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Introduction to Bathtub Science

Question: What do you get when you mix 300 pounds of cornstarch and water with an antique claw-foot bathtub?

Answer #1:  an astounding mess.

Answer #2: A very practical introduction to the science of “Rheology

Some useful terms (loosely defined):

RheologyThe study of how materials deform when stressed

StressThe forces acting on a material

StrainHow the material responds (deforms) when stressed

The difference between Stress and Strain:

We all have an intuitive understanding of stress and strain…stress is all the forces that bear down on us in our everyday lives…strain is how we deal with them.  Many people experience  the same sources of stress (school, work, telemarketers…) however,  different people respond to these stresses differently…if you’re elastic like a spring, you’ll bend with the stresses and bounce back with no permanent damage…if you’re brittle like glass,  you'll shatter and break.

Physical materials also have different responses to stress.  some simple rheologies that are important in Earth science are: (it would be great to find some simple animations of springs and fluids and  glass etc.)

Elastic: behave like a spring

Viscous:  flow like water or honey

Brittle:  break by cracking (as rocks do in earthquakes)

The Rheology of Rocks (and other complex materials)

Most of the solid earth is made of solid crystalline rocks (picture) which turn out to have many different rheologies depending on their temperature and the rate at which they deform. In fact the same rock can be brittle, elastic and viscous at different times.

Near the surface, rocks are cold and brittle and break by fracturing in faults and earthquakes (put in links to other exhibits?). 

For small deformations, rocks can actually behave elastically and propagate seismic waves (links?)

However, deeper in the Earth’s mantle where the temperatures and pressures are much higher…solid rocks can actually flow viscously like fluids.  This ability to flow is what gives rise to mountain building and plate tectonics.

So Why Cornstarch and Water?

The rate at which rocks flow, however, is comparable to the rate your fingernails grow (~1-10cm/year) which isn’t terribly exciting (unless you’re  a geophysicist).  However, the strange properties of Cornstarch and Water suspensions allow us to demonstrate complex rheologies on a much shorter time scale.

The Recipe

1-3 lb boxes of cornstarch

~1 cup of water/box of cornstarch

1 medium size mixing bowl

(Bathtub not included)

Place the dry cornstarch into the bowl and slowly add water to the starch, mixing as you go.  Add water until the mixture “does its thing” (you can’t miss it).  Always add water to the starch NOT starch to the water (or you’re likely to end up with watery soup).

If you’re feeling ambitious (and rich) increase the amount to a large swimming pool.  The current Lamont record is 300 pounds of cornstarch.

The Science

Cornstarch and water forms a suspension that is “shear-thickening” i.e. it’s viscosity increases, the harder you try to deform it.  Actually, this material is “visco-brittle”: if deformed slowly, it can flow like a viscous fluid (i.e. honey), however if you hit it hard enough, or try to pull it apart, it can actually propagate fractures like a brittle rock. 

The mechanics of why this works (and for example why flour and water doesn’t) is not well understood, however, it is similar to the behavior of sand-water mixtures such as “quick-sand”.  These materials actually expand or “dilate” when sheared, which drives the water away from the particles of starch or sand.  The drier material is harder to deform because you have to slide the drier particles past each other.  When these materials are deformed slowly, the water allows the particles to be lubricated.


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