8/19/2023 0 Comments NewtoniansTwo containers were used, each with the same length (22 cm) and depth (22 cm) but with different widths (2.4 and 22 cm). The object was a plastic sphere with a mass m b of 0.2 g and radius r b of 3 mm, being shot vertically into the liquids with impact velocities around 100 m/s using a pressurised CO 2 propulsion system (Smith & Wesson MP40). The object velocity as a function of time is determined by dividing the change in object position between consecutive frames by the time in between frames. We subsequently measured the temporal decrease in velocity of a high-speed object in water, cornstarch and PVA using high-speed camera footage (Fig. This confirms that PVA is a shear thinning and cornstarch a shear thickening fluid. For cornstarch the opposite effect can be seen, with both moduli increasing strongly when the strain exceeds a critical value. The results show that for PVA, both G′ and G″ stay relatively constant as a function of the applied deformation, after which they both decrease when some critical strain is exceeded. In this measurement, cornstarch was carefully density matched using CsCl to avoid the influence of particle sedimentation 22, 23, 24. An oscillatory shear experiment was used to avoid complications due to e.g. In order to assess the properties of these fluids, their storage ( G′) and loss ( G″) moduli were measured in a rheometer using a cone-plate geometry with an angular frequency ω of 10 rad/s (Fig. 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. 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.
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