REPLICATION OF LASER INDUCED PERIODIC STRUCTURES, GROOVES AND SPIKES ON PDMS FOR ROBUST WETTING APPLICATIONS

REPLICATION OF LASER INDUCED PERIODIC STRUCTURES, GROOVES AND SPIKES ON PDMS FOR ROBUST WETTING APPLICATIONS C. RUIZ DE GALARRETA¹, F. FIEDLER2,3, B. ZIELINSKI2,3, I. MEHDIOGLU2,3, M. KAHLMAYER4, S. BÖHM4 AND C. FLORIAN2,3* ¹Instituto de Ciencia de Materiales de Barcelona, ICMAB - CSIC, 08028, Barcelona, Spain ²Institute of Materials Engineering, Kassel Universität, 34125, Kassel, Germany 3Institute of Physics, Kassel Universität, 34125, Kassel, Germany 4Institute of Materials Engineering, Kassel Universität, 34125, Kassel, Germany *[email protected] ABSTRACT Surface functionalization through laser-induced periodic surface structures (LIPSS) has been applied to a variety of materials to modify their wetting properties. Previous studies have shown that surface wetting is influenced not only by surface topography but also—often more critically—by surface chemistry. In laser-irradiated surfaces where material ablation occurs, the newly formed surfaces not only exhibit chemical changes in the way of oxide creation on metallic samples or carbonization in the case of polymers, but also time-dependent wetting behaviour may occur by the gradual adsorption of hydrocarbons, which create new anchoring sites over time and ultimately govern long-term wettability. Such time-dependent changes present a challenge for wetting functionalization in materials sensitive to chemical changes using LIPSS, as the final wetting state cannot be precisely defined through laser irradiation alone. In this study, we propose an alternative approach to produce constant wetting with water over extended times based on the fabrication of master samples with three types of structures—LIPSS, grooves, and spikes—using a Yb-doped high-repetition-rate femtosecond laser (1035 nm wavelength, 400 fs pulse duration). Master samples were produced on crystalline silicon over areas of 1 × 1 cm². These structures were then successfully replicated in PDMS, demonstrating that surface patterns can be transferred with high fidelity and that wettability can be tuned by keeping the surface chemistry of raw material and only altering its surface topography. We achieved negative replications for all structures, as well as positive replication for LIPSS and grooves. Master samples were characterized by SEM, while replicated structures were analyzed using AFM. Contact angle measurements with water were performed to evaluate the wettability, confirming the robustness of this replication-based approach for wetting applications. 1. INTRODUCTION Laser-induced periodic surface structures (LIPSS) are periodic surface modifications typically generated by short or ultrashort pulsed laser irradiation under multiple-pulse exposure conditions. It has been shown that LIPSS can be fabricated using infrared laser systems on virtually any material. Large-area fabrication has enabled applications in fields such as optics, medicine, tribology, and wetting. For wetting applications on metals, LIPSS have proven effective in finely controlling the wettability of regular steel substrates [1], including the biomimetic replication of natural wetting phenomena [2]. However, when applied to oxidation-prone materials, laser irradiation often induces surface oxidation. The combined effects of chemical modification and increased topography result in wettability that evolves over time. Several strategies have been explored to stabilize wetting behaviour, such as laser processing in controlled atmospheres, post-irradiation thermal stabilization, and application of chemical coatings [3]. These studies highlight the difficulty of achieving long-term wetting control through LIPSS using a single-step process. In polymers, laser-induced surface texturing presents additional challenges. Due to the low melting temperatures, laser irradiation often causes permanent chemical modifications, such as bond scission or rearrangement. Under extreme ablation conditions, carbonization or graphitization of the polymer surface can also occur [4]. For wetting control, however, preserving the intrinsic chemical properties of the base material is essential. 2. RESULTS In this study, we propose an alternative fabrication method for polymer samples that relies on a replication approach that successfully reproduced both positive and negative micrometric structures on polydimethylsiloxane (PDMS). The process relied on fabricating master samples on crystalline silicon, using laser-induced periodic surface structures (LIPSS), grooves, and spikes generated by laser irradiation. A Trumpf TRUMARK 7000 station was employed, delivering 400 fs pulses centered at a wavelength of 1030 nm. After fabrication, the master samples were characterized by 3D optical profilometry to determine homogeneity and average roughness. Topography was further analyzed by scanning electron microscopy. The replication procedure began with silanization of the silicon masters using perfluorooctyltrichlorosilane. Negative replicas were then prepared by casting soft PDMS onto the silanized silicon masters, followed by oven curing under standard conditions. This method reliably reproduced LSFL, groove, and spike structures. For positive replicas, an intermediate step was introduced. A hybrid inorganic/organic polymer with glass-like properties after UV curing (OrmoStamp®, Microresist) was used. A droplet of OrmoStamp was placed on the master surface, then covered with a clean glass slide serving as a backing substrate. The stack was UV-cured, resulting in a negative replica of the silicon master. Positive PDMS replicas were subsequently obtained by casting PDMS onto these OrmoStamp negatives. LSFL and groove structures were successfully replicated in this way. However, spike structures proved difficult to replicate in positive form due to the high surface roughness, which caused detachment failures during demolding from the soft PDMS. The morphology of the masters and their replicas (both negative and positive) was examined using AFM to assess replication fidelity. Wetting properties were characterized by depositing 3 µL water droplets on the PDMS replicas and comparing the resulting contact angles with those measured on flat, nonreplicated PDMS, providing a baseline for evaluating the effect of the structures. Fig. 1 (a) SEM characterization of grooves structures on silicon, PDMS positive and negative replicas. (b) AFM characterization of the produced structures. We demonstrate that this replication method offers a robust alternative for reproducing laser-induced microstructures in polymer materials such as PDMS. The replicated surfaces exhibit distinct wetting behaviors determined by their topography, consistent with Cassie–Baxter and Wenzel wetting models. Moreover, the wetting response remained stable over time, confirming both the accuracy and the reliability of the replication process. REFERENCES [1] C. Florian, E. Skoulas, D. Puerto, A. Mimidis, E. Stratakis, J. Solis, J. Siegel, "Controlling the Wettability of Steel Surfaces Processed with Femtosecond Laser Pulses", ACS applied materials & interfaces 10 36564–36571, (2018). [2] E. Stratakis, J. Bonse, J. Heitz, J. Siegel, G.D. Tsibidis, E. Skoulas, A. Papadopoulos, A. Mimidis, A.-C. Joel, P. Comanns, J. Krüger, C. Florian, Y. Fuentes-Edfuf, J. Solis, W. Baumgartner, "Laser engineering of biomimetic surfaces", Materials Science and Engineering: R: Reports 141 100562, (2020). [3] N. Joy, A.-M. 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