Simulation analyses

Simulation studies were performed and revealed the diculties and limits of three-dimensional simulation yet. The wetting behaviour of nanosecond laser structures was studied using Comsol MultiPhysics c . The design of the laser patterns was made using HyperMesh c , a nite element pre-processor, since a three-dimensional resolution was necessary to simulate the eects of the pattern orientation (perpendicular and parallel) and of the geometrical parameters (depth and period). As shown in gure 68, the eect of the

struc-Figure 68: Simulated uid velocity gradients as a function of the uid thick-ness and depending on the period of the laser groove in a parallel pattern orientation. As the period of the structure increases, the velocity decreases, which conrms the Jurin's law. The speeds were calculated using the Comsol Multiphysics c software.

ture period was demonstrated for a parallel conguration. Indeed, simulation

curves proove that as the period increases, the uid velocity decreases. This observation conrms that the spreading of uid in laser grooves is related to the capillarity eect represented in the Jurin's law. Finally, the simulation

Figure 69: Simulated uid velocity gradients as a function of the uid thick-ness and depending on the laser pattern orientation. The parallel orientation shows a higher speed gradient than the perpendicular orientation. This ob-servation conrms that the perpendicular patterns are energetic barriers to the uid spreading and slow down its velocity.

revealed that the orientation of the pattern is a key parameter to understand and control the wetting phenomena, since it is shown in gure 69 that the

uid velocity is slowed down by the presence of obstacles (i.e. the perpendic-ular orientation). The simulation conrmed that these phenomena might be related to the capillarity eects and the obstacles induced by topographical grooves oriented parallel to the triple line.

First, errors occurred due to the three-dimensional modelization. Limitations appeared to design precisely the grooves due to the complicated geometrical dimensions and irregularities such as the radius of curvature that were taken into account. Moreover, the simulation was performed assuming that a lim-ited volume would be representative of the eects occurring at the surface of the irradiated samples while the uid was spreading on it. The limited volume dimensions are an important issue since they did not allow ner meshing and simulations. Secondly, the boundary conditions applied such as the free wall and more precisely the uid velocity are a reason of the incomplete achieve-ment of this algorithm. Indeed, the uid velocity applied was extrapolated from the videos taken during the contact angle measurements. The speed was calculated using the original diameter of the deposited drop subtracted from the drop size at a certain time divided by the corresponding time period.

Since the contact angle measurement itself requires the identication of the syringe diameter, the diameter calculated at each time could be converted from a pixel scale to a meter scale as it is represented in the gure 70. The

uid velocity was then calculated at the basis of the droplet and precisely represents the speed evolution of the triple point. Nevertheless, is has to be considered that, since in the algorithm the speed was applied inside the uid and even if the height of uid volume simulated was not consequently high, this dierence might induce a probable "scale" problem.

Figure 70: Snapshot used for the determination of the uid velocity at the deposition time. The diameter of the syringe (d) is known and recognized before each measurement. D, the diameter of droplet is converted, from a pixel to a mm scale using d as a reference for the conversion. At various times, D varies and then the velocity can be extrapolated.

5.3 Conclusions

In this section it was shown that the nanosecond laser and steel interactions have an inuence on the concentrations of oxides and carbides at the surface of the laser irradiated. Since the LIMET process involves the melting of the irradiated matter and its transfer (from high intensity to low-intensity regions) under ambient atmospheric conditions, oxidation phenomena occur leading to a chemically homogeneous surface. The wetting analyses showed an anisotropy in the spreading of the uid according to the theory developed by de Gennes. The geometrical parameters such as period, height i.e. slope of the grooves and of course the regularity of the patterns control the wetting of the surface by acting as barriers or capillary channels. Finally, uid ow simulations were performed in order to conrm the phenomena involved in the wetting of surfaces irradiated using LIMET. While boundary conditions can still be discussed and the algorithm can also be improved, the velocity curves conrm theoretical and experimental phenomena. First, the increase of the period shows a decrease of the uid velocity in the parallel cong-uration, which proves that the wetting of parallel oriented laser grooves is linked with the capillarity behaviour of the Jurin's law. Then, the eects of the groove orientation were highlighted since the uid spreading velocity was slowed down by the perpendicular laser lines. It also conrms that they act as energetic barriers and obstacles in comparison to parallel oriented grooves.

6 Optimized structures: femto- and

nanosec-ond structure combination