Abstract
The development of a robust, cost-effective, scalable and simple technique that enables the design and construction of well-controlled large area superhydrophobic surface structures which can be easily tuned from lotus-leaf to rose-petal state is essential to enable progress in realising the full applied potential of such surfaces. In this study, we introduce the tuneable carbon nanotubes-based electrohydrodynamic lithography (CNT-EHL) to fabricate unique multiscale structured cones and nanohair-like architectures with various periodicities and dimensions, successfully enabling surface energy minimization. The possibility of contact-less lithography via the CNT-EHL morphology replication combined with the electric field coupling to smaller self-assembled patterns within the film, provides a way for hierarchical structure control spanning many length scales along with tuneable wetting capabilities. By controlling the hierarchy of micro- to nano cones and spikes, these morphologies provide a range of architectures with sufficient roughness for very low wettability, with the highest contact angle achieved of 173° and their properties can be easily switched between lotus-leaf to rose-petal behaviour.
Highlights
The development of a robust, cost-effective, scalable and simple technique that enables the design and construction of well-controlled large area superhydrophobic surface structures which can be tuned from lotus-leaf to rose-petal state is essential to enable progress in realising the full applied potential of such surfaces
It has been acknowledged that it is important to consider wetting modes beyond the classical and the broadly described Wenzel/Casie–Baxter states recognising existence of the ‘Lotus-Leaf ’ effect and of a strong adhesion combined with super-hydrophobicity known as ‘Rose-Petal’ state[6,7] and distinguishing wetting regimes of a surface with a single level of roughness and the hierarchical ones (Fig. 1b). The existence of such a spectrum of the wetting states can be understood through the competition of forces acting on the solid–liquid in terms of surface energy, which is inversely proportional to the contact angle adhesion of water molecules to the rough surface as well as impregnation of the hierarchical structures by water or air
carbon nanotubes-based electrohydrodynamic lithography (CNT-EHL) method, elucidated below, requires assembling a miniaturised capacitor-like device comprised of a bottom electrode topped with a thin nanofilm, spun-cast from the polymer to be patterned and the topographically structured top electrode with a pattern of interest to be replicated
Summary
Tunable superapolar Lotus-to-Rose hierarchical nanosurfaces via vertical carbon nanotubes driven electrohydrodynamic lithography†. The red rose petals maintain spherical droplets on their surface which do not roll off even if the petal is turned upside down (Fig. 1b) exhibiting both superhydrophobicity along with high adhesive force with the water While technologically valuable, both high-density hierarchical fibrillar adhesives and precisely orchestred periodic micro-to-nano super apolar structures are difficult to manufacture in a straightforward manner from a material of choice and no scalable low-cost approach yet exist to create the required tuneable geometries. Utilising low-energy materials, the CNT-EHL fabricated micro to-nanoscale roughness allows precisely tailoring and controlling hierarchical geometries by adjusting the patterning parameters and significantly influencing the surface wetting properties and mimicking the various regimes found in nature This method enables tuning and alternating between the lotus-leaf and rose-petal behaviour due to the controllable experimental approach and the ultimate morphologies generated while patterned from the same initial material. CNT-EHL opens a new avenue for the generation of a broad spectrum of highfidelity superhydrophobic patterns in a straightforward and low-cost fashion, requiring no vacuum processing, no hazardous organic compounds with possibilities of exploiting biodegradable or environmentally-friendly apolar polymers, rendering this technique even more technologically appealing
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