It’s the most useful word you’ve never heard of*, and it’s something you probably use everyday: Rheology[1].
Rheology is the study of flow. And I know what you’re thinking ‘But wait, don’t we already have ‘fluid dynamics’ for that’, and yea we do.
But will fluid dynamics tell me why instead of making a vortex when I stir cake batter, it climbs up the beaters? Or why silly putty both bounces like a ball AND forms a puddle when I leave it out on my kitchen table? No, but Rheology will.
See Rheology, as a subset of Fluid Dynamics, is concerned with investigating flows of complex fluids and soft solids: A silly-putty-grey area of fluid flow where materials behave both like a liquid (viscous-ly) and a solid (elastic-ly). These fluids are often, very creatively, known as viscoelastic fluids.
So a rheologist will tell us that the cake batter climbs the mixer’s rods because the semi-elastic nature of cake batter means that it is being put under tension by the stirring motion which results in a normal stress that drives the batter up the rods.This is commonly referred to as the Weissenberg effect. Named after Karl Weissenberg, who first noted the phenomena.
The rheologist will also be able to tell us that silly putty acts like a solid on a short time scale, like the few seconds it takes to hit a wall and bounce back, but flow over the the course of the day because it’s primary ingredient is very large molecules of Polydimethylsiloxane (PDMS) which take a very long time to relax after deformations like being molded into a ball[2].
But how do they know all this? They stir, poke, prod, and squeeze the material until it tells them what they want to know. Literally.
They use machines, such as Dynamic Mechanical Analyzers, to twist material samples at different frequencies and gather information about the different time scale behaviors. These machines also have attachments that allow samples to be squeezed or stretched. Rheologists can even use laser tweezers[3] to probe extremely short time scales. Because, science always needs more lasers.
And these are all useful things to know because complex materials are used all the time in industrial processes.
Squeezing a material between two plates can help model blow molding processes[4] used for things like making plastic bottles or trash bags.
Geologists use Rheology to model events such as soil liquifaction, which is when your normally solid ground starts acting like a liquid.
Food manufacturers use it to ensure your delicious snacks always taste the same.
Cosmetic companies do it too. Your shampoo or soap wouldn’t be very useful if it just flowed out of your hands like water would it?
NASA EVEN USES RHEOLOGY TO MAKE FAKE POOP[5,6]**. Because, turns out, pooping in space is not that easy. (Also while we’re on the subject, diarrhea and rheology share a common root in the greek word rhein, ‘to flow’.)
And of course you use rheology too: When you’re picking out a fruit and you squeeze it to determine if it’s still good. When you decide your meat is undercooked because it’s chewy, or that using anything other than whole milk will make your milkshakes too ‘thin’, you are using rheology to inform your decisions.
So if you're ever feeling stressed out remember to think like a Rheologist and give yourself the appropriate time to relax and just go with the flow.
*Said every scientist ever about their work.
**If SciShow were the sort of show you could swear on, this is where I would follow up with ‘I am not even sh*tting you guys’. #MissedOpportunities. Although, if you did a SciShowSpace on pooping in space this would be a good cross promotion opportunity.
Citations:
[1]Barnes, Howard A. A Handbook of Elementary Rheology. Aberystwyth: The University of Wales Institute of Non-Newtonian Fluid Mechanics 2000.
[2]I’m sure this is true but I’m off campus and can’t get any papers right now because PAYWALLS
[3]http://web.stanford.edu/group/blocklab/Optical%20Tweezers%20Introduction.htm
[4]Venerus, T.-Y. Shiu, and T. Kashyap, “Continuous lubricated squeezing flow: A novel technique for equibiaxial elongational viscosity measurements on polymer melts” J. Rheol. 54, 1083 (2010)
[5]Roach, Mary. Packing for Mars. New York: W. W. Norton & Company, Inc. 2010
[6]https://www.youtube.com/watch?v=uT6Rui1998o
