Gradient wettability induced by deterministically patterned nanostructures

Abstract

We report a large-scale surface with continuously varying wettability induced by ordered gradient nanostructures. The gradient pattern is generated from nonuniform interference lithography by utilizing the Gaussian-shaped intensity distribution of two coherent laser beams. We also develop a facile fabrication method to directly transfer a photoresist pattern into an ultraviolet (UV)-cured high-strength replication molding material, which eliminates the need for high-cost reactive ion etching and e-beam evaporation during the mold fabrication process. This facile mold is then used for the reproducible production of surfaces with gradient wettability using thermal-nanoimprint lithography (NIL). In addition, the wetting behavior of water droplets on the surface with the gradient nanostructures and therefore gradient wettability is investigated. A hybrid wetting model is proposed and theoretically captures the contact angle measurement results, shedding light on the wetting behavior of a liquid on structures patterned at the nanoscale.

Recent Growth of Wettability Gradient Surfaces: A Review

“…Dry WGSs establish the contact with the liquid droplet through a dry interface, which can be further divided into sticky and slippery classes. An example of dry WGS [58] is the formation (of concentric nanopillars) onto the cyclic olefin copolymer substrate, consisting of a central hydrophobic wetting regime which tends to be less hydrophobic towards the edges.…”

“…The fabrication of WGSs is supposed to be the primary key, deciding the potential usage later in device erection either for lab-scale or industrial applications. Commonly, WGSs are created after passing through two steps [61] or multiple steps in a single trial or several different methods are combined and carried out [58,64]. The fabrication processes, including the plasma treatment, imprinting, vapor deposition, and soft-lithography, become complicated owing to the stringent process constraints or control parameters [12,41].…”

“…For example, a secondary substrate is frequently required to create the footprints on the target substrate, whereas that secondary substrate is often difficult to be designed at the lab scale, thus increasing the fabrication cost when buying it from professional designers. Likewise, high-cost methods such as reactive-ion etching have also been suggested to be replaced by other facile steps [58], as well as the laser cutting and photolithography have been declared complex and timeconsuming [85]. Therefore, there is an urgent need to pay special attention on simplifying the fabrication processes to create the robust WGSs by either alternative methods or by modifying the existing methods.…”

Estimation of surface free energy at microstructured surface to investigate intermediate wetting state for partial wetting model

“…Wetting states different from these two states have also been widely observed during wetting transitions [21][22][23][24][25][26][27] and gradient wetting. [28][29][30] Min et al proposed a hybrid wetting model involving a locally suspended and pillar/hole arrays for a nanostructured surface. 29 Another study proposed a partial wetting model for an intermediate wetting state between the Cassie-Baxter and Wenzel states in the case of static wetting.…”

Durable and Flexible Hydrophobic Surface with a Micropatterned Composite Metal–Polymer Structure

“…For instance, Ma et al 19 prepared hierarchical structures on Pd-based metallic glass surfaces by thermoplastic forming with an anodic aluminum oxide (AAO) template and a micropatterned silicon mold, showing superhydrophobic effects. Min et al 20 created a surface with continuously varying wettability by fabricating a variable intensity nanohole/pillar pattern on a cyclic olefin copolymer using interference lithography. Kang et al 21 demonstrated superhydrophobic surfaces made of mushroom-like micropillar arrays on poly(dimethylsiloxane) (PDMS).…”