Borax (sodium tetraborate) acts as a weak crosslinker between the PVA polymers 21. Using high-speed camera footage and three kinetic-energy dissipation models, we show that both the shear thickening and viscoelastic fluids respond viscoelastically to the object’s impact.Īs typical shear thickening and viscoelastic fluids we selected cornstarch dissolved in water and a polyvinyl alcohol (PVA) borax solution, respectively. In this study, we compare how a high speed spherical plastic bullet is decelerated in three fluids: water, a purely shear thickening fluid and a viscoelastic shear thinning fluid. The possibility exists that for the liquid body amour studies, where the impact velocities are two magnitudes higher compared to the study of Waitukaitis and Jaeger 13 ( v ~ 100 m/s), the mechanism could be different. Here, the characteristic timescale of the developing visco-elastic stress depends on the impact velocity of the object. In practice, most suspensions are also viscoelastic and can therefore react elastically on an external stress, depending on the timescale 20. However, shear thickening might not be the only mechanism for describing the enhanced stopping power of suspensions. With this model, they were able to predict the deceleration of an object impacting in shear thickening fluids. This plug is subsequently pushed downward by the object and causes a downward flow in the surrounding liquid, increasing the mass that the object has to displace over time. In the model, the shear thickening liquid underneath the object solidifies during impact due to the formation of so-called jamming clusters 16, 17, 18, 19, creating a solid ‘plug’ underneath the object. They proposed that the kinetic energy dissipation inside a shear thickening fluid could be described using an ‘added mass’ model 14, 15. To describe such kinetic energy dissipation in a shear thickening fluid, Waitukaitis and Jaeger recently investigated relatively low velocity impacts ( v ~ 1 m/s) on cornstarch dissolved in water 13. These studies proposed that the shear thickening properties of the suspension, an increase in viscosity when subjected to a shear stress, is responsible for the enhanced kinetic energy dissipation in the Kevlar vests. For instance, a recent study has shown that the object stopping power of Kevlar body vests can be significantly enhanced by impregnating the Kevlar layers with a suspension 7, which is the foundation of liquid body armour designs 8, 9, 10, 11, 12. However, it has been known for some time that the use of non-Newtonian fluids can provide efficient ways of changing the impact dynamics, which could have many useful applications. For Newtonian fluids, the kinetic energy of the impacting object is dissipated by either viscous or inertial forces, depending on the impact velocity of the object 6. Understanding the forces and mechanics for the kinetic energy dissipation is key for understanding the fluid behaviour during impact. This is what causes mudslides and avalanches.Īuthor: Fred Senese Chemistry Online! What is a non-Newtonian fluid?Ĭomments & questions to Revised 02/23/18.The impact of solid objects on liquid surfaces has been studied intensively over the last century due to its relevance in the water entry of military projectiles 1, 2, the construction of naval vehicles 3, 4 and the motion of water-walking lizards 5. Until you push hard enough, and the substance begins to flow like a normal liquid. Quicksand will make it even harder to escape.įor some fluids (like mud, or snow) you can push and get no flow at all. That's why stirring gravy thickens it, and why struggling in It may less thanĭouble (like ketchup), or it may more thanĭouble (as in the case of quicksand and gravy). They're non-Newtonian fluids because doubling the speed that the layers slide past each other does not double the resisting force. The slower one layer slides over another, the less resistance there is, so that if there was no differenceīetween the speeds the layers were moving, there would be no resistance.įluids like water and gasoline behave according to Newton's model, and are called Newtonian fluids.īut ketchup, blood, yogurt, gravy, pie fillings, mud, and cornstarch paste DON'T follow the model. If you want one layer to slide over another twice as fast as before, you'll have to overcome a resisting force that's twiceĪs great, Newton said. The resistance to flow arises because of the friction between these layers. Picture a flowing liquid as a series of layers of liquid sliding past each other. The liquid will squirt out of the pistol. Newton devised a simple model for fluid flow that could be used to relate how hard you have to pull the trigger to how fast To make the water to come out faster, you have to pull the trigger harder.įluids resist flow. You have to pull the trigger on a water pistol to get the water to squirt out.
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