"Everything flows and nothing abides; everything gives way and nothing stays fixed."
-Greek philosopher 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 universe.
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 in lightning.
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
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 will stiffen 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.
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.
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.
Garbage Bag Hug
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!
- 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 Move
The pressure in a moving fluid decreases as its speed increases, and increases as speed decreases.
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 umbrella.
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.
The Bernoulli Bag
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!
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.
Did you know...
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.
Galileo Takes the 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.
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.
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 asrheology.
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.
When some 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:
- Put cornstarch in bowl
- Slowly and while stirring (hands are fine) add the water.
- Add food coloring as desired.Once your students have made their Oobleck, you may wish for them to try the following experiments.
Have them test the Oobleck by hitting it hard, then softly. They should then stir it quickly, then very slowly.After 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.Have students 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. Have students 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. For something 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 video above 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.