A new model combining fluid dynamics with granular dynamics can provide important insights into discontinuous shear thickening – a curious effect that causes some fluid-like materials to suddenly behave like a solid. Developed by physicists in the US, the model agrees with key experimental observations and could help researchers develop new technologies based on shear thickening, such as flexible body armour.
Suspensions of tiny solid particles dispersed in fluids can exhibit a range of bizarre properties that have proven difficult for physicists to fully understand using traditional fluid dynamics. Most notably, they do not have a constant viscosity: some suspensions become runnier when they are stirred faster or pushed harder, while others become thicker. Albert Einstein first evaluated the effect of suspended particles on viscosity in 1906, and there have been numerous theoretical models since then. None of these, however, has adequately explained discontinuous shear thickening: the abrupt solid-like stiffening that is easily observed in a suspension of corn starch in water.
In the new research, Ryohei Seto and colleagues at the City College of New York considered the essential nature of a suspension – a mixture of solid grains and liquid medium. This allowed them to combine ideas from fluid dynamics with those from granular physics. In fluid dynamics models, suspended particles are forbidden to touch, and all interactions are mediated by the fluid medium. In contrast, there is no medium in granular physics, or the medium is a gas that offers little resistance to particle contact. As a result the macroscopic dynamics of granular physics are the product of individual interactions between the granules.