Study of high velocity projectile impacts on non-Newtonian fluids may help us make liquid body…
If I had known firing bullets at non-Newtonian fluids was a job option I might have made different career choices.
If I had known firing bullets at non-Newtonian fluids was a job option I might have made different career choices.
Liquid body armor conjures images of a metallic fluid encasing a person, absorbing bullet impacts like a gelatinous force field. The reality is a little different. Instead, Kevlar vests are impregnated with a suspension of a non-Newtonian fluid. The fluid layer significantly improves the vest effectiveness.
Non-Newtonian fluids are ubiquitous on TV shows like Mythbusters as well as science classrooms. Usually made from copious amounts of cornstarch mixed with water, they have a fascinating property that they become harder to penetrate the harder you hit them. While your hand can slip easily into the fluid, if you try to punch it, you can’t break through the surface.
Given this miraculous ability, you might wonder how far it goes. If I fire a bullet at such a fluid, will it stop it?
A trio of researchers did exactly this in a 2019 study published in Scientific Reports and available on Nature.com.
All fluids have a property called viscosity, which is a measure of how thick a fluid is. The thicker a fluid is the more viscous it is. Newtonian fluids have a viscosity that doesn’t depend on what is impacting them. It is just a constant of the fluid. Non-Newtonian fluids, however, change viscosity depending on the forces applied to them. One of these forces is called shear stress, the force of a object trying to penetrate the fluid.
While pure water is a Newtonian fluid, a lot of fluids are not including ketchup, toothpaste, paint, blood, and melted butter.
When a solid object impacts a Newtonian fluid, the water stops it by dissipating its energy by a combination of inertia and viscosity. Basically water gets pushed out of the way and that absorbs energy but the water also adds shear stress, essentially friction, to the object as it passes through the water. That depends on the viscosity. Thicker fluids absorb more energy through viscosity.
A non-Newtonian fluid, however, can actually increase its viscosity when something attempts to pass through it, so it absorbs more and more energy the harder something tries to penetrate it. That makes it a good body armor because it is able to absorb and dissipate a lot of energy without the fluid being displaced. Non-Newtonian fluids that become thicker as things pass through them are called shear thickening fluids.
There are also fluids that actually become thinner when objects pass through them. These are shear thinning fluids.
How exactly this works however is something of a mystery. Why exactly do these non-Newtonian fluids behave this way? Understanding that could be the key to creating fluid body armors that are even more effective.
One theory is called the “adding mass” theory which suggests that when a bullet tries to penetrate the fluid the liquid solidifies under the object by forming what are called jamming clusters. This is a property of shear thickening fluids.
An example is if you fill a funnel with beads and try to drain them out the bottom. As the beads escape, they can become jammed together to form an arch shape that resists any more beads exiting the funnel. Or think of a spice shaker where the spices have caked together as you try to shake them out. Eventually, no more spices come out unless you clear the jam.
It is like too many people trying to escape a burning building all at once. They jam in the door and then nobody can get out.
As the mass of the jam increases under the bullet, the bullet has to push harder to move the jam out of the way.
Another theory is that these fluids are not just viscous but elastic as well. This property, call viscoelasticity, occurs when molecules are rearranged in the fluid.
As an example: a fluid full of long chain molecules called polymers experiences stress from a penetrating object. The stress forces the chains to rearrange themselves and become tangled together. This rearrangement is called creep. The polymers resist being rearranged and that creates a reverse stress which, when it matches the applied stress, stops the creep. When the penetrating object stops, the stress stops, and the polymers returns to their original form, untangling. The tangling part is viscous while the untangling is elastic.
The elasticity could also be responsible for stopping objects faster as well.
To try to understand whether jamming clusters or viscoelasticity was more important the study authors fired bullets (made of plastic) into three different fluids: water, which is the control, a shear thickening fluid of cornstarch mixed with water which is also viscoelastic, and a viscoelastic fluid shear thinning fluid, polyvinyl alcohol (PVA) borax solution.
Mythbusters style they used a high speed camera to record their observations.
You can see some still shots here: https://www.nature.com/articles/s41598-018-37543-1/figures/2
The results came in:
Water is the worst absorber, no surprise there.
Both the cornstarch and PVA solutions stopped the bullets rapidly.
The authors attempted to fit models of added mass and viscoelasticity to the profiles of the bullets velocity vs. penetration depth, but it seems obvious given the stopping power of the shear thinning PVA that thickening is not a big contributor.
They conclude that the viscoelasticity is responsible for the bullet stopping power of non-Newtonian fluids.
Since viscoelasticity is a property of shear thickening and thinning non-Newtonian fluids, this may lead to better body armor as the viscoelastic properties of fluids can be increased and exploited to make Kevlar vests even more impregnable.
de Goede, Thijs C., Karla G. de Bruin, and Daniel Bonn. “High-velocity impact of solid objects on Non-Newtonian Fluids.” Scientific reports 9.1 (2019): 1–8.