Sensing the Mechanical Response of Interfaces with Gradient of properties at nanometric scales – MERIG

An increasing number of situations in our everyday life rely on the mechanical response of interfaces. The characterization of the mechanical response of interfacial layers having eventually gradients of mechanical properties, and the understanding of the correlations between the molecular structure of the layers and their mechanical response, represents crucial steps for the design of innovative materials and solutions in the domain of friction, lubrication, mechanical reinforcement in composite and nano-composite materials, surface coatings, bio-compatible materials, integrated micro-electro-mechanical or fluidic devices. We propose a program based on the use of three complementary techniques: the Surface Force Apparatus in the dynamic and elasto-hydrodynamic mode (DSFA), the instrumented JKR friction test, and Near Field Laser Velocimetry (NFLV), to tackle the direct determination of interfacial mechanical properties at the sub-micrometric scale. We will ascertain our approach on controlled model systems, such as self assembled monolayers and various grafted polymer films specifically designed to exhibit increasing complexity in their mechanical properties (purely elastic or viscoelastic) in order to directly address the question of the correlation between the molecular organization and the interfacial mechanics. Contrary to previous experiments performed with a surface force apparatus, we shall not directly put two layers into contact to indent or shear them, but we shall use a liquid flow as a probe to mechanically solicit the layers. In order to control the boundary condition for the flow at the interface between the layer and the liquid, we shall use the near field velocimetry technique available in one of the associated teams to directly access this velocity on similar interfaces. There are several difficulties to overcome and challenges to address: i) Sensing the mechanical properties of nanometric interfacial layers is intrinsically a difficult task, ii) in order to be able to relate the observed mechanical response of the surface layers to the molecular mechanisms responsible for the stress transmission through the interface, one needs to compare the mechanical behavior of series of surface layers with growing complexity, keeping the molecular parameters under control.. This project is at the interface between the physical chemistry of surfaces, low Reynolds number hydrodynamics in the lubrication regime, adhesion and friction science. This project involves a strong knowledge in the control of physical chemistry of surfaces including grafting of polymers and polyelectrolytes, self-assembled monolayers. It also needs to improve and develop new well-controlled surface with a defined geometry combined with very precise measurements of the mechanical properties at the nanoscale. Finally this project will allow us to construct a new surface forces apparatus with better mechanical properties (stiffer) and a controlled environment.