“Everything flows and nothing abides;
everything gives way and nothing stays fixed.”
Heraclitus of Ephesus
We live in a world of fluids, i.e., substances that can flow.
Unlike solids, fluids have no definite form but instead assume the
shape of their containers. Fluids include all liquids and gases and
a rather strange state of matter called plasma, an ionized gas that
scientists believe accounts for 99% of the matter in the visible
The importance and pervasiveness of
fluids cannot be overstated. The Earth's atmosphere, oceans and core
are fluids. We breathe and drink fluids. We sail ships in them and
fly planes in them. We are entertained by images on our plasma
televisions, illuminated by the glowing plasma in fluorescent
lights, and awed by the amorphous streams of charged particles found
Fluids are described by properties such
as density, viscosity, and compressibility and are responsible for
familiar phenomena that include pressure, buoyancy, and aerodynamic
lift. The characteristics associated with fluids derive from the
relatively weak interactions between their constituent particles.
Atoms and molecules found in fluids are not bound to fixed positions
as they are in solids
and are therefore free to roam.
Some common materials known as
non-Newtonian fluids don’t follow conventional laws of flow. With
cornstarch and water, bread dough and peanut butter,
resistance to flow changes with applied force. Silly Putty will
ooze like a viscous liquid but, when pulled apart quickly,
it willstiffen up and break
cleanly in two. Other non-Newtonian fluids such as paint and
mayonnaise flow more readily when disturbed.
The advances made in the understanding
of fluids have been substantial, but we have much to learn.
Understanding the transport of fluids across biological membranes,
the airflow around the outer surfaces of airplanes and rockets, and
the dynamics of our oceans, atmosphere, and convection in the
Earth’s mantle provide ongoing challenges.
The study of fluids is one of the oldest
branches of the physical sciences. Despite its long history, it
continues to fascinate scientists and lay people alike. In this
edition of CoolStuff we offer ways to engage your students in the
study of fluids.
Spanish TV Star Pablo Motos promised his audience that he would
walk on water...Well, not quite. They filled a pool with a mix of
cornstarch and water (a non-Newtonian fluid). When stress is applied
to the liquid it exhibits properties of a solid.
View the Video
Key Concept: Pressure is force per unit area. Atmospheric pressure
is defined as the force per unit area exerted against a surface by
the weight of the air above that surface.
Atmospheric Pressure Mat
A simple rubber
mat provides an extremely effective means of demonstrating
atmospheric pressure. The demonstration, devised by John MacDonald
of Boise State University, employs a sheet of soft rubber with a
handle at its center. The rubber sheet is square, about a foot on
each side, with a 50-gram mass hanger poked through the center.
When the rubber sheet is placed on any perfectly flat surface - the
top of a lab stool works extremely well - students find that
picking up the rubber by a corner is an easy task. Lifting the
corner allows air to get under the mat as it is lifted, thus
equalizing the pressure on the top and bottom of the mat. But
lifting the mat by the middle is another story. As the middle is
raised, a low-pressure region is formed because air cannot get in.
The greater external atmospheric pressure forces the mat and stool
together. In essence, the rubber sheet behaves as a suction cup, and
the entire stool is lifted when the handle is raised.
At sea level, the Earth's atmosphere presses against each square
inch of an object with a force of approximately 14.7 pounds per
square inch. The force on 1,000 square centimeters (a little larger
than a square foot) is about a ton! We generally go about our daily
business unaware of this persistent pressure. The Garbage Bag Hug
just might change all that!
Image: Students of physics teacher Shannon Hughes are shown here
sharing a "Garbage Bag Hug".
Barrington High School ~ Barrington IL.
Have a student step inside a heavy duty garbage bag (see figure
above). Make sure the student sits down with arms crossed. After the hose from a Shop Vac
is inserted in the top of
the bag, the student should hold the neck of the bag tightly around
their throat so as to form a tight seal.
(Note: this demo should never be done with the
bag covering the head.)
Once the student is inside the garbage bag, use the Shop Vac® to
remove the air from inside the bag. Removing air from the bag essentially vacuum seals the
student. The external air
pressure is often so great that the occupant of the bag is
completely immobilized. Safety dictates that a spotter be prepared to catch the encased student
should tipping occur.
Bernoulli’s Principle: Fluids on the
Key Concept: The pressure in a moving
fluid decreases as its speed increases, and increases as speed
A sheet of paper may be used to
illustrate Bernoulli’s Principle. Hold the bottom of the paper with
both hands. Now blow over the top of the paper. Since air moving
over the top of the paper exerts less pressure than the still air on
the underside, the greater pressure below will result in lift and
the paper will rise. This is essentially how lift for an airplane
wing is produced.
A rather maddening consequence of Bernoulli’s Principle occurs when
high winds pass over the top of an umbrella. The fast moving air
exerts less pressure on the upper surface of the umbrella than the
relatively calm air exerts from below. The result: an inverted
There are so many wonderful demonstrations of
Bernoulli’s Principle that we found it hard to choose. Here are four
that are sure to please your students.
A long plastic
bag nicely illustrates Bernoulli’s Principle. If you were to blow
the bag up by placing it firmly to your mouth, many lung-fulls of
air would be needed. But when you hold it in front of your mouth and
blow, air pressure in the stream you produce is reduced, entrapping
surrounding air to join in filling up the bag. So you can blow it up
with a single breath! This is especially effective after your
students have counted many of their own breaths attempting to
fill up the bag!
Key Concept: A fluid exerts an upward
force on every object in it equal to the weight of the fluid
displaced by the object. This is basis of buoyancy.
Most everyone is familiar with the Cartesian Diver, named after the
French philosopher Rene Descartes (1596-1650). A vial of some sort (a medicine dropper or pen cap work well) is filled with water until
it just floats. The vial (diver) is then placed inside a 2-liter
soda bottle and the bottle sealed. When the walls of the bottle are
compressed, the diver sinks. When pressure is removed from the walls
of the bottle, the diver rises.
The Cartesian diver is a wonderful teaching tool for it not only
demonstrates Archimedes' Principle (i.e. buoyancy), but the
variable compressibility of gases, the relative incompressibility of liquids, and
implications of Pascal's Principle as well. Squeezing on the top of the
sealed plastic container decreases the volume and therefore
increases air pressure above the water. By Pascal's Principle, that
pressure is transmitted to all parts of the container. This
increases the pressure inside the small glass vial. The increased
pressure decreases the volume of air at the top of the vial, and in
so doing, decreases the amount of water displaced by the vial. This
decreases the buoyant force on it enough to cause it to sink.
Cartesian divers that use packets of condiments rather than glass or
plastic vials offer an engaging, no-cost way to get students engaged
in learning about buoyancy, and the properties of fluids in general.
Have your students bring in empty 2-liter soda bottles and packets
of condiments from restaurants. Encourage students to bring a
variety of condiments. Catsup, mustard and soy sauce are easily
found. (Note: Individually wrapped Miniature Milky Ways are reported
to also work. Why not have your students experiment!)
Students should test their packets’ worthiness as divers by placing
them in a bowl of water. Good divers are those that barely float.
After a good diving candidate has been identified, it should be
placed in a 2-liter plastic bottle (The packet may need to be folded
in half lengthwise to get in through the opening). After filling the
bottle to the brim with water, students should screw the cap on
tight and squeeze the sides of the bottle.
In 1593, Galileo found himself in dire
need of money. In fact, he was only a few steps away from Debtor's
Prison. Out of necessity, he needed an invention that would net him
a tidy profit. To that end he proceeded to develop a device to raise
water from aquifers. During the process, he stumbled upon the basis
for a rudimentary thermometer. In the end, his water pump found no
market, but he did succeed in finding a method for measuring
variations in temperature.
A beautiful instrument, known as a Galilean thermometer, relies on
buoyancy to measure temperature. The device consists of a sealed
vertical glass cylinder mostly filled with a clear liquid. In the
liquid are colorful glass bulbs, each having a precise density and a
tag indicating a particular temperature. As the temperature changes,
the glass bulbs rise and sink. The temperature is read by looking at
the tag attached to the lowest floating bulb.
Like a Cartesian diver, or any other
object in a fluid, the only factor that determines whether an object
will float or sink is the object's density in relation to the
density of the fluid displaced by the object when submerged. If the
object's density is greater than the density of liquid displaced,
the object will sink. If the object's density is less than the
density of liquid displaced, the object will float. If the object
and liquid have the same density, a condition called neutral
buoyancy, the object will remain suspended at a certain depth
without rising or sinking. Neutral buoyancy is achieved
by fish, sunken logs, scuba divers and submarines.
The density of liquids varies slightly with temperature. As the
temperature increases, the density of a liquid will decrease and
vice versa. This is the key to the operation of the Galilean
thermometer. As the liquid changes temperature it changes density
and the suspended weights rise and fall to stay at the position
where their density is equal to that of the surrounding liquid. The
lowest glass ball has the greatest density, so this sinks first as
temperature rises. The correct temperature is read from the lowest
floating ball in the top half of the thermometer.
Up, Up and Away! Building Your Own
Hot Air Balloon
When I was in high school, a friend of
mine and I saw an intriguing ad in the classified section of
Popular Science magazine. The headline on the ad read “Build Your Own
Hot Air Balloon!” To say that we were intrigued by the offer would
be an understatement, so we immediately ordered the $1.00 plans for
the project. Little did we know that the investment would bring us
hundreds of hours of fun, fascination and fleeting fame. In all, we constructed six tissue paper balloons ranging in height
from 9 to 28 feet.
The images below show a "Student Built"
Paper Hot Air Balloon. Below Balloonist "Conner" is inflating his
balloon with a hair dryer.
image shows pre-launch hair dryer inflation technique. Above shows
launch. Images by Karin Laurel of Woodinville WA, via
You may wish to share this
engaging example of buoyancy with your students. Building hot air
balloons fashioned from tissue paper may be used as a class activity
or an extra credit project and need not cost much money. See Brian
Queen’s Building and Flying Paper Hot Air Balloons at
complete instructions on building a hot air balloon.
Mini Hot Air Balloon
Would you rather
demonstrate the workings of a hot air balloon on a much smaller
scale? If so, then the Tea Bag Hot Air
for you! Watch this student made video on Tea Bag Hot Air
Fluids Behaving Strangely
Non-Newtonian fluids are so named because their properties cannot be
described in terms of the concepts of classical fluids. Unlike
normal Newtonian fluids, these materials possess properties
that depend on how gently or strongly they are stirred or pulled.
The study of the flow of materials that behave in this unusual
manner is known as rheology.
Quicksand is a common example of a
non-Newtonian substance that tends to solidify when placed under
stress. The harder a person thrashes around to get out, the worse
matters become. Ketchup, on the other hand, behaves in the opposite
way. The more it’s shaken, the more readily it flows.
non-Newtonian fluids experience a sideways force known as shear,
they tend to solidify. A mixture of cornstarch and water is such a
fluid. Known to many as Oobleck, this strange substance offers
students an opportunity to become amateur rheologists.
To make Oobleck,
students will need 1 cup of cornstarch, 1/2 cup of water and, if
desired, food coloring. Instruct them to:
1. Put cornstarch in bowl
2. Slowly and while stirring (hands are fine) add the water.
3. Add food coloring as desired.
students have made their Oobleck, you may wish for them to try the
test the Oobleck by hitting it hard, then softly. They should
then stir it quickly, then very slowly.
pouring some of the Oobleck on the table, have students push on the
puddle with the side of their
hand. The Oobleck will become a solid with the application of a force, but
will return to its liquid state as soon as the force is removed.
attempt to pick up some Oobleck. Once they have it in their hands,
ask them to try
to keep it in solid form by continually kneading it.
play catch with Oobleck. They will notice that as soon as they stop kneading
the Oobleck it will return to its liquid state. This is very
obvious as it flies through the
air and is caught.
that is sure to delight your students you may wish to have them….
Run on Oobleck
After learning of the amazing properties of Oobleck, students in my
physics class thought they should be able to walk on the stuff…and
they did! They made a large batch of Oobleck in a plastic container
intended for mixing cement. After taking off their shoes and rolling
up their pant legs, off they went! Even though they knew it should
work, students were still thrilled to see that they could run across
the Oobleck without sinking. As the YouTube.com video below
shows, stopping or even slowing down while on the stuff can lead to
a sinking sensation!
Shake it Up: Animating Oobleck with Sound Waves
As you have seen, when stress is applied
to a mixture of cornstarch and water it exhibits properties of a
solid. Especially interesting is the effect produced when the
mixture is disturbed at certain frequencies.
To produce effects that have to be
witnessed to be believed, you will need a function generator, an
amplifier, a subwoofer and a dish or pie tin containing a mixture of
cornstarch and water (Oobleck). You may wish to begin with a mixture
of two parts cornstarch and one part water.
Support the container containing the
Oobleck over the top of the subwoofer. Begin your experimentation
with a 50 Hz signal and adjust until fingers of Oobleck begin to
rise from the surface of the liquid. Here are some video examples of some incredible
phenomena resulting from the acceleration of Oobleck with sound.
are aware of the importance of polarized light in modern life. Thousands of us visit amusement parks each year
and enjoy 3-D movies but don't realize that the life-like projected
images are created with polarized light. Even more people take
multi-colored liquid crystal displays, made possible by polarized
light, for granted.
The next issue of CoolStuff will examine polarized light, what it is
and why it continues to play such a prominent role in 21st
century science, technology and society.
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