Femtosecond laser microfabrication of open Janus wettability channels on silicon for pump-free passive fluidic transport

We report a femtosecond laser microfabrication strategy for creating open microfluidic channels on silicon substrates with programmable Janus wettability, defined as the coexistence of superhydrophilic (water contact angle, WCA ≈ 0°) transport channels and a superhydrophobic (WCA = 178.9°) background within a single silicon wafer. Three sequential steps are employed: (i) global femtosecond laser surface structuring (3.0 W, 10 mm s−1, 25 μm hatch spacing) to generate hierarchical micro/nanostructures including laser-induced periodic surface structures (LIPSS), yielding a superhydrophobic background of WCA = 178.9° after silanization; (ii) vapor-phase fluorinated silane functionalization; and (iii) digitally controlled selective laser re-irradiation (100 mm s−1) to define superhydrophilic open channel networks by locally modifying fluorinated surface species and micro/nanostructures, restoring WCA ≈ 0°. Channel depth is independently controlled from approximately 25.4 μm to complete through-ablation by adjusting the processing pass number under fixed laser power, scanning speed, and hatch spacing. The superhydrophobic state (WCA = 178.9°) remained near 179° over 3 years of laboratory storage, indicating long-term storage stability. A 3.3° roll-off angle was observed in five tests with 7.8 μL droplets. Two functional demonstrations are presented: (i) a bioinspired leaf-venation hierarchical channel network, operated with a glass cover placed over the channel region, achieving spontaneous capillary-driven propagation through three channel generations within 2.4s, with measured transport velocities of 21.7 ± 1.2, 14.8 ± 0.9, and 6.0 ± 0.5 mm s−1 for the main channel (G0), first-generation (G1), and second-generation (G2) branches, respectively, decreasing monotonically with narrowing channel width; and (ii) a fully open, traditional Korean pagoda (Dabotap)-inspired multi-inlet network enabling ten independently guided dye streams without visible lateral colour mixing over approximately 11s. This mask-free, digitally programmable laser-enabled approach is potentially compatible with future silicon microelectromechanical systems integration and provides a versatile platform for passive open microfluidics, autonomous fluid management, and lab-on-chip applications.