3D Micropatterned Functional Surface Inspired by Salvinia Molesta via Direct Laser Lithography for Air Retention and Drag Reduction

Bioinspired functional surfaces are attracting increasing interest in surface engineering, mostly in the field of wettability. The Salvinia-effect is a remarkable example of superficial air retention and drag force reduction caused by selective chemical coating (super-hydrophobic wax and hydrophilic dead cells) and peculiar 3D hierarchical morphological structures. The replication of Salvinia-like patterns at the microscale has always been prevented by limitations in microfabrication techniques, thus hindering relevant technological applications at this dimensional scale. Integrating 3D laser lithography and traditional microfabrication techniques, dimensionally downscaled, 3D micropatterned surfaces inspired for the first time by both morphology and chemical coating of the hairs present on the Salvinia Molesta leaves are reproduced. The effect of design and different surface energies (bare hydrophilic, hydrophobic, selective hydrophilic/hydrophobic coating) on the wettability are modeled and investigated. Bioinspired surfaces demonstrated to be super-hydrophobic in terms of apparent static contact angle (up to 170°) and provide tunable adhesion with roll-off angle from less than 10° to 90°. They successfully proved remarkable underwater air retention capability, sustained by stable Cassie-Baxter state under external hydrostatic pressure up to 4 atm. The proposed surfaces are tested in hydrodynamic conditions: drag force reduction is successfully demonstrated with up to 40% of energy saved.