Fluids are a necessary part of everyday life. After all, without water you couldn’t live, and without chocolate syrup, well, what would be the point? But have you ever noticed that not all liquids behave the same? In fact, some of them do some pretty bewildering stuff under the right circumstances, and even after decades of research scientists are no closer to understanding why.
One question about strange fluids may have finally been answered thanks to some glass beads and — I always love saying this — laser beams.
All liquids that we encounter in the real world can be boiled down to one of two kinds. They’re either Newtonian, or non-Newtonian. Newtonian fluids are pretty easy to grasp. I mean, not literally in some cases—a Newtonian fluid like water would squirt right out of your hand. But chocolate syrup is also Newtonian even though it would slowly dribble out of your hand, maybe as you frantically lick your palm to get as much of that rich goodness before it drips away.
In other words, chocolate syrup is much more viscous than water. Viscosity is a measure of a fluid’s resistance to flow. It’s basically the friction between molecules in a fluid. The higher the viscosity, the slower the flow. A Newtonian fluid obeys Newton's Law of Viscosity which means its viscosity is constant. It doesn’t change when a force is applied to it. Non-Newtonian fluids, on the other, less sticky hand, do change their viscosity…which can lead to some pretty zany shenanigans.
There are different sub-categories of non-Newtonian fluids depending on how their viscosity changes. There are dilatants whose viscosity increases as force is applied. Examples of dilatants include quicksand, silly putty, and the cornstarch-water mixture known to 1st grade classrooms as oobleck. The tiny corn starch particles in oobleck can flow freely with the water molecules if you gently dip your fingers in, but give it a good smack and the cornstarch locks up, giving oobleck a surprisingly solid character. You could straight up run across a swimming pool if you dumped enough cornstarch in it. But, there are vandalism laws so, y’know… don’t.
Some fluids get more viscous when force is applied, but the opposite can also be true. Ketchup is in a category of non-Newtonian fluids called pseudoplastics. When no force is applied, it just sits there. Not doing anything. But when you give the back of the bottle a whack the viscosity decreases and the ketchup comes out. Inside the sauce at a molecular level what’s happening is long chains of atoms called polymers get tangled together and hold fast, but when smacked or shaken, they stretch out and align, allowing the gooey red paste to slide around, hopefully onto your french fries but probably on your pants too.
Still, there are many more weird non-Newtonian behaviors scientists don’t have answers for. They may have just solved one riddle that’s stood for over 50 years.
The problem was first noticed in the 1960s when engineers were attempting to extract oil from the ground with fluids that contained long-chain polymers. Pumping these so-called “pusher fluids” into the ground below a certain rate worked fine, but pumping them faster would cause them to become much more viscous, like oobleck.
The fluids would only behave this way when flowing through the microscopic spaces between soil; when not confined to the twisty windy paths in a porous medium, the fluids’ viscosity would actually drop as more force is applied, like ketchup. For a while, scientists thought maybe the polymers were clogging up the pores in the soil, but that couldn’t explain how the fluids flowed easily when the flow rate dropped again. It wasn’t until a new study was published in late 2021 that scientists think they might have cracked it.
Part of the problem they’ve had is soil and other porous media aren’t see-through, so it’s hard to tell what’s going on down there. To solve this they created a custom medium out of glass beads. And they concocted a polymer solution with the same refractive index of the glass, meaning the liquid and solid would both bend light exactly the same way. To see the windy paths fluids would follow in the spaces between the beads, the researchers added a red dye to the solution that would give off a certain wavelength of light when hit with a laser.
To visualize how the fluid was moving, they added tracer particles that would emit a different color when excited by another laser. With this extremely complicated setup in place they observed the fluid flowing at different rates and found that the long polymers in it started tumbling around as the fluid moved faster. This movement pushed on other nearby molecules in the liquid and created a phenomenon called “elastic turbulence,” creating eddies and slowing the whole fluid down.
The researchers think this new understanding of why pusher fluids suddenly become so viscous could be useful for purifying groundwater. It may aid in the development of new polymer-containing solutions that can force water through rocks, trapping contaminants in the process.
But there’s more work to be done because elastic turbulence itself isn’t fully understood. Maybe that’ll be the next riddle solved.