Thursday, March 05, 2009

Some hints on the Rheology of Ketchup

The most common interactions people have with rheologically complex materials is in their food. Having a few moments of free time, I sampled some of the ketchup sitting in the company cafeteria refridgerator, which as you would expect, has been sitting in there for who-knows-how-long, enduring who-knows-what-temperature-cycles and surviving countless power outages of verying lengths. It passed the SOS test (sight-odor-slime) so I'd eat it, and therefore it's good enough to test in the dynamic mechanical analyzer.

While it seems that endless rheologist-wanna-be's endlessly state that ketchup is "thixotropic", it is far more complex than that simple statement. Let's define terms: thixotropic: "A material that shows a decrease in viscosity over time while being exposed to a constant shear rate." This is commonly confused with shear thinning (aka pseudoplasticity): "A material that shows a decrease in viscosity when the shear rate increases." Got it? In one case (thixotropy) you measure the viscosity at a constant shear rate and look for changes over time. In the other case (shear-thinning), you make measurements at a variety of shear rates and look for a negative correlation.

The plot below shows that the sampled ketchup is shear-thinning. The viscosity decreases as the shear rate increases.

It also shows the storage and loss moduli with the latter begin less than the former, but both are non-zero. This means that the ketchup shows both solid and liquid characteristics, but that the solid-like characteristics are greater than those of the liquid. The moduli are also pretty insensitive to the shear rate.

The next figure shows what happens as ever increasing strains are applied to the sauce. Between a strain of 1 and about 20, very little happens, but after that, things get interesting. All three curves drop, but the storage modulus drops faster than the loss modulus, eventually crossing over at about 60. All the while the viscosity keeps dropping too.

That's all for today, but more will be on the way.

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