Nonfluorinated Superomniphobic Surfaces through Shape-Tunable Mushroom-like Polymeric Micropillar Arrays

Background

Surfaces that repel both water and low-surface-tension liquids are usually made by combining micro- and nanohierarchical structures with perfluorocarbon coatings. These coatings are vulnerable to chemical and physical damage, so liquid repellency degrades, and their use raises environmental and health concerns. Springtail skin repels liquids through doubly re-entrant structures, mushroom-like features with downward-facing edges, but earlier mimics relied on inflexible silicon substrates, additional fluorinated coatings or slow 3D direct laser writing.

Design and approach

Flat mushroom-like micropillars are molded in PDMS (5:1 base to curing agent, Young’s modulus ≈ 3.5 MPa) from a photoresist mold, in which top-side UV exposure defines the pillar holes and a mask-free back-side exposure defines a thin cap layer. Thermal evaporation of Al then produces compressive residual stress that bends the cap edges permanently toward the polymer, forming doubly re-entrant micropillars (D-MPs). Liquid entering from above is pinned under the downward edges, where the negative cap edge angle lets a contact line form even for liquids that fully wet the Al. The geometry was set with the Cassie-Baxter equation: a solid fraction below ∼6.7% is required, and with a 23 μm cap an interpillar spacing of 80 μm gives ∼6.5%.

How it was evaluated

The authors compared flat-cap micropillars (MPs), perfluorocarbon-coated MPs (F-MPs) and D-MPs by static contact angle (CA) and sliding angle (SA) of ∼2 μL droplets of liquids from DI water to fluorinated solvents, supported by scanning electron microscopy and numerical simulation of cap bending. Pressure resistance was examined by following an evaporating ethanol droplet. Repellency was tested under uniaxial strain up to ∼50%, repeated stretching, bending and twisting, and after oxygen plasma treatment (100 W for 10 s) that makes the surface chemistry hydrophilic. A horse blood droplet was placed on a D-MP surface attached to the finger of a latex glove.

Key results

Caps were stable without collapse at a diameter of about 23 μm and thickness of about 2.2 μm; caps over 30 μm in diameter or less than 2 μm thick collapsed, and caps under 20 μm in diameter or over 3 μm thick rarely bent after Al deposition. The bending angle was ∼28° after 5 nm of Al and saturated at ∼84° above 50 nm, in good agreement with simulations.

Both MP and D-MP surfaces repelled DI water (72 mN/m), but a 50% ethanol solution (∼28 mN/m) fully wetted the MPs while the D-MPs remained super-repellent. The MPs could not repel liquids below ∼32 mN/m, and the F-MPs were wetted below ∼25 mN/m, whereas the D-MPs repelled all liquids tested, including Krytox 100 (17 mN/m) and FC-770 (14.8 mN/m), with CAs above 150° and no chemical treatment. FC-770 spread completely on flat Al-coated PDMS (θi ≈ 0°) but sat on the D-MP surface at θ* = 152.2°. An evaporating ethanol droplet stayed on the surface until its radius fell to ∼490 μm, corresponding to a Laplace pressure of ∼85.7 Pa, and then wetted it.

When stretched by ∼50%, the square array became rectangular and the calculated solid fraction fell from ∼6.5 to ∼5.5%; measured CAs of 70% ethanol (∼25 mN/m) were very similar to the calculated values, and repellency was maintained under strain and over more than 1000 stretching cycles. Repellency also persisted after stretching, bending and twisting. After oxygen plasma treatment, which increased the XPS O 1s signal, DI water wetted flat Al-coated PDMS and MPs (θi < 10°), while the D-MPs stayed super-repellent (θ* > 150°). A ∼3 μL horse blood droplet rolled off the highly curved D-MP surface on a latex glove without leaving a stain.

How it connects to other research

Slippery liquid-infused porous surfaces (SLIPS) repel liquids by locking a lubricating fluid into a porous structure; the D-MP surface instead holds an air layer beneath downward-curved solid cap edges, so its repellency depends on the solid geometry rather than on a retained lubricant or a fluorinated coating.