Mask-assisted electrospray for superoleophobic surfaces: An experimental and numerical study

This paper presents both experimental work and numerical simulations of formation of superoleophobic surfaces created by mask-assisted electrospraying, followed by a second layer overlay and fluoropolymer treatment. The primary electric field focusing in the mask-assisted electrospray effectively guides the electrosprayed particulates into the mesh openings, forming characteristic pyramid-shaped pillars. The secondary focusing occurs during the overlay deposition when the electrosprayed particulates favorably deposit onto the pre-patterned pillars. Systematic studies were conducted on the effects of mask-substrate-gap and duration of the overlay deposition on the pattern morphology and wetting performance. A shorter mask-substrate-gap results in a stronger focusing effect and pillars with a larger aspect ratio. The overlay deposition firstly increases the pillar height and then changes the pillar shape from pyramids to domes with overhangs due to electrostatic interactions. All the surfaces are superhydrophobic, however, superoleophobicity varies. Surfaces that have tall pillars and overhang structures demonstrate robust superoleophobicity when compared to their counterparts with shorter pillars and absence of overhang structures. The primary and secondary electric field focusing effects exerted by the mask and the pre-patterned pillars, and their roles in pattern formation have been numerically investigated by COMSOL Multiphysics simulation. A reasonable agreement has been obtained between the numerical predictions and experimental results.