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Name: Carol
Status: educator
Grade: 4-5
Country: Canada
Date: Spring 2014


Question:
Is Jell-o considered to be a thixotropic mixture?



Replies:
Hi Carol,

Thanks for the question. I would consider Jell-o a thixotropic mixture, at least under certain conditions. The extent of the thixotropic properties can be changed by temperature as well as by amount of proteins used in the Jell-o formulation.

I hope this helps. Thanks Jeff Grell


Hi Carol,

Jell-o does not even flow at all, so it cannot be considered thixotropic. Thixotropic substances flow only very slowly or not at the all, when undisturbed, but will flow more freely once shaken or stirred. Jell-o does not act like this. The most common example of a thixotropic substance, is plain ordinary Catsup.

The following article should help you understand what thixotropy is....

http://en.wikipedia.org/wiki/Thixotropy

Jell-o is, technically-speaking, a "gel".

Regards, Bob Wilson


Certainly there is data showing that dilute solutions of gelatin are thixotropic:

http://www.sciencedirect.com/science/article/pii/0141813088900190

This, however, does not actually answer your question: Jell-o is a specific, much higher concentration of gelatin. Furthermore, I assume you are talking about the gelled state of Jell-o rather than its liquid state, which further complicates matters. As anyone who has made Jell-o before knows, the mechanical properties of Jell-o change dramatically with temperature, handling, and concentration (have you ever added too much/not enough water to a Jell-O packet?). This makes it hard to say definitively whether 'Jello-O' as a whole demonstrates thixotropy or not. In some formulations and conditions it most certainly does (see the above cited paper), but it is quite possible that in others it does not. That may be especially true if Jell-O every forms a true solid, rather than a Bingham fluid - if it ever matches the definition of solid (a somewhat technically complex concept), it cannot be considered to have thixotropic properties, since it will not continuously flow under shear.

In this context, it is useful to think about what thixotropy actually means. Contrary to popular belief, thixotropic fluids are NOT the same thing as shear thinning fluids. Shear thinning fluids, put simply, decrease their viscosity (or resistance to flow) as you INCREASE the shear rate. That means that if you have a fluid moving through a pipe, it will flow more easily if you increase the flow rate. Thixotropy, on the other hand, is observed for fluids continually flowing at a FIXED shear rate; if the viscosity drops over time, the material is thixotropic. Bingham fluids, also commonly confused with the two properties above, are materials that act as solids, not flowing at all, until a threshold shear stress is applied, after which is behaves like a normal fluid.

Many materials exhibit a combination of all three properties; I would imagine that many testing conditions for gelatin (temperature, concentration, shear rates) would show any and all of these properties. I find it unlikely, however, that all conditions would show thixotropy.

Dr. Shimon Unterman


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