Monday, February 22, 2010

Playing with Cornstarch

Last night, my Chef and I were wondering whether a cornstarch-thickened fluid would lose viscosity when subjected to shearing forces, so this morning I started doing some research. Unfortunately, my limited efforts turned up technical abstracts and youtube videos, and nothing much in between. On a side note, the youtube videos were pretty cool. "A Pool filled with non-Newtonion liquid" anyone?

So, I decided to do my own sloppy little experiment. First, I made a relatively concentrated cornstarch and water slurry and cooked that until milky-translucent and very thick. After noting the viscosity, I turned off the heat and started shearing the mix with a stick blender. The change in viscosity was obvious just from the way the blender worked. At first, with the blender submerged there was no movement on the fluid surface. After half a minute or so, the fluid was thin enough that immersing the blender caused great distortion even at the surface. Shearing had diminished viscosity considerably. Curious about whether letting the fluid sit would allow the gelling network to reform itself, I removed the blender and let the fluid rest in the still-hot cooking pot. it regained all or almost all of its viscosity in a few minutes. Unfortunately, I didn't design my experiment to separate the temperature and shear variables very well, so some of the thickening may have been due to cooling, rather than the absence of shear. I wondered about how further cooling would effect viscosity, so I transferred a small sample to a ice water bath. Both the sample in the ice bath and the sample in the pot cooled to a solid, soft, opaque gel that I could pick up with my hands.

A morning well spent.

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