Axis 1 : HIERARCHICAL SURFACES

The aim of research axis 1 "HIERCHICAL SURFACES" is to develop surface structuring focused on multi-scales. Surface structuring means changing the topography at different controlled scales as well as modifying physical and chemical properties.

The process of generating hierarchical surfaces is addressed in four main steps :

1. Choice, definition, characterization of the initial surface
2. Understanding the structuring mechanisms, induced either optically (laser) or mechanically (indentation, scratch, manufacturing processes)
3. Design of multi-scale structured surfaces
4. Identification of potential functions based on the structured surface – Applications

Figure 1: Schematic diagram of the objectives of research axis 1.

The four key steps defined for this axis are :

The first step before functionalization obtained by mechanical or laser processing is identifying the characteristics of the initial surface as precisely as possible. These characteristics may play a critical role in the response of the material to mechanical stress and / or photonic excitation.
Initial surface conditions, particularly the topography, can affect the final texturing.

The recent renewed interest in nanostructured surfaces can be partly explained by the remarkable effects of "Laser Induced Periodic Surface Structure" (LIPSS). These structures show a periodic arrangement at micron or submicron scales [Fig. 2] also known as "ripples" as their shape resembles that of wavelets.
New surface functionalities are enabled by periodic texturing.

Figure 2 : Surface nanostructuring (LIPSS) by femtosecond laser irradiation to functionalize surfaces.


One of the objectives of axis 1 is to understand the physical mechanisms involved in the the formation of ripples. Following localized photonic excitation, the region immediately below the surface undergoes transient phase transformation at nanoscale. A better understanding of the mechanisms responsible for this self-organization of matter, will make it possible to control the final morphology of the ripples more precisely. Our approach is consequently based on the development of fundamental tools to control laser-matter interaction.

Femtosecond laser irradiation is used to structure surfaces at multiple scales of roughness. Figure 3 shows two examples of stainless steel surfaces textured by femtosecond laser. The left image shows a single roughness scale characterized by the existence of a single type of submicron ripple. The right image shows the same kind of impact with higher power. Two scales of ripples are nested in the second example.

Controlling the laser irradiation conditions makes it possible to generate multi-scale surfaces with controlled topography.

Figure 3 : Multi-scale texturing of stainless steel surfaces using femtosecond laser.

Multi-scale surfaces have many applications, for instance wettability properties, themselves directly related to adhesion and tribological properties. Figure 4 shows the wetting behavior of a titanium alloy surface textured by femtosecond laser. Both surfaces were coated with a thin hydrophobic film after laser irradiation. These two observations reveal that topography plays an essential role in surface wettability.

Figure 4 : Anisotropy spreading of water drops on a textured polymer surface.