Re: What causes a shear thickening fluid to act the way it does?

Hi Rob,

To tell you straight away: I probably can't give you an entirely satisfactory answer, because the problem is among the tougher ones. I have had shear-thickening materials in my hands during my research, so I know the effect is real, yet it depends on subtle details in the properties of the components.

Shear-thickening fluids are also called "dilatant". This is one of several possible deviations from Newtonian flow behavior. To my knowledge, the property is only observed in mixtures (dispersions) of relatively high concentrations of certain solid particles (some tens of micrometres in diameter) in low-viscous "solvents", never with pure solvents. We thus have to deal, not with a purely "molecular" phenomenon, but with the interaction of molecules with "big" chunks of matter. The trouble is that outwardly very similar systems sometimes display the opposite, shear-thinning behavior.

If we want to understand the phenomenon, it is useful to recall that viscous forces are due to momentum transfer between the fluid and boundary walls (see e.g. L.D.Landau and E.M Lifschitz, Hydrodynamik, Akademie Verlag Berlin 1974). Imagine a teeming mass of very small points - the solvent molecules - and much bigger spheres - the dispersed particles. Both components are in constant random thermal motion, the former much faster than the latter. They constantly exchange momentum. In typical dilatant systems, the solid particles are not aggregated, but free to move "independently". If a directional movement is now imposed on the system (by shear forces), it can easily be imagined that the velocity distribution between the particles and the molecules is changing. Under low shear, most of the movement is done by the solvent, but then more and more particles are accelerated and made to bump into each other. Because they were moving slowly at the start, more momentum has to be transferred to them by the boundary walls, i.e. viscosity rises. The exact conditions under which this rise occurs, depend on the specific interactions (attractive or repulsive) between the solvent and the solid particles, as well as among the solvent molecules. It is also possible that the solid particles need to have a gel-like "shock-absorbing layer" on their surface, as e.g. in the case of the often-quoted starch grains.

Shear thinning is usually explained by the breakdown of "structure" formed by attractively interacting solid particles. Quite clearly, shear thickening cannot be due to the formation of such a (static) structure, since it occurs at increased energy input. It is thus a predominantly dynamical phenomenon, involving interactions between two mobile components of very different size.

Best Regards
Werner Sieber