Surfaces with finely stratified micro/nanostructures exhibit supersurface-like properties that are solely determined by their geometry. Alfa Chemistry specializes in fabricating mimicked natural surfaces using manufacturing processes based on structural properties of biological nanostructures. The following are some of the precise ways we've utilized to build microarray surface surfaces that have been successful in producing surface structures of astonishing complexity.
Photolithography
Our most popular micro/nanofabrication approach for producing microarray-like surfaces with predetermined geometries is photolithography. Photolithography makes it possible to create reentrant structures, which are required for totally sparse surfaces. Here are a few examples of how to put this into practice.
- Lithography can be used to create microhoodoo reentrant geometries with independently controllable pitch, height, radius, and width.
- Inverse trapezoidal microstructured surfaces of polydimethylsiloxane (PDMS) can be fabricated by replication and diffusion lithography techniques, and their holophobicity can be demonstrated by coating the surface with a fluoropolymer.
- Two-step lithography can be used to create microarrays of overhanging discs. The resulting re-entrant microdisk array surfaces can then be fluorinated to achieve full sparsity.
- Straight microcolumns can also be modified to produce mushroom-like microarrays, for example by spraying straight columns with a mixture of PDMS and fluoropolymers.
- In addition to the reentrant surfaces for omniphobicity, microgroove structures can also be easily fabricated by lithography-based techniques for directional liquid spreading and transportation.
Fig 1. Surfaces fabricated by photolithography. a) Schematic diagram of lithography. b) Surfaces prepared by photolithography. c) SEM image of the surface produced by lithography. d) SEM image shows an accurately defined surface microstructure formed by photolithography. e) SEM image of a surface with spray-coated PDMS/F-POSS on PDMS pillars formed by photolithography. f) SEM image of the small slip angle of water and ethanol. g) Anisotropic microtextured surface produced by photolithography. (Kong T, et al. 2019)
Nanoimprint Lithography
Nanoimprint lithography is a straightforward method for applying stencil tiny designs to a substrate by deforming an imprint resist at the right pressure and temperature. The approach is low-cost, has a high resolution, and is easy to repeat.
- Photolithography or chemical etching can be used to create the original templates with embossed patterns. A microarray of two-layer silicon structures, for example, can be produced as a template by photolithography and then embossed on a photocrosslinkable flexible polymer substrate to create a surface inspired by a spring-tailed skin.
- Imprinted lithography can also be used to prepare highly adhesive surfaces. For example, to mimic the layered arrays on gecko feet, a two-step nanoimprinting technique assisted by sacrificial layers was used.
Etching
Etching is a strong micro/nanofabrication process that permits intricate 3D microstructures to be created. Depending on whether the etchant is a liquid or a reactive gas, this approach encompasses both wet and dry etching. Dry etching often eliminates material in an anisotropic way, allowing structures with high aspect ratios, fidelity, and resolution to be created. Despite the fact that chemical contamination is a severe disadvantage, etching techniques are routinely utilized to produce sophisticated 3D micro/nanostructures.
Fig 2. Surfaces fabricated by etching. a) Schematic of a liquid suspension on different microstructures; b) SEM images showing doubly reentrant serif-T-shaped micropillars fabricated by etching. (Liu T. L, et al. 2014)
An important breakthrough in the fabrication of fully sparse surfaces is the ability to repel fluorinated oils with surface energies below 20 mN m-1. Superhydrophobicity for all available liquids, including perfluorocaprolactane, is obtained by anisotropically etching wettable silica substrates, and delicate double-folded liner-T-shaped microarrays are the key to producing superhydrophobicity. Dry etching makes it possible to make microarrays that would otherwise be impossible to make, such as asymmetric arrays of deflected nanopillars.
Femtosecond Laser Ablation
Femtosecond lasers have been used to pattern the surface of a wide variety of materials, including silicone, glass, polymers, metals, and even ceramics. The laser beam is focused on the processed surface through an objective lens; the beam ablates from the surface and removes the material at the focal point. The surface is then moved along a pre-designed path, such as row-by-row, to create the pattern by controlling the 3D translation phase.
Direct Laser Writing
Direct laser writing, also known as two-photon laser writing, is a technique for creating nanoscale features in photocrosslinkable resins without any masks. By using a laser of the appropriate wavelength, the transparent resin is polymerized at the focus of the laser. The movement of the laser inside the photocrosslinkable resin results in highly accurate multi-scale structures.
The resolution and complexity of the structures fabricated by this technique is exceptionally high, but the technique can only produce nanometer-sized and submicron-sized objects. Triple-folded structures can be fabricated by two-photon laser writing, and triple-in structures exhibit superhydrophobicity. The superhydrophobicity of the resulting structures remains chemically stable even after the material is chemically superhydrophilic by oxygen plasma.
References
- Kong T, et al. (2019). "Bioinspired Superwettability Micro/Nanoarchitectures: Fabrications and Applications." Adv. Funct. Mater. 29: 1808012.
- Liu T. L, et al. (2014). "Repellent Surfaces. Turning A Surface Superrepellent Even to Completely Wetting Liquids." Science. 4: 1096-1100.
