If you sometimes wander on youtube, you have probably already seen a video about experiments on cornstarch mixed with water. They were quite popular a few years ago. These videos show the surprising properties of the mixture (also known as oobleck, as in the book of Dr Seuss): you can walk on it (see Figure 2), cycle on it, hit it and harm your fist or even make a solid ball out of it in your hands… just before you leave it still and it turns back into a liquid. The behaviour of this fluid is always fascinating! In the most science oriented videos, the behaviour is described as non-Newtonian, but that is the more accurate you can get.
Figure 2: Someone walking on a fluid! (from http://gph.is/1PEpUBc)
Recently, I went through a paper that explores experimentally a dense suspension of solid particles. Just like oobleck, this suspension showed an interesting response under mechanical stress. Hsu et al.  studied the sudden increase of viscosity of the fluid: this transition is known as discontinuous shear thickening. It is known that the transition can also reach shear jamming, where the fluid suddenly behaves like a proper solid. The physical phenomena behind the radical behavioural changes above mentioned has only been identified recently, the authors say: between 2013 and 2017 different papers began to explain the cause.
And that is when tribology gets involved! In these dense suspension of solid particles, the observed behaviour is the one of a classical liquid under limited shear stress: the solvent allows hydrodynamic lubrication to occur between the particles. But as soon as stress is applied, the particles start to interact with one another with more intensity and the solvent lubrication ability decreases. Boundary lubrication or even dry contact occur.
Figure 3: Smooth and controllable rough particles
Now that the main idea is identified by the research community (a transition of the lubrication regime between the particles), it needs further investigations to actually understand it and to be able to predict it. While it is easy to prepare different suspensions with different solvent viscosity and find the threshold shear rate, Hsu et al. decided to use a single solvent and taylored silica particles with controllable roughness (see Figure 3). The first type of particle is smooth and the second one is rough. However, both were coated with a thermo-responsive polymer layer. This layer changes its conformation according to temperature and swells above a given threshold. With these different silica particle sizes and the tunable roughness, Hsu et al. have a well designed tool box to “test the relative contributions of friction, adhesion and surface roughness”.
A first step is to determine successively the adhesion and the coefficient of friction of the different particles in the presence of the solvent. For this purpose, a single particle is attached to an AFM cantilever and lifted from a flat surface with the same roughness and swell properties. Then it is rubbed against it. The rough contact friction force is up to 20 times larger than the smooth contact friction force and a weak adhesion force is measured.
With the tribology of the particle identified, the solvent-particle suspension is characterised in a rheometer. The authors show that the roughness and the adhesion forces clearly control the triggering of the discontinuous shear thickening. However, it is not possible experimentally to completely distinguish the roles of roughness and adhesion. The paper is concluded by a call to further develop the numerical work on this topic: within the well controlled frame of equation solving, it is possible to virtually decorrelate the two effects.
If you want to know more about this open access work, feel free to read the full article , visit the author’s webpage or tweet @soft_isa.
 Hsu, CP., Mandal, J., Ramakrishna, S.N. et al. Exploring the roles of roughness, friction and adhesion in discontinuous shear thickening by means of thermo-responsive particles. Nat Commun 12, 1477 (2021). https://doi.org/10.1038/s41467-021-21580-y