Such fabrication methods for architectures with random roughness are typically simple, low cost, and scalable, but do not provide fine control over micro/nanostructures. These methods provide limited control over the average roughness by varying processing parameters, such as the composition of the working solution. Due to random roughness, surfaces often need to be chemically modified with low surface energy substances to obtain larger θ*. Alfa Chemistry specializes in fabrication methods related to the structural features of biological nanostructures to create simulated natural surfaces. Below are some of the specific methods we use to fabricate random rough surfaces.
Spray and Spin Coating
These two methods are typically used to form thin films coated on a flat substrate from a polymer solution or nanoparticle suspension. The solution is sprayed onto the substrate and cured into a thin film by the spray method; or a drop of the solution is poured onto the substrate and then spin-coated in the spin-coating method to form a thin film of uniform thickness. To enhance the adhesion between the sprayed silanized nanoparticles and the substrate, the glue is sprayed onto the substrate prior to spraying the particle suspension. The glue layer can subsequently be dissolved to obtain a free-standing, fully hydrophobic film.
In addition to hydrophobic nanoparticles, edible wax-in-water emulsions are used to coat glass substrates and the resulting microscale structure and wax coating lead to superhydrophobicity.
Fig 1. The spray-coating and spin-coating methods. Schematics of the a) spray-coating and b) spin-coating processes. c–h) SEM images show the surfaces with random roughness fabricated by spray-coating and spin-coating. (Kong T, et al. 2019)
Dip Coating
Dip coating, also known as the solution immersion process, involves dipping a substrate into a solution to produce roughness. By dipping a copper substrate into a fatty acid solution, for example, the substrate can be covered with flower-like clusters of Cu(CH3(CH2)12COO)2 with large water contact angles and small sliding angles. Alternatively, aqueous suspensions containing low surface energy fluorocarbon surfactants and nanoparticles can be used as impregnating solutions to obtain superwettability, such as TiO2, SiO2 or Al2O3 nanoparticles. Repeated impregnation methods by pH-controlled polymer aggregation or in situ nanoparticle formation can produce highly layered lotus-like structures with micron- and nanometer-sized structures.
Electrostatic Spinning
Superhydrophobic and fully hydrophobic surfaces are made by electrostatically spinning solutions of hydrophobic polymers such as polyvinylidene fluoride (PVDF), polystyrene (PS) and polyaniline. Nanofibers are interwoven to form membranes with the desired roughness and chemical hydrophobicity, resulting in full hydrophobicity. The deposition and alignment of electrospun nanofibers is difficult due to the chaotic nature of electrical instability; therefore, we classify electrospinning as a technique to create surfaces with random roughness, which means that the average roughness can be adjusted, but the detailed structure cannot be precisely controlled.
By increasing the polymer concentration, the final structure on the substrate can vary from bead-only, bead-string and fiber-only. Both the beaded fiber surface and the straight fiber surface exhibit full sparsity, with slightly less hysteresis on the beaded fiber surface than on the straight fiber surface. Hybrid solutions of polymers and nanoparticles can also be electrospun to produce superhydrophobic fiber mats. For non-spinnable materials, coaxial electrostatic spinning can be applied. Using this method, poly(caprolactone) (PCL)/poly(tetrafluoroethylene) coaxial fiber membranes can be fabricated and exhibit full hydrophobicity. Photo-responsive superwettable surfaces can also be obtained by electrostatic spinning by incorporating thermally responsive polymers such as poly(N-isopropylacrylamide) (PNIPAAm) and photo-responsive TiO2 nanoparticles.
Fig 2. Schematic diagram of electrospinning. Reproduced with permission. (Jung J. W, et al. 2016)
Casting
Casting is a wet processing technique that uses a suspension of nanoparticles, prepolymers, crosslinkers, or bubbles that solidify into highly porous scaffolds. This technique is most commonly used to create lubricant-infused sliding surfaces by infiltrating lubricants into the scaffold. The method is simple, economical and can be applied to a wide range of material combinations.
Layer-by-Layer Deposition
Layer-by-layer deposition is a method of coating a substrate with multiple layers of polyelectrolyte films of nanometer thickness. Typically, the process involves adsorption of alternating polyelectrolyte layers of opposite charges onto the substrate until a multilayer film of the desired thickness is obtained. For example, Cu2+/alginate multilayers are deposited on modified PAA g-PVDF films to obtain submerged superoleophobic films. Polymer-nanoparticle multilayer films can also be fabricated by this method.
References
- Kong T, et al. (2019). "Bioinspired Superwettability Micro/Nanoarchitectures: Fabrications and Applications." Adv. Funct. Mater. 29: 1808012.
- Jung J. W, et al. (2016). "Electrospun Nanofibers as A Platform for Advanced Secondary Batteries: A Comprehensive Review." J. Mater. Chem. A. 4: 703-750.
