Prototyping of Superhydrophobic Surfaces from Structure‐Tunable Micropillar Arrays Using Visible Light Photocuring

Abstract

A new approach is reported to fabricate micropillar arrays on transparent surfaces by employing the light‐induced self‐writing technique. A periodic array of microscale optical beams is transmitted through a thin film of photo‐crosslinking acrylate resin. Each beam undergoes self‐lensing associated to photopolymerization‐induced changes in the refractive index of the medium, which counters the beam's natural tendency to diverge over space. As a result, a microscale pillar grows along each beam's propagation path. Concurrent, parallel self‐writing of micropillars leads to the prototyping of micropillar‐based arrays, with the capability to precisely vary the pillar diameter and inter‐spacing. The arrays are spray coated with a thin layer of polytetrafluoroethylene (PTFE) nanoparticles to create large‐area superhydrophobic surfaces with water contact angles greater than 150° and low contact angle hysteresis. High transparency is achieved over the entire range of micropillar arrays explored. The arrays are also mechanically durable and robust against abrasion. This is a scalable, straightforward approach toward structure‐tunable micropillar arrays for functional surfaces and anti‐wetting applications.

Fabrication of Tapered 3D Microstructure Arrays Using Dual-Exposure Lithography (DEL)

“…Superhydrophobic properties of natural surfaces, known as the lotus effect, have been extensively mimicked for many industrial and biological applications [ 55 ]. MSAs are ideal candidates for such superhydrophobic surfaces because of the densely packed microstructures [ 1 , 9 ]. A direct measure of the MSA’s superhydrophobicity is a high contact angle (>150°) [ 56 ] and a low roll-off angle (2–30°) [ 57 ], which determine the wettability of the surface [ 9 , 58 ].…”

“…Microscale roughness, according to the lotus effect, is mainly responsible for the level of hydrophobicity of the surface [ 1 , 9 , 59 ]. With our MSAs, the microscale roughness was directly related to the height of the microstructures, given the same grid distance.…”

“…Surface hydrophobicity analysis was performed as previously described [ 9 ]. Briefly, a 2 µL volume droplet was placed on the MSA surface.…”

“…Measured contact angles were plotted as a function of the microstructures’ height. Further, the roll-off angle and hysteresis were calculated, as reported previously [ 1 , 9 ].…”

“…MSAs, with these micron-sized (~1–15 microns) structural units, bestow unique tunable material properties, such as wettability, optical characteristics, and cell adhesion [ 6 , 7 , 8 ]. These MSAs have been widely employed in studies like anti-wetting surfaces [ 9 ], optical waveguide fabrication [ 10 ], cell mechanics studies [ 11 ], stem cell engineering [ 12 ], and other scientific material applications [ 2 , 13 ]. MSA applications also include structural support in the fabrication of microelectrode array for brain interface studies [ 14 , 15 ], microprobes for neural depth recordings [ 16 , 17 ], microneedles for drug delivery [ 18 ], and other micro-optic studies [ 19 , 20 ].…”