles for festive occasions. However, in the
past couple of decades, the factors affecting the flow of bubbles
out of the host liquid have been recognized. It is now realized
that lowering the surface to volume ratio lowers the rate of loss
of bubbles. Hence, today''s sophisticates serve the bubbly in
relatively tall fluted glasses for a prolonged bubbly sensation.
Those who prefer beer also are acquainted with bubbles. Brian
Trumbore is certainly much more of an expert than I am
concerning this brew. However, I gather that there is a division
of opinion as to whether beer should be served in a manner to
promote a large head and whether the bubbles should be fine or
large. In any event, I''m sure that those imbibers will have noted
at some point, staring at their glasses, that steady streams of
bubbles tend to arise at certain points on the interface between
glass and liquid. Those in a more pensive mood may even
wonder why? The answer, I''m afraid, may show the different
approaches to cleanliness and attention to detail by those who
serve champagne versus your typical barkeep.
In the relatively few bars I''ve frequented over the years, I''ve
always been impressed by the rather casual approach to cleaning
used glasses. It seems that the process generally consists of a
rather cursory dip in some sort of cleaning fluid and perhaps a
quick rinse. Just the opposite of the fastidious champagne server,
as noted above. The origin of the steady stream of bubbles is a
case of nucleation. To form a bubble, the process has to start
somehow and it is common that in all kinds of chemical processes
that there is what is known as an "activation energy" for such
things as bubble formation, the growth of crystals, formation of
rain droplets (possibly from ice crystal), etc. I''ve probably
mentioned this term before but it bears repetition.
This activation energy can be illustrated by a rock sitting in a hole
on the side of a hill. The rock obviously would like to roll down
the hill but it has to get out of the hole to do so. Someone or
something has to use some energy, activation energy, to lift or
push the rock out of the hole, after which the rock rolls down the
hill on its own. Similarly, most processes need this push to get
started. In chemistry, catalysts are substances or things that can
lower this activation energy typically by providing attractive sites
for the process to get started on. A little bit of dirt or residue of
some sort is enough of a catalyst to promote the bubble stream
seen in the beer stein. This process could be called heterogeneous
nucleation; that is, it involves a foreign substance. In the
champagne glass, you may note that the bubbles tend to arise
within the liquid itself. This may be heterogeneous nucleation
involving fine particles not seen or homogeneous nucleation,
spontaneous growth of bubbles in the liquid. Ok, it is possible
that scratches in the glass or mug also provide nucleation sites.
Again, however, the champagne sophisticate will probably be
more inclined to treat his stemware more gently? Lest you think
this piece to be somewhat snobbish in nature, let me assure you
that my wife continually has to remind me to not put our good
stemware in the dishwasher.
We tend to think of bubbles as being innocuous, ephemeral
objects. I imagine most of you have fond memories of dipping a
wire contraption into a solution and wafting bubbles into the air.
In my day, we didn''t have such advanced technological devices
but relied on the bubble pipe. I remember mine as having three
bubble chambers! Actually, in certain circumstances, bubbles are
not at all gentle. The August issue of Discover magazine presents
some nice pictures of bubbles in action and credits Lord Raleigh
in England with showing how bubbles in the ocean can erode a
ship''s propellers. When I was a real chemist, I used to employ
ultrasonic cleaning to keep my glassware spotless. This process
employs bubbles to advantage. The Discover article also
mentions that surgeons use bubbles to liquefy the fat they deftly
suck out of certain portions of the anatomy. Yuk!
How does the bubble accomplish such tasks? When a bubble is
made to expand, perhaps to a hundred times its original size, the
pressure inside the bubble is lowered drastically. Then, when it
collapses back and it''s on a surface, a tiny jet of water is launched
through its center. This tiny jet can be going hundreds of miles an
hour, just the opposite of the old form of torture where a person
is held under a slow drip, drip, drip for hours or days. The fast
moving jet is more like a spear hitting a surface, say of a ship''s
propeller, and the cumulative effect of prolonged pounding by
millions of bubbles can be quite devastating.
Bubbles have even been touted as a means to achieve nuclear
fusion, the Holy Grail as an energy source when oil runs out.
This possibility is based on a phenomenon known as
"sonoluminescence". When bubbles are exposed to high pitched
sound waves they glow, giving off light. The origin of this light is
the subject of debate in the scientific community. One school
believes that the ultrasound forms a shock wave so concentrated
that the local temperature is a couple million degrees Fahrenheit!
This strips electrons from the gas atoms to form a plasma, which
emits light. This purported high temperature is what leads to the
proposal that bubbles could produce nuclear fusion. Andrea
Prosperetti of Johns Hopkins University proposes instead that our
tiny jet of water slams into the wall of the bubble at 4,000 miles
an hour and "cracks" the water film as if it were a solid. This
energy is then released as light. Prosperetti thinks the
temperature is only about 10,000 degrees Fahrenheit, not
millions. If true, no nuclear fusion. Time will tell.
At Bell Labs many years ago, there was great interest in another
kind of bubble, the magnetic bubble. The magnetic bubble device
involved the magnetization of very small areas in a material.
These areas, known as magnetic domains could be oriented in an
up-and-down direction in thin films of certain garnets. Applying a
vertical magnetic field to these materials causes the areas of
opposite fields to shrink to circles or "bubbles". By changing the
orientation and strength of the field in a small spot the bubble can
be created or destroyed. I don''t know if memories based on
magnetic bubble patterns are being used much these days. In the
old Bell System, one use was to record certain automated
messages. That voice you heard came from a garnet, a gemstone
in other settings such as on that charm bracelet you or your
spouse wears on special occasions.
Bubbles play a role in another kind of charm. I''m referring, of
course, to Lucky Charms. I myself have never knowingly eaten a
Lucky Charm. Yet, Lucky Charms have been one of General
Mills leading children''s cereals for 35 years. Another article in the
same issue of Discover magazines notes that the charming
component of this cereal is formed by extruding a foam (to me
this means lots of tiny bubbles) consisting of sugar, gelatin and
corn syrup. The extruded material is cut and, as we scientists
would say, heat-treated to form "marbits", essentially bits of
marshmallow. Well, it appears that the control of the flow of this
compressible foam to engender the formation of a marbit is not a
simple matter. This is especially true for the more complex
marbits developed over the years through evolution. The original
pink hearts, yellow moons, orange stars and green clovers have
mutated into shapes such as blue diamonds, purple horseshoes
and even the Eiffel Tower! I don''t know if Bernoulli is relevant
here.
As I finish this column, I''m listening to what else? Don Ho and
"Tiny Bubbles" on my computer''s CD. Dan from Hawaii assures
me that Mr. Ho is still performing there. Mahalo, Dan. Aloha.
Allen F. Bortrum
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