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By C. L. Stong May, 1969
Edited and expanded by Shawn Carlson
3 December 2004
ABOUT TWO MONTHS AGO I blew a big soap bubble inside
a glass jug and put the jug on a bookshelf. The bubble
is still there, iridescent and shimmering. It may
last for years.
Since then I have blown other bubbles
that lacked the protection of a glass cover. They
have survived for a few minutes or even a few hours,
depending on such conditions as drafts and the amount
of dust in the air. In any case, the bubbles have
lasted long enough for experimentation. For example,
I have made bubbles into inexpensive instruments that
measured the viscosity of gases, determined the surface
tension of fluids or served as ultra-sensitive manometers
for measuring atmospheric pressure. I have blown other
bubbles to demonstrate simple geometric propositions
and to solve mathematical equations related to electrical
circuits and optical lenses. Mostly, however, I have
been blowing bubbles for fun: spheres up to 60 cm
(2 feet) in diameter, small bubbles blown inside larger
ones, "weightless" bubbles that float in
containers of heavy gases, configurations of adjoining
bubbles that resemble the lattice structure of crystals,
and so on.
It all began early last winter when
I visited A. V. Grosse, president of the Research
Institute of Temple University in Philadelphia. Much
of Grosse's career has been devoted to nuclear research
and investigating high temperatures. For this reason
I was scarcely prepared, on entering his office, to
see a huge display of soap bubbles. They were on tables
and bookshelves, and one bubble, suitably protected,
even served as a paperweight. Since the career of
most bubble blowers ends at about age 10, I wondered
why soap bubbles continued to interest the administrator
of a prominent research establishment. In answering
that question Grosse not only revived my own interest
in bubbles but also agreed to share his enthusiasm
with readers of this department. He writes:
"During a pleasant discussion several years ago
with my former colleague, Willard F. Libby, we concluded
that as members of the older generation of atomic
scientists we might well spend part of our spare time
devising simple new experiments for the entertainment
and education of the younger generation. The discussion
eventually led me to the investigation of soap bubbles.
This field of experimentation seemed appropriate because
soap bubbles are inexpensive and very pretty and present
many unanswered questions. I set up a small bench
in my basement at home and have been blowing bubbles
ever since. So many new facts and phenomena turned
up that until now I have neglected my original objective
of passing my findings along to amateurs.
"A soap bubble behaves somewhat
like an inflated rubber balloon. The skin of the bubble
is stressed, because of surface tension, and like
stretched rubber it compresses the air trapped inside.
As soon as the bubble is blown the compressed air
begins to make its way through the permeable skin.
The bubble shrinks. Eventually the bubble collapses
to a flat film stretched across the blowpipe. I call
this interval—the time required for the bubble
to shrink to a flat film—the natural lifespan
of the bubble.
"My first bubbles all died in
infancy. Few lasted more than a minute. They were
blown with much the same kind of soap that Isaac Newton
used for his classic bubble experiments. I turned
to the literature and found that a much better soap
was described more than a century ago by the blind
Belgian physicist Joseph A. F. Plateau, who laid the
foundations of our present knowledge of soap bubbles.
Bubbles with a radius of four or five centimeters
blown with Plateau's solution will last for several
minutes in an ordinary room and for several hours
with proper protection. [See the sidebar for this
recipe. Note: it takes eight days to prepare!]
[Editors note: Here's something that is much easier
to try. Sodium oleate is an emulsifiant—it allows
the water and glycerin to combine without separating.
Dishwashing detergents also contain emulsifants; that's
how they dissolve hydrophobic substances, like grease,
off your pots and pans. So a perfectly adequate bubble
solution can also be obtained by dissolving four parts
glycerin with two parts liquid dishwashing detergent.
A trusted friend of my mine once told me that adding
one part white Karo Syrup to this recipe increased
the bubble's stability, though I have never tested
that myself. I hope you will experiment with these
and other bubble making formulas and let me know what
you discover so Forrest and I can share your results.
Quantifying the qualities of different bubble formulas
found by systematically altering the ratios of different
quantities would make for an excellent article in
TCS. SC.]
"About 10 years ago [ca.
1959. SC] the Canadian chemist A. L. Kuehner described
a solution that makes more durable bubbles. He first
purified oleic acid by a painstaking procedure and
then, by the addition of bromine, converted it into
9,10-dibromostearic acid. When this acid is carefully
neutralized with sodium hydroxide and mixed with glycerin,
it forms bubbles that will last for months if they
are protected by an enclosure.
"Having experimented with Kuehner's
solution, I decided to take advantage of recent developments
in polymer chemistry in the hope of compounding a
solution of even greater interest. The result is what
I call a 'double bubble' solution: two solutions that
are mixed just before use. Bubbles can be blown with
either solution, hence the name 'double bubble.' Large
bubbles blown with the mixture have lasted for three
years so far and are likely to last for many more.
From the rate at which one bubble is shrinking I estimate
a natural life of more than 20 years.
"One of the two solutions consists
essentially of Kuehner's dibromostereate soap. The
other one is a solution of polyvinyl alcohol, water
and glycerin. Bromine is an extremely hazardous substance
before it has reacted with the acid, and compounding
these mixtures calls for controls that are not available
in the home. For these reasons amateurs should not
attempt to make double bubble solution. It can be
bought from the Techno Scientific Supply Company,
Inc., [Apparently, this company is no longer operating
under this name. If anyone can finds a source of this
mysterious solution, please
share it with the rest of us! SC] The solution
costs $4 a pint, a quantity sufficient to blow thousands
of bubbles.
"Bubbles can be blown with almost
any pipe. As was pointed out by the English experimenter
C. V. Boys, however, moisture from the breath tends
to condense inside the pipe and drain into the soap
film, where it dilutes the solution and hence reduces
the life of the bubble. Pipes that are blown by mouth
should have a trap of some kind to catch the exhaled
moisture. The carbon dioxide in the breath may also
be harmful to the bubble. Therefore it is best to
do the blowing with compressed air. Small compressors
of the kind used for supplying air to aquariums are
ideal for blowing bubbles. The rate at which the bubbles
are blown can be adjusted by placing a pinch clamp
on the tubing that leads from the compressor. To blow
big bubbles, it is advisable to interrupt the blowing
periodically. [Presumably, this is to allow a few
seconds for the fluid to flow about so as to equalize
stresses and fill in any sections that irregularities
in the blowing process may have caused to thin out
faster than the rest of the bubble. SC]
"Dust is the archenemy of bubbles,
particularly dust that contains crystals of salt.
It is interesting to set up a bubble in one corner
of a room filled with clean air and with a perfume
atomizer inject a puff of brine into the air at the
opposite corner. Microscopic crystals of salt from
the evaporated mist will make their way across the
room; within a minute or two they will break the bubble!
[Why should this be the case? Another excellent opportunity
for amateur experimentation. SC]
For this reason I blow my bubbles
with 'glycair,' which is air that has been filtered
through glycerin. An adequate filter can be made by
cutting the bristle sections from four to six test-tube
brushes and inserting them into a glass tube they
fit snugly. The tube is closed at the ends by perforated
rubber stoppers. An ounce or so of glycerin is placed
inside the tube, which is kept in the horizontal position.
The bristles are moistened with glycerin periodically
by rotating the tube. [I suggest that a loosely rolled
strip of paper towel that filled the tube might serve
as well. Please do the experiment and let me know
what you discover. SC]
"Big bubbles require more solution
than small ones. A relation also exists between the
ultimate size to which a bubble can be blown and the
diameter of the blowpipe from which it is blown, since
the bubble is held only by the surface tension between
the solution and the rim of the pipe. Bubbles up to
about 10 centimeters in diameter can be blown with
a pipe one centimeter in diameter, but I prefer a
ratio of about five to one. Bubbles that are intended
to last their full natural life must be blown inside
a protective enclosure, preferably one with a spherical
shape, such as a Florence flask.
"I first wash the container
carefully, put about 10 milliliters of bubble solution
in it and flush it with glycair for a few minutes
to remove as much dust as possible. A blowpipe is
pushed into a perforated stopper that fits the neck
of the container. I dip the lower end of the pipe
into soap solution, insert it in the container and
blow the bubble. The stopper is pressed firmly into
the neck of the container and a stopper seals the
free end of the blowpipe.
"Bubbles made in this way may
break after a short time because of dust trapped in
the container. The bubble traps the dirt, however,
and purifies the air of the flask. Therefore, without
opening the flask to the dusty air outside, I push
the blowpipe through its supporting stopper down into
the solution inside the container, slide it back up
and blow another bubble. The second or third bubble
should last for its natural lifetime. The container
is stored on a mat of foam rubber for protection against
mechanical shock. The diameter of each bubble is measured
at weekly intervals and the shrinkage is plotted against
time. From these data I estimate the ultimate life.
"Bubbles can be blown inside
one another by several methods. An easy way is to
blow a small bubble, about the size of an orange,
and put it on the rim of a paper cup that has been
moistened with bubble solution. Detach the blowpipe
from the bubble by turning the pipe at a right angle
to the film and lifting it gently. The film will close
and peel away from the pipe.
"Immerse the end of the pipe
in bubble solution to a depth of a few centimeters
and push it into the top of the bubble. It will penetrate
the bubble without breaking the film. As you blow
the inner bubble, the outer one will expand in proportion.
Detach the inner bubble by pulling the pipe upward,
abruptly but not violently. This is a knack that comes
with practice.
"Concentric bubbles can also
be blown by means of coaxial pipes telescoped together
and supported by spacers of rubber or plastic. Place
a soap film across the end of the outermost pipe and
blow the outer bubble. Slide the next smallest pipe,
with a film, into its supporting pipe axially and
blow the second bubble, which enlarges the first one.
Additional bubbles can be blown. I have had triple
bubbles, with beautiful colors, that have lasted for
more than a year.
"Foams are also easy to make.
Tie a bag of terry cloth around the end of a blowpipe,
soak the cloth in bubble solution and blow. The walls
between adjacent bubbles make interesting geometric
patterns. To observe the walls clearly, blow foam
between two sheets of window glass, spaced about an
inch apart, that have been moistened with bubble solution.
“The air pressure inside a
bubble increases as the bubble shrinks. For this reason
the wall that is shared by two adjoining bubbles,
as in foam, always curves toward the larger of the
two bubbles by an amount that can be predicted by
simple geometry. For example, make a dot (a) on a
sheet of paper. From the dot draw three straight lines
spaced at angles of 60 degrees. Then draw a straight
line that intersects the three lines (points b, c
and d). Place the point of a compass at each of these
intersections successively and, with the dot as the
radius, draw three circles. The two smaller circles
mark the boundary that would be formed if two bubbles
of this size were placed in contact with each other.
The smaller circles intersect the largest circle at
two points. The arc of the largest circle included
between these two points marks the position of the
wall that would be shared by the adjoining bubbles.
"This theory was worked out
by Plateau. You can prove it by experiment. Place
a diagram so drawn under a sheet of window glass moistened
with bubble solution. Blow a pair of hemispherical
bubbles against the glass, about the size of the two
smaller circles. Detach the blowpipe from each bubble
and with the end of the pipe center the bubbles over
the circles. The size of the bubbles can be adjusted
by adding air or letting air escape through a small
blowpipe, such as a soda straw. When you have matched
the size of the bubbles to the diagram, you will find
that the curvature of the shared wall conforms to
the geometric prediction. This combination of bubbles
can be used to find the total resistance of an electrical
circuit consisting of parallel resistors and to predict
the distance at which a scene will come to focus behind
a lens of known focal length.
"Observe also that any three
walls of the bubbles make equal angles of 120 degrees
at the point where they come together. This must be
so because the surface tension of the three soap films
is equal: all three films exert pull on the point
where they meet. They balance only when the three
pulls are exerted in mutually opposing directions.
To prove that surface tension acts
uniformly in all directions, cut two straight wires
about 10 centimeters long, bend small loops at the
ends and tie the mating pairs of loops together with
silk threads about 10 centimeters long. The resulting
structure is a rectangle with ends of wire and sides
of thread. Attach a bridle thread to one of the wires
and dip the assembly in bubble solution. When you
pull it out of the solution, a soap film will cling
between the wires and the threads. The film will have
straight ends and curved sides. If the sides curve
inward excessively, hang wire weights to the bottom
member.
"Tie a length of silk thread
into a loop, dip the loop in bubble solution and toss
it onto the film. Touch the film inside the loop with
a point of dry paper. The film will break, and surface
tension outside the loop will snap the thread into
a perfect circle. The circle is the only possible
shape the loop can assume when it is pulled outward
by a uniform radial force. [Do it. This is a super
cool demonstration! SC]
"As mentioned, surface tension
compresses the gas inside a bubble. If you punch a
small hole in the bubble, air will flow out. The bubble
will collapse at a rate that increases with the size
of the hole and with the surface tension and decreases
with the density of the gas. To make a bubble deflate
slowly, twist a length of fine wire into a row of
three small loops that lie in a common plane. With
silk thread tied to the end loops, dip the assembly
in bubble solution and suspend the wire against the
bubble. To let the air out, puncture the soap film
in the middle loop with a splinter of dry wood.
"I recently devised a similar
experiment that amateurs can use for measuring the
surface tension of soap film or the viscosity of a
gas if either quantity is known. If both quantities
are known, the experimenter can accurately predict
in advance of the experiment how long it will take
the bubble to deflate from any size to any smaller
size. All you need for the experiment is bubble solution,
a slender blowpipe, a fixture to support the pipe,
a small aquarium and a watch with a second hand.
"Blow the bubble, plug the blowpipe
with a stopper and measure the diameter of the bubble.
Remove the stopper. Time the interval during which
the bubble shrinks. Measure the diameter of the shrunken
bubble. With these data and the known dimensions of
the blowpipe you can easily determine the quantities
of interest.
"My blowpipe is 29 centimeters
long with an inner radius of 0.182 centimeter. (It
is a length of four-millimeter glass tubing.) The
ends were pushed into perforated rubber stoppers and
the stoppers were in turn pushed into three-centimeter
lengths of 10-millimeter glass tubing. These large
glass nipples at the ends prevent bubble solution
from entering the bore of the small tube. Either nipple
will also accept a stopper for plugging one end of
the blowpipe. The dimensions are not critical. Any
slender tube of about this size will work. It is essential,
however, to determine the actual dimensions as accurately
as possible. The length can be measured with a ruler.
To find the radius, fill a portion of the slender
tube with mercury and measure the length of the filled
portion in centimeters. Transfer the metal to a container
and determine its net weight in grams. Divide the
weight of the metal by 42.55 times the length of the
column. The square root of this quotient is equal
to the radius of the tube in centimeters. For example,
a 10-centimeter length of my tube holds 14.2 grams
of mercury. Therefore its radius is (14.2 / (42.55
x 10))1/2 = (0.0333)1/2, or 0.182 centimeter.
[Caution: Mercury works very well
here because it will not "wet" that glass.
That is, you can recover it all without worrying about
what sticks to the sides of the tubes. But, of course,
mercury can be difficult to get and dangerous to use
if you don't know what you are doing. I don’t
know of any good substitute fluid to use here, and
would welcome any suggestions. However, there is another
way to do this. Try packing the end of the tube with
a small amount of wood putty. After it sets, you can
push out the plug with a length of stiff wire inserted
into the opposite end of the tube. The outer diameter
of the plug equals the inner diameter of the tube.
It can be easily (but gingerly) measured
with a set of calipers. SC]
"I blow the bubble inside a
small aquarium of the type used for keeping tropical
fish. (Any large glass beaker or battery jar will
do.) The aquarium is covered with a sheet of transparent
plastic that has two narrow slots along the middle
that lead almost to the hole in the cover through
which the blowpipe is inserted; the slots are thus
on a straight line through the center. Through the
slot I suspend two small plumb bobs made with a silk
thread and a penny. To measure the diameter of the
bubble I slide each plumb bob toward the bubble until
the thread is within .5 millimeter of the soap film,
as judged by eye. This brackets the bubble and gives
its diameter within one millimeter.
"The experimental procedure
is simple. Blow a bubble of any diameter. Remove the
air hose from the blowpipe, plug the pipe and measure
the radius of the bubble. Remove the stopper from
the blowpipe and time the interval during which the
bubble shrinks. Insert the stopper and measure the
smaller radius.
"It turns out that the time
of efflux varies as the fourth power of the radius
of the bubble. [An important but little known fact
of fluid dynamics! SC] Raise each radius to the fourth
power. Subtract the smaller figure from the larger
one. Assume, for example, that you let the bubble
contract from a radius of two centimeters to a flat
film across the end of the blowpipe. The figure would
be 24 - 04 = 2 x 2 x 2 x 2 – 0 x 0 x 0 x 0 =
16. If the bubble had contracted from a radius of
20 centimeters to one of 18 centimeters, the corresponding
figures would be 204 – 184 = 160000 –
104976 = 55,024.
"Assume that you want to determine
the surface tension in a soap bubble that contracts
from a radius of two centimeters to a flat film in
4.94 seconds. The next consideration is viscosity,
which is a measured in units of poises [after Jean
Louis Poiseuille (1799-1869). In this strange system
of MKS units, the basic unit is the Poiseuille. One
Poiseuille equals 10 poises, and one poise is about
the viscosity of olive oil at room temperature. SC]
The viscosity of air at 20 degrees Celsius is 1.83
x 10-4, or 0.000183 poise. The surface tension of
the two-centimeter bubble is equal to twice the length
of the blowpipe, multiplied by the viscosity of the
air and the fourth power of the radius as determined
above, divided by the product of the time in seconds
multiplied by the fourth power of the radius of the
blowpipe.
[It's so much easier to read this
when it is expressed as an equation. Taking T to be
the surface tension and t is the time in seconds:
SC]
The example works out as follows:
(2 x 29 x 1.83 x 10 -4 x 16) / (4.94 x 11.31
x 10 -4) = 30.4 dynes per centimeter. (A
force of one dyne is about equal to the weight of
a hungry mosquito.)
Both the surface tension and the viscosity of the
air are now known, so that the efflux time can be
predicted. With the same blowpipe, how long would
a bubble take to shrink from a diameter of 40 centimeters
to one of 36 centimeters? The difference in the fourth
power of the radii is 55,024. To compute the efflux
time, multiply twice the length of the blowpipe by
the viscosity of the air and divide the product by
the surface tension multiplied by the fourth power
of the radius of the blowpipe.
[Again, for the non-math phobic:
SC]
The resulting quotient is a constant: (2 x 29 x 1.83
x 10 -4) / (30.4 x 11.31 x 10 -4)
= 0.309. Multiply the difference in the fourth power
of the radii by the constant to get the time in seconds.
In this example, 55,024 x 0.309 = 17,002 seconds,
or about 4.7 hours.
Assume that the surface tension of the bubble is known
and you want to determine the viscosity of a gas,
such as hydrogen. Blow the bubble with hydrogen and
proceed as above. Experiment would reveal that the
efflux time needed for a hydrogen-filled bubble to
shrink from a radius of 20 centimeters to 18 centimeters
is about 8,100 seconds. The viscosity of the gas is
equal to the product of the surface tension, the fourth
power of the radius of the blowpipe and the efflux
time, divided by the product of difference of the
fourth power of the radii divided times twice the
length of the blowpipe:
[The equation is… SC
SC]
(30.4 x 11.31 x 10 -4 x 8,100) / (2 x 29
x 55,024) = 8.73 x 10 -5 poise. I find it
interesting to blow bubbles with different gases,
such as a variety of Freons [Oh, the good old days.
SC], and determine their viscosity by this procedure.
"A more ambitious project involves the construction
of a useful manometer in which a nearly flat soap
film functions as the sensing element. The instrument
is capable of indicating directly changes in air pressure
of less than a millionth of an atmosphere. Cut a circular
hole, 45 millimeters in diameter, through the bottom
of a plastic Petri dish to take a No. 10 rubber stopper.
Perforate the stopper axially to accommodate a sleeve
of 20-millimeter glass tubing four centimeters long.
Push the sleeve through the stopper from the smaller
end until it is flush with the top, and insert the
assembly into the Petri dish from the top. Plug the
top of the sleeve with a No. 2 rubber stopper perforated
by two holes and plug the bottom by a similar stopper
that has one central hole.
"The assembly rests on a tabletop, preferably
one that is covered with hard, smooth plastic. Drill
a hole in the table to take the glass sleeve and couple
the sleeve to a five-gallon carboy under the table.
The carboy must be well insulated against heat, preferably
by a covering of rock wool 12 centimeters thick.
"Place about 10 milliliters of bubble solution
in the Petri dish and blow a soap film across the
edge of the dish. Insert a round-headed pin through
the film and into the doubly perforated No. 2 stopper.
Drill a second hole in the tabletop near the Petri
dish to admit a snugly fitting glass sleeve and
couple the lower end of the sleeve to a glycair
filter. Invert a cake plate over the Petri dish
and insulate it with at least five centimeters of
rock wool. Make two holes in the rock wool insulation
for observing the relative distance between the
pinhead and the soap film. As the barometric pressure
changes, the soap film will move up or down in relation
to the pinhead. You can observe the changes through
a small telescope. I constructed the device to demonstrate
one of many possible applications of soap films.
The details of construction are obviously amenable
to modification, as is the method of reading the
instrument.
"In conclusion, I invite both amateurs and
experts to help me solve a fascinating puzzle that
turned up during a recent experiment. I wondered
how a bubble would behave if it were blown with
hydrogen in an atmosphere of hydrogen at low pressure.
I blew the bubble in a round-bottomed flask that
had a side arm and stopcock connected to an air
pump. The blowpipe was inserted through a glass
sleeve in a perforated stopper that fit the flask,
and the telescoping joint between the blowpipe and
the sleeve was sealed with a thin rubber tube. Bubble
solution was placed in the bottom of the flask.
The flask was exhausted to the limit of the air
pump, flushed with hydrogen twice and again evacuated.
The pump was shut off. Hydrogen was admitted to
the flask until a manometer in the system indicated
a pressure of 40 torr. The blowpipe was lowered
into the bubble solution and withdrawn, and hydrogen
was admitted to blow a bubble with a radius of about
two centimeters.
"Next I shut off the hydrogen supply, intending
to observe the rate at which the diffusion of hydrogen
through the soap film caused the bubble to shrink.
To my amazement the bubble began to expand! Within
about four hours it doubled in size. Thereafter
it slowly shrank to a flat film as expected. I then
blew the flat film into another bubble. The cycle
was repeated.
"The experiment has been performed with other
apparatus and under other conditions but always
with the same result. The bubble expands without
apparent cause! None of the obvious explanations
is satisfactory. My stopcocks do not leak, the vacuum
is held constant for weeks and the apparatus is
trustworthy in other respects. I can only conclude
that much remains to be learned about soap bubbles,
and I urge amateurs to join me in the fun of solving
the puzzle."
C. V. Boys, Soap-Bubbles. Their Colours And The
Forces Which Mold Them. Dover Publications, Inc.,
1959.
A. V. Grosse, The Natural Life Of Soap Bubbles,
Report RITU 1967-1. The Research Institute of Temple
University, 1967. 
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