Improved bubble release from cylindrical sensor devices via 5-axis laser micromachining

Micro-scale sensors designed for aqueous applications are often hampered by attached air bubbles, which can interfere with their measurements. This is especially true for sensors fabricated by laser micromachining, which typically increases the hydrophobicity of metallic substrates. To address this problem, we developed and compared three different femtosecond laser micromachining strategies for cutting eyelets into the face of steel tubes, which represent sensor housings. By using the full potential of a 5-axis motion stage (3 linear plus 2 rotary axes), we fabricated eyelets with Straight, Funnel, and Round-shaped wall profiles. Then, immersion tests in an aqueous bath were performed to characterize the bubble release properties of each design. We found that all 3 eyelet designs exhibit excellent bubble release properties immediately after micromachining. However, due to the gradual chemisorption of carbonaceous species onto the steel, the substrate's surface chemistry experiences hydrophobic recovery, which gradually reduced the bubble release performance of all samples. Accordingly, we executed a series of low-temperature heat treatments on the steel samples to purposely accelerate and study this process. By performing immersion experiments at different heat treatment time points, our results verify that eyelets with rounded side walls exhibit improved performance compared to the other designs. This difference can be attributed to a lack of sharp edges present on the Round eyelets, allowing for more facile liquid advancement across the solid interface due to the Gibbs inequality criterion. • Novel laser micromachining strategies using 5-axis stage for cylindrical substrates • Improved bubble release properties of fabricated sensor housings • Tunable wettability of microstructured devices via heat treatment