More About Liquids: Thick and Thin

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Liquids: Thick And Thin

The Point of the Experiments

In the Science Lab experiment Viscosity of Liquids we examine the properties of different liquids beginning with a comparison of corn syrup and water.

In Ketchup: Thick or Thin? and Cornstarch: Is it Liquid or Solid? we look at two examples of what are called non-Newtonian liquids, which exhibit some odd behaviors: their viscosity changes depending on whether they are agitated or sitting still.

Ketchup becomes thicker, or more viscous, when it sits still. If you stir it up or shake it, it becomes thinner, or less viscous. We see this by dropping weights through ketchup and timing how long they take to sink down to the bottom. We do this first with ketchup that has been sitting still for a while, and then again after the ketchup has been stirred.

Ketchup is a thixotropic liquid. It becomes less viscous when agitated. It is similar in this respect to Visplex* a substance used in drilling fluid to enable it to suspend rock cuttings when the circulation of the drilling fluid stops. This process is discussed in Drilling Fluid: Lifeblood of the Well.

Another non-Newtonian liquid is a mixture of cornstarch and water. It also acts differently depending on whether it is still or agitated. But the behavior is the opposite of the way ketchup and Visplex act. You can slowly stick you finger into the cornstarch water mixture and it gives like a liquid, but if you jab it, it feels hard and your finger won't go in very far. You can pour it slowly, but you can grab a clump of it, almost as if it's a solid. This kind of fluid is called dilatant. It becomes more viscous when agitated or compressed.

How can these phenomena, of thixotropic and dilatant liquids be explained?

The Exploration

Ketchup: Thixotropy
Ketchup Ketchup that has been standing still is thicker -- more viscous -- than ketchup that has been stirred or shaken. Schlumberger chemist Chris Sawdon says that at least part of this thixotropy comes from the macerated tomatoes. The solid part of the fruit must form suspended microfibers when ground up. On standing still the fibers in such a suspension increasingly make contact with each other and and stick together. This forms a 3-D structure or gel throughout the fluid, the strength of which increases with time.

The gel structure is broken by agitation, reducing the viscosity. It would not be surprising if ketchup also contained a thickener like xanthan gum. This dissolves in water to produce a thixotropic gel. The explanation is similar to the microfiber theory above, except that the xanthan gum polymer molecules are very much smaller. The rod-like polymer molecules also build a structure with time. There may also be some gels arising from pectin if there is any in the tomato pips. Pectin is also a soluble polymer that has the power to form a cross-linked gel with sugars.

Cornstarch and Water: Dilatancy
To start thinking about why the cornstarch and water mix behaves the way it does it may help to realize that it is not a simple liquid like water, oil or corn syrup. It is a suspension. The tiny granules of cornstarch do not dissolve in the water. Rather they are mixed in with the water but remain intact and solid. If you let the mixture stand for a while you will see that the starch settles to the bottom and there is a layer of clear water on top.

Compare this with salt and water. The salt dissolves in the water. There are no little chunks of salt floating around in the water.

The most generally accepted explanation for the behavior of the cornstarch water mix is that when sitting still the granules of starch are surrounded by water. The surface tension of the water keeps it from completely flowing out of the spaces between the granules. The cushion of water provides quite a bit of lubrication and allows the granules to move freely. But, if the movement is abrupt, the water is squeezed out from between the granules and the friction between them increases rather dramatically.

This is one of the explanations offered in "Oobleck: What do Scientists Say?"** but author Cary Sneider suggests some other possibilities:

Cornstarch molecules are in long chains called polymers that get stretched when the mixture is compressed. They may also get tangled so as not to slide easily against each other. It would make sense that stretched fibers would offer more resistance to movement, just like the resistance of a taut rubber band or a coil spring under tension. But the tangling argument doesn't explain why rapid motion increases viscosity. Wouldn't the fibers be tangled when the mixture is moving slowly or still? In fact, rapid motion might break the fibers. (Look at the discussion of Ketchup above for some more thoughts about this.)

Another problem with this model is that the starch is not separated into molecules, but rather exists as much larger (though still tiny) granules, which are essentially spherical. These granules will dissolve with heat. Now that's a different phenomenon. A cornstarch and water mixture poured into a sauce during cooking will thicken it. This will only happen if the sauce is hot.

We should also note that a mixture of fine sand and water exhibits behavior similar to the cornstarch and water mix, but sand molecules are not polymers.

The Oobleck publication presents another possible explanation involving static electricity. When the starch particles rub together they become charges and attract each other. This attraction would increase viscosity.

Chris Sawdon finds the first argument to be the most convincing. He adds that starch granules are monodisperse, meaning that they are all of about the same size. This is known to increase dilatancy, perhaps by allowing more rapid drainage of water on pressing than would be the case with polydisperse (broad particle size distribution) particles which can pack together more closely.