Black Silicon Microstructuring via Plasma Cryogenic Etching for Microfluidic Applications in Geoscience

In this work, we present a novel approach to black silicon (BSi) microstructuring using the STiGer process, specifically tailored for microfluidic applications in geoscience. In contrast to traditional approaches that rely on secondary coatings to modify surface wettability, our research focused on optimizing the STiGer process parameters to achieve fine-tuned control over both the surface morphology and wettability of the resulting BSi structures. By carefully adjusting the SiF₄/O₂ ratio, passivation time, and bias voltage within the STiGer process, we demonstrate the ability to create surfaces ranging from strongly hydrophilic to highly hydrophobic, all without the need for secondary coatings. Notably, we have investigated the microstructuring of vertical sidewalls within microfluidic channels, a feature previously underexplored, which allowed to mimic the roughness observed on natural rock minerals. Through systematic variation of the process parameters, we could effectively etch BSi microstructures on all the vertical sidewall of the microfluidic channel by controlling the plasma sheath thickness relative to the channel opening. This approach mimics the conditions of natural porous media and will open a new way to quantify the effect of the heterogeneity of the microstructure of the inner surface of the microchannel on two-phase flows (e.g. water/air, CO2/brine, hydrocarbon/water). The presented results highlight the high sensitivity of BSi formation to parameter tuning, underlining the importance of bias, SiF4 ratio and other parameters for achieving the desired roughness and wettability. By achieving vertical sidewall roughness within microfluidic devices, we aim at bridging the gap in current microfluidic modeling, which often oversimplifies the heterogeneity of vertical sidewalls within microfluidic channels in geological studies.