## Impact of Particle Shape on Shear Thickening

Irregular, cornstarch-like particle shapes increases shear thickening

A shear-thickening fluid (STF) behaves like a solid when it encounters mechanical stress or shear. This can be exploited, e.g. in liquid body armor and non-linear torque coupling applications, but can also lead to difficulties, e.g. during mixing of ceramic components. An STF hardens and relaxes in a few milliseconds. It is the opposite of a shear-thinning fluid, like paint or blood, which becomes thinner when it is agitated or shaken.

Coninuous shear-thickening (CST) is not yet fully understood and a number of theories are currently discussed, e.g. hydroclustering and order-disorder transitions (break-up of layers). Discontinuous shear-thickening (DST) is a stress-induced transition from a flow of lubricated near-contacting particles to a flow of frictional contact networks of particles. DST stems from the strong sensitivity of the jamming volume fraction to contact forces between suspended particles. Fluid flow on micro and mesoscopic scales plays an important role.

Cornstarch, an example of STF used in research

We carried out simulation studies of shear-thickening (ST) in polydisperse suspensions of spheres and “cornstarch” particles suspended in water in a range of volume fractions $\phi$. The constarch was modelled as polydisperse clusters of smaller spheres.

Snapshots from simulations of spherical (top) and “cornstarch” particles (bottom) with (left) (volume fraction $\phi$, shear rate $\dot{\gamma}$) = (0.3, 1/s) and (right) (0.5, 1000/s). Colors indicate the pressure field (slice through the simulation box) and normal stresses (particles) during shear. The vectors show the flow velocity of the fluid after subtraction of the Newtonian shear flow field.

As can be seen in the images above networks of heavily interacting particles can be observed for the “cornstarch” particles. We also find that a non-spherical particle shape not only enhances the viscosity in general but also leads to DST for larger volume fractions where spheres still show CST.

Relative apparent viscosity for spheres (left) and “cornstarch” (right) with otherwise identical parameters. KD is the Krieger-Dougherty prediction for Reynolds numbers approaching 0.