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Word of the Week: Viscosity

By Paul Adams Published

Liquids don’t always go with the flow.

Working at the intersection of food and science, we come across a LOT of scientific jargon and terminology, from chemical processes, to physical phenomena, to species names. We’ll be sharing many of these fun words with you in this ongoing series, the Cook’s Illustrated Word of the Week. Get ready to impress at your next cocktail party.

I recently subscribed to an entertaining email newsletter for users of the Bostwick Consistometer texture measuring device.

Paul, Do You Know the Difference Between Consistency and Viscosity? read one subject line, and I thought, sure I do. Then viscosity was suggested as a Word of the Week by a reader, and I did a little reading and found myself in the thick of it.

Viscosity is a characteristic of fluids, and it’s (deceptively) easy to get a handle on intuitively: honey is more viscous than water, and olive oil is somewhere between the two. But—as with many kitchen phenomena—when you look closely, there’s a lot more to it.

The Bostwick Consistometer is a foot-long metal ramp with a ruler along its length and a gated compartment at the top. Fill the compartment with your fluid foodstuff of choice, then open the gate and let it flow down the ramp! The distance it flows in 30 seconds is a measure of its consistency, rather than its viscosity, which is a related but more complicated phenomenon.

That’s a very simple scenario. On the ramp, your nacho cheese sauce or what-have-you is just moving downhill, propelled by the steady force of gravity. The complexities of viscosity come in when different forces are applied: stirring, swirling, scooping.

Back in 1686, Sir Isaac Newton mathematically described the physics of viscosity in his usual elegant way, giving an equation that approximates how a fluid moves when a force is applied to it. The more viscous the fluid, the more it resists moving. However, the real world is not quite that elegant, and every fluid is different. A few, such as water, are called Newtonian fluids because their behavior is actually quite similar to what Newton described. But a lot of the materials we actually deal with from day to day—from toothpaste to whipped cream—behave in more complicated ways, and are called non-Newtonian fluids. These come in a few different types.

Shear-thinning fluids flow more readily the more force you apply to them. The classic example is ketchup: turn the bottle upside-down and you’ll be waiting all week, but give it a hard shake and the sauce becomes instantly less viscous and flows all over your burger, plate, and leg. (You shook it too hard.) On a microscopic level, ketchup is held together by a fine mesh of pectin molecules from the tomatoes, as well as xanthan gum molecules. When you whack it, the xanthan molecules lose their grips on each other, freeing the fluid to flow.

Shear-thickening fluids are the reverse. Mix cornstarch into a little water and slowly poke your finger into it. Then do the same but quickly. Under the greater force of the fast poke, the stuff thickens up dramatically. Microscopic corn starch particles suspended in water have time to slide past each other when they need to—until they’re pushed too hard and traffic-jam together, slowing the flow of the whole mixture.

Then there are materials that slowly thicken or thin more the longer they’re under force: rheopectic and thixotropic fluids respectively. Hold an open jar of low-fat mayo upside-down. For a long while, nothing happens, as cumulative stress builds up. Then suddenly it’s the dog’s lucky day: the cumulative stress thins the mayo just enough for it to slump out onto the floor. That’s thixotropy. There don’t seem to be any rheopectic foods, but some engineered rheopectic fluids are used as fillings for body armor. They’re flexible until they absorb a blow, then they instantly get rigid, and only gradually soften up again.

Then you’ve got viscoelastic materials, like bread dough. When you apply force to one of these, it flows somewhat but also stretches somewhat and springs back to where it was.

Phew. Now if you think viscosity is surprisingly complicated, just wait till we get to rheology.

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Graphics by Sophie Greenspan.