MANUTECH-SISE > Axe Scientifique 1 > Prsentation


Axe 1 : HIERARCHICAL SURFACES_Eng



The research axis 1 "HIERCHICAL SURFACES" aims to develop surface structuring with a focus on multi-scale aspects. Surface structuring means changing the topography at different controlled scales as well as modifying physical and chemical properties.

This objective is addressed by considering four main steps in the process of generating hierarchical surfaces:

  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. Determination 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:

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


2. Understanding the structuring mechanisms
 
The recent renewed interest in nanostructured surfaces can be explained in part by the remarkable effects resulting from the so-called "Laser Induced Periodic Surface Structure" (LIPSS). These structures present a periodic arrangement of micron or submicron scales [Fig. 2] and are also known under the term of "ripples" as their shapes are similar to wavelets.
New surface functionalities are offered by the periodic texturing of such surfaces.

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 ripples formation. Following localized photonic excitation, the region immediately below the surface undergoes transient phase transformation at the nanoscale. Deepening the understanding of the mechanisms responsible for this self-organization of matter, it will be possible to control more precisely the final morphology of the ripples. Therefore, our approach is based on the development of fundamental tools of laser-matter interaction.


3. Design of multi-scale structured surfaces
 
The femtosecond laser irradiation is used to structure surfaces at multiple scales of roughness. Figure 3 shows two examples of textured stainless steel surfaces by femtosecond laser. The left figure shows a single roughness scale characterized by the existence of a single type of submicron ripples. The right figure shows the same kind of impact for a higher power. On this second example, two scales of ripples are nested.

By controlling the laser irradiation conditions, it becomes possible to generate multi-scale surfaces with a controlled topography.

Figure 3: Multi-scale texturing induced by a femtosecond laser on stainless steel surfaces


4. Determination of potential functions of the structured surface - Applications
 
Performing multi-scale surfaces offer many applications. They concern 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 have been coated with a thin hydrophobic film after laser irradiation. These two observations reveal that the topography plays an essential role on the surface wettability.
Figure 4: Anisotropy spreading of water drops on a textured polymer surface