Traditionally, tribology focuses on the optimisation of interfaces comprising two surfaces with closely-controlled properties and a lubricant. From the outside, a fluid stream in an enclosed duct, containing a high density of suspended hard particles, doesn't appear to be a tribological system.
Recently, two research groups have published near-simultaneously their findings, which provide an insight into why the apparent viscosity of the fluid/particle mixture increases in response to shearing. Cornflour in water is a good everyday example of this phenomenon, which is fundamental to many industrial processes.
1. Research by ETH, Zurich: "Why concrete tends to thicken"
http://www.ethlife.ethz.ch/archive_articles/131001_zement_su/index_EN
"...the researchers have discovered that the onset of the viscosity increase is linked to the transition between two types of lubrication between grains. At sufficiently low speeds, the contacts between particles in the suspension are lubricated by a liquid film (hydrodynamic regime). As the particles flow faster, this lubrication film breaks down and the surfaces of the grains come into direct contact and their friction is analogous to the case of dry grains—a regime known as boundary lubrication."
2. Research by City College, New York: "Friction’s Role in Shear Thickening"
http://physics.aps.org/articles/v6/125
"...they have, for the first time, accurately reproduced the DST transition with numerical simulations by including direct friction between the solid particles"
"...particle contact networks can dynamically form and break up because of what is called “dilatancy,” which is the tendency of granular materials and sufficiently concentrated suspensions to expand under shear. Dilation causes the particle networks to press against the boundaries, which then in turn support the networks. The force that can be supported is then only limited by the stiffness of the particles or the boundary, whichever is weakest"
The achievements of these research teams form a good example of the power of creatively analysing complex systems by applying common tribological principles. It's also interesting that both groups have combined analytical modelling, numerical simulation and physical testing to produce their conclusions.