Chemical Education Journal (CEJ), Vol. 15 /Registration No.
15-111/Received August 31, 2013.
URL = http://chem.sci.utsunomiya-u.ac.jp/cejrnlE.html
Abstract
This experiment demonstrates the phenomena created when different
types of eggs collide with cornstarch aqueous solutions. The eggs
used in this experiment include raw eggs, unshelled eggs, and
century eggs (i.e., preserved eggs). The cornstarch solution
is formed by mixing cornstarch with water (1 part water to 1.5-2
parts cornstarch). This non-Newtonian fluid is called an "oobleck"
or "ooze". When an egg collides with the oobleck, it
suddenly depresses the contact surface. Like billions of little
springs, the surface compresses and stretches, mitigating the
force of impact and causing the object to bounce off without damage.
The intermolecular interactions and van der Waals forces (such
as dipole-dipole forces, dipole-induced dipole forces, and London
dispersion forces) can be recorded and watched with rapid photography.
Keywords: non-Newtonian fluid, oobleck, egg, van der
Waals force
At a stand at a fair, there was a vendor who challenged the audience, "Whoever can break an egg by throwing it into this basin filled with cornstarch slurry will win drinks and candy." This challenge attracted a group of people, and each of them tried. No matter how hard they threw the egg, though, no one could break it. Finally, a basketball club vice-captain threw an egg so hard that it bounced out of the basin, and it broke when it hit the ground. This sparked the curiosity of students, thus inspiring the following series of experiments.
Oobleck [1], a kind of non-Newtonian fluid [2], becomes a fluid when agitated and returns to a solid or semi-solid state when allowed to stand. If one slowly squeezes an oobleck, it will flow between the fingers like a liquid; if the substance falls onto the floor, the force of gravity on the oobleck will cause it to behave as a solid. Thus the oobleck demonstrates thixotropic [3] properties. The instant contact between the surface of the materials generates intermolecular van der Waals forces.
A plastic basin (size: 60.5 cm x 48 cm x 16 cm), 5.0 kg cornstarch [4], two raw eggs, a century egg [5], water, 1.0 M hydrochloric acid, and a 250 mL beaker. The century egg (called pidan in Mandarin Chinese) is a duck, chicken, or quail egg that has been preserved in a mixture of clay, ash, salt, quicklime, and rice hulls for several weeks to several months.
1. Mix cornstarch with water (1 part water to 1.5-2 parts cornstarch)
to create an oobleck on the plate, about six centimeters thick.
2. Prepare an unshelled egg by soaking a raw egg in 150 mL 1.0
M hydrochloric acid in a 250 mL beaker for about an hour (see
Figure 1). This will dissolve the
egg's outer shell and leave only the membrane.
3. Drop a raw egg onto the oobleck (see Figure 2) and observe the egg bounce off the oobleck.
4. Drop the unshelled egg from Step 2 onto the oobleck and
observe the unshelled bounce off the oobleck.
5. Drop the century egg onto the oobleck (see Figure
3) and observe the century egg bounce off the oobleck.
6. Take a wad of oobleck and drop it onto the oobleck in the plastic basin. Observe the bouncing phenomenon.
When a raw egg, unshelled egg, or century egg is dropped onto an oobleck, the egg will bounce off undamaged. This phenomenon can be observed using rapid photography and uploaded to YouTube [6].
1. A raw egg dropped onto an oobleck will bounce off
without breaking (see Figure 2). A
raw egg is covered with a protective eggshell, and the main component
of the shell of the egg is calcium carbonate (an ionic compound).
Cornstarch has high amylose content, the main component of straight
chain _-glucose polymers with spring-like helical structure (polar
polymer molecules, including hydrogen bonds and polyether linkage).
The raw egg tilts the surface of the oobleck upon impact and thus
bounces. The forces at work between the surfaces are mainly dipole-induced
dipole forces.
2. An unshelled egg dropped onto an oobleck will also bounce off
undamaged. The unshelled egg is protected by a membrane whose
main ingredient is protein (polar polymer molecules containing
amide bonds and hydrogen bonds). The forces at work between the
surfaces are mainly dipole-dipole and hydrogen bonding forces.
3. A century egg dropped onto an oobleck will bounce off,
only this time even higher (see Figure
3). The shell of a century egg is comprised primarily of a
short-chain protein (polar molecule aggregation materials, including
amide bond, hydrogen bond). The forces at work between the surfaces
are mainly dipole-dipole and hydrogen bonding forces.
4. Dropping a wad of oobleck onto an oobleck surfaces produces
bouncing followed by fusing of the wad into the solution in the
basin. Oobleck is made of a cornstarch suspension, polyvinyl alcohol-based
glue and borax.
5. These types of materials could be used in car bumpers.
Car bumpers are generally made of chrome plated hard shells. If
these were replaced with short-chain polymer materials with hydrogen-bonding
short-chain amide bonds, vehicles could park more easily in tight
spots. In the event of a collision, there would be no damage to
the vehicle or structure. If the vehicle were to be violently
hit, the polymer bumper would automatically "bounce,"
thus preventing damages and casualties.
The author thanks Dr. Yang-Min Lin, the teachers of National Yilan Senior High School, and Chieh-yu Su, Ci-chen Sun, the Students of New Taipei Municipal Hsintien Senior High School, for they made experiments respectively along with author.