You’re full of amazing liquids. How can a liquid be amazing? One way is to have properties that change depending on how strongly the liquid is pushed. The speed at which a fluid flows depends on the force that’s driving it, and the proportionality between force and flow is known as the viscosity of the fluid. (This isn’t quite the technical definition of viscosity, but it’s good enough.) Corn syrup has a higher viscosity than water — it takes more force to generate a flow of a given speed in corn syrup than in water. For conventional (“Newtonian”) fluids, the viscosity is independent of circumstance — if we double the applied force, we double the resulting flow speed. If we triple the force, we triple the flow speed. The proportionality remains the same. For some fluids, however, the viscosity itself depends on how hard the fluid is being pushed. For a “shear-thinning” fluid, the viscosity decreases with force. Tripling the force we apply, for example, might quadruple the speed at which the fluid flows.
Ketchup is a common example of a shear-thinning liquid. Left alone, with little applied force, its viscosity is large. It doesn’t readily fall out of an upturned bottle, as you probably know from experience. Slapping or shaking the bottle, however makes the ketchup flow more readily. (Great things have been written about the science of coaxing ketchup from its bottle; see “Getting tomato sauce out of the bottle –– minus the mess”, for example.)
Your tears are another shear-thinning liquid. Left alone, the fluid coats your eyes. When you blink, the liquid viscosity decreases, driven by the force of your eyelids, making it especially easily for the eyelids to clear away the tears and any associated debris. The shear-thinning properties are a consequence of lipids and proteins the tears contain, and these “non-Newtonian” characteristics are mimicked by artificial eye drops. Food for thought the next time you eat ketchup.
I don’t discuss the fluid mechanics of tears in my pop-science biophysics book, though tears come up at one point as I describe a terrible idea I had to use onions in an in-class DNA purification exercise. Fluid mechanics of other sorts are featured in Chapter 10 (“A Sense of Scale”), especially the ways in which physics constrains the swimming styles of small creatures. Here are the usual book links: My description, Publisher, Amazon.)
This is #8 in my series of “What is biophysics?” posts. (Here are posts one and seven in the series.)
Today’s illustration: A ketchup bottle, of course, which I painted from this photograph in a very good New York Times article about ketchup: https://www.nytimes.com/2017/02/13/us/ketchup-bottle-pour.html
— Raghuveer Parthasarathy; July 7, 2022