Optimization of polypropylene surface texturing with ultrafast lasers. A semi-empirical and computational methodology for wettability control

Over the last decades, the study of surface functionalization regarding the concept of wettability has been on the rise. The number of papers published on the modification of a surface´s natural ability to retain or repel a droplet has grown exponentially from the 1980s to the present day. Through purely superficial changes in inherent material properties such as wettability, laser surface texturing has emerged as one of the most interesting technologies for applications related to the generation of superhydrophobic surfaces that promote easy cleaning or reduce the adhesion of water in liquid and/or solid state. 

However, the process of achieving surfaces with specific wettability properties, i.e. with a desired minimum contact angle, can consist of a large number of trial-and-error steps including:

• Parametrization of the laser micromachining process 
• Characterization of the introduced surface modification: topography and morphology 
• Functional characterization of the surface. 

This process must be repeated for each manufactured texture, which leads to an increase in manufacturing time, with its associated economic cost. For this reason, wettability prediction models for surfaces undergoing both physical and chemical modifications have been proposed over the last decades to reduce this cost. 

In this thesis, the main objective is to provide a guide for future materials and feature geometries, showing the steps to follow to study the effect of different topographical parameters that define a texture on the contact angle, as well as introducing a simulation modelling tool to predict which type of texture and/or dimensions offer the best results. 

This is intended to reduce the window of dimensions to be considered and, consequently, to reduce the experimentation time. Thus, the process of selecting textures to achieve specific wettability objectives would be streamlined not only at the laboratory level but also at more relevant environments (at a more industrial levels), which could lead to throughput improvements in terms of surface functionalization. 

In order to achieve the objective described above, this thesis proposes the elaboration of a Design of Experiments in which a number of textures are fabricated by means of ultrashort pulse laser technology in a polymeric material with a relatively high use in the industry such as polypropylene. 

Beforehand, a complete study is performed regarding the comparison between two pulse emission modes (Single Pulse and Pulse Train or Burst Mode). The main advantages and disadvantages of both operation modes have been evaluated and those parameters that allow an optimal quality/throughput ratio for the considered application have been identified. Once selected the optimal parameters, the corresponding textures have been developed. The Design of Experiments focuses on the evaluation of the probabilities of each factor in the modification of the contact angle, and a simplified prediction model obtained by regression is proposed. 

The experimental results for the Design of Experiments serve as validation parameters for the simulation of wettability at different textures, performed with COMSOL Multiphysics. A finite element model is proposed with the CFD (Computer Fluidic Dynamics) module, which makes use of the union of two physics such as the kinetics of the fluid (Laminar Flow) and the identification of two different phases in a medium, divided by an interface (Two Phase Flow, Phase Field). In this case, the effect of different simulation parameters on the results is studied and the optimal mesh size is established to provide results (in terms of contact angle) similar to the experimental ones. The results allow understanding the effect of each topographical factor defining a texture on the final surface wettability and provide a map of results similar to those observed experimentally.