Wetting on surfaces that are disordered at the nanometer scale
Romain Lhermerout (LPS)

Wetting is the study of the spreading of a liquid on a solid surface. This phenomenon is ubiquitous in everyday life, and its control has major industrial implications, from the cooling of reactors to microelectronics and nanofluidics. Despite having been studied for over two centuries, a quantitative understanding is still lacking. A principal difficulty is that the physics of the contact angle involves a wide range of length and time scales. On one hand, the hydrodynamic flow in a liquid wedge is singular at the contact line and each decade of length scale contributes equally to the viscous dissipation, from the macroscopic scale, typically millimeters, to the molecular scale, typically nanometers. On the other hand, real surfaces are rough and chemically heterogeneous, and when left at rest the line is pinned by this disorder on the surface, and relaxes very slowly towards an equilibrium, that is never reached in accessible time scales. This phenomenon is called contact angle hysteresis.
During this thesis, we first developed an experimental set-up to measure contact angle dynamics with a record precision of 0.01° over 7 decades of velocity of the triple line, a range never before attained. For the first time, numerically solving the lubrication equations has allowed us to deduce the contact angle at the microscopic scale from these macroscopic measurements, and thus enabled the multi-scale hydrodynamic problem to be disentangled from the physics of the contact line at small scales. With these tools we have shown that the dynamics can be completely piloted by a pseudo-brush -a nanometric layer of polymers-, producing the lowest ever reported hysteresis (< 0.07° !) and giving rise to a huge source of dissipation originating from the viscoelasticity of the coating. This study points the way towards nano-rheology, to probe extremely fast dynamics ( 100 ns) of polymers confined at the nano-scale. Thanks to a fruitful collaborative work, we then developed a model that provides a single quantitative framework to account for hydrodynamic dissipation, hysteresis and thermal activation. Finally, a great deal of effort has been made to produce nano-defects whose size, shape and density are controlled. The dynamics appears to be insensitive to this scale of disorder, and the presence of defects is observed to only modify the hysteresis. These results have been interpreted semi-quantitatively with scaling laws, and we expect that the complete characterization of the defects should eventually allow the development of new models to quantitatively describe the relation between the microscopic features of the surface and the dynamics of wetting at the macroscopic scale.