Jean Comtet (EPFL). Mechanics at the Nanoscale: From Gold Nanocontacts to Macroscale Suspensions

New paradigms, challenging existing theoretical foundations, are expected to emerge for mechanics at the nanometer scale, stemming from the breakdown of continuum concepts. But the mechanical properties of matter at the nanoscale can also illuminate and account for the macroscopic behavior of materials. I will illustrate these two points experimentally using an Atomic Force Microscope based on a quartz tuning-fork.

I will first uncover the yielding and plastic properties of gold nanocontacts of only few atoms. In particular, I will show that these molecular systems exhibit a liquid-like dissipative response under large deformations. This behavior challenges existing frameworks for plastic flow in defect-free crystalline systems, but exhibits surprising similarities with the yielding behavior of macroscopic soft disordered materials [1].

Second, I will show that contact mechanics and friction play a critical role in the rheological behavior of non-Brownian suspensions of particles in solvent. I will focus on non-Newtonian shear-thickening suspensions, which exhibit a dramatic increase in viscosity at large shear rates. I will demonstrate through local tribological measurements, that the rheological flow behavior of these suspensions can be rationalized by a frictional transition occurring between individual particles at the nanoscale [2, 3].

[1] Atomic rheology of gold nanojunctions. J Comtet, A Lainé, A Niguès, L Bocquet, A Siria. Nature, 1476- 4687, (2019).

[2] Pairwise frictional profile between particles determines discontinuous shear thickening transition in non- colloidal suspensions. J Comtet, G Chatté, A Niguès, L Bocquet, A Siria, A Colin. Nature Communications 8 (2017).

[3] Shear thinning in non-Brownian suspensions. G Chatté, J Comtet, A Niguès, L Bocquet, A Siria, G Ducouret, F Lequeux, N Lenoir, G Ovarlez, A Colin. Soft Matter (2018).