March 14th, 2011 § 18 comments § permalink

Oobleck Goop 1Can we ever so briefly have a celebration for oobleck?  It deserves a surprise party, or at least a pat on the back, and a sweet-smelling chocolate cosmos, scabiosa pod, and hydrangea bouquet.  On days when I have scarcely been able to brush my teeth due to little grabby hands and empty bellies, I simply pour oobleck into a mixing bowl, pop in some little hands, and beam myself just about anywhere I want to be.  Keep in mind that the entire process is meant to be messy.  Prepare to clean up a bit.

This project requires the following: cornstarch, water, a bowl and small hands willing to get messy.  That’s it.  Fancy schmancy oobleck (some of you are fancy, I know) requires an eyedropper, tempera paint, and a mixing spoon in addition to the above.  The cornstarch to water ratio will most likely need some tweaking to get the ideal consistency, but 1½ cup cornstarch to 1 cup water is a good jumping off place.  Pour the cornstarch into a large mixing bowl; slowly add the water and mix.  Add smidgens of additional water with a teaspoon or eyedropper.  Ultimately, the ideal suspension will feel like molasses and will “tear” a bit when small fingers stroke its surface.

Oobleck Goop 2The viscosity of oobleck is not constant.  It behaves like a solid or a liquid depending on how much pressure is applied.  Squeeze some in your palm and it will form a solid ball.  Release the pressure and it will flow out between your fingers.  A material that behaves this way is called a “non-Newtonian liquid”—its viscosity changes depending on the stress or force applied to it.  If large force is applied (compression, agitation), it becomes viscous and stationary.  If a teeny force is applied (a gentle pour), it flows like molasses.  Why does oobleck behave the way it does?  When sitting still, the starch granules are surrounded by water.  The water’s surface tension keeps it from completely flowing out of the spaces between the starch granules.  The cushion of water provides lubrication and allows unconstrained movement of the granules.  But, if large force is applied, the water is squeezed out from between the granules and the friction between them increases considerably.  When cornstarch is heated (mmmm.  gravy, for instance), the molecular chains unravel, allowing them to collide with other starch chains to form a mesh, thickening the liquid.  Hope I have not lost you….

Oobleck Goop 3Oobleck is a “suspension,” not a solution.  Cornstarch does not dissolve in water like salt or sugar.  Instead, the tiny cornstarch particles are suspended in the water.    If you allow oobleck it to sit in a bowl for long enough, the cornstarch and water will separate.  Because of this, it is important to not wash it down the sink.  Don’t gum up those pipes!  In fact, while you were poring through your latest issue of Physical Review Letters, Volume 106, Issue 5, perhaps you came across the related article “Viscoelastic Suppression of Gravity-Driven Counterflow Instability” and then moved on to “Complex Fluids at Work.”  And then, you were super surprised hear NPR’s Weekend Edition discussion “Could Cornstarch Have Plugged BP’s Oil Well?

Maybe not.  You seriously should consider checking these out.

What is cornstarch, really?  It is obtained from the corn seed’s endosperm.  The endosperm of the seed surrounds the embryo (developing plant) and provides nutrition in the form of protein (starch) for the sprout.  Corn endosperm makes cornstarch, wheat endosperm makes flour, and barley endosperm makes, well, beer.  To see the endosperm of a corn kernel, soak a seed in water overnight and cut it lengthwise.  You will most likely be able to identify the seed coat or “pericarp,” embryo or “germ,” and the endosperm.

Corn Seed DissectionAnd the name?  The name oobleck originates from the 1949 Dr. Seuss book “Bartholomew and The Oobleck.”  In the story, the King of Didd, bored with mundane weather, asks his royal magicians to whip up something a bit more dramatic.  Soon thereafter, sticky, gummy green goo falls from the sky and wreaks havoc on his tiny village.  A fantastic story with a typical Seuss-ish moral:  There are some things in life that are best left as they are.

Something to think about.


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