Study: Non-Newtonian Fluids Don't Work Like We Thought, Ketchup Still Annoyingly Viscous
Scientists have been trying to crack the mystery of why some complex fluids don’t behave the way one might expect for some time. These fluids, called non-newtonian fluids, have different viscosities depending on how much energy is applied to them. You know those videos of dudes running across vats full of corn starch with water and then slowly sinking once they stand still? That’s what we’re talking about here. Now, a team from Cornell University led by Xiang Cheng believe they have figured out the secrets of these strange goos.
Non-newtonian fluids have two unusual properties that interest scientists. The first is “shear thinning,” where a highly viscous substance flows easier when energy is applied to it, like ketchup. The other is “shear thickening,” where the fluid behaves like a solid when energy is applied, like the aforementioned cornstarch slurry. Up until recently, scientists had believed that these effects were caused by layers of particles moving within non-newtonian fluids. The established theory held that the trajectory of these particles within the layers were altered by the application of force on the fluid.
The Cornell team says that this just isn’t the case. Their study used a confocal microscope coupled with a viscosity meter to observe silica spheres moving through a water and glycerin mixture. After observing the thickening and thinning behavior of the fluid, the team concluded that layering did not affect the fluid enough to account for the changes in behavior.
In place of the existing model, the team suggests a new theory. Theirs is based around something called Brownian motion, which is a probabilistic model that describes the near-random dispersal of particles in a fluid. I don’t really understand it either, but the Cornell team believes that shear thinning comes from the random motion of particles being overwhelmed by force, thus making it flow. On the flip side, they maintain that shear thickening occurs when particles move too quickly and get log-jammed from the applied force. I’d be lying if I said I completely understood that, too.
If their theory survives further review, it could give scientists a far greater understanding of how these fluids work. This could have applications in industrial production, and in makers of cool videos like the one below.