Direct Laser Interference Structuring as a Tool to Gradually Tune the Wetting Response of Titanium and Polyimide Surfaces

Abstract

Adjusting the wetting behavior of materials by modifying their surface properties can be convenient for many industrial processes such as coating, drying, and adhesion applications. Total wetting of hydrophilic surfaces has its benefits in painting and lubricated processes, hydrophobic surfaces on the other hand are desirable for liquid flow, dewetting, and low friction concerns. [1][2][3] Especially, the development of superhydrophobic (even superoleophobic) surfaces and the stabilization of this sensitive wetting state have recently drawn a noticeable amount of attention. [4,5] The underlying parameters, which define, whether a solid will be covered in liquid or show fluid-repelling behavior, are the surface energies of the substances involved and the surface topography. [6] Apart from chemical modifications, artificial surface roughness has been found to be most effective in tailoring specific wetting conditions on solid materials. Topographical surface design directly affects the solid-liquid contact area and the triple line movement in dynamic wetting conditions. Numerous physical and chemical surface treatments have proven their efficiency in this field, yet most of them are time-consuming and extensive. In this work, we have created moderate aspect ratio surface structures by direct laser interference structuring (LIS) of titanium and polyimide in order to gradually tune their wetting response. LIS is a fast, contactless surface structuring technique, based on the interference of one or more coherent, superimposed, and high-power pulsed laser beams. It allows the creation of periodic surface structures with a well-defined long-range order on the micro/nanoscale in one single preparation step. During irradiation, photo-thermal interaction between laser and material according to the intensity distribution of the re-combined laser beam induces distinct local heating of the surface. In the case of metals, metallurgical processes such as melting, recrystallization, recovery and defect, and phase formation are initiated on the lateral scale of the microstructure itself. Therefore, LIS of metals is also referred to as laser interference metallurgy (LIMET). [7] In contrast, the interaction of high power laser light with polymers is governed by significant ablation processes. [8,9] In order to cover both the domains, bulk titanium and polyimide have been processed with two kinds of surface patterns each at various energy densities and structure periods. Polyimide has been chosen with regard to its notable chemical resistance, as all samples had to undergo extensive chemical cleaning and drying prior to contact angle determination. Statistical static contact angle measurements have been carried out with bi-distilled water on flat reference specimen and structured surfaces. Scanning and transmission electron microscopy (SEM, TEM) and white light interferometry (WLI) allowed the characterization of surface topographies and their implementation in established models for wetting behavior of rough solid surfaces. Further...

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