Mimicking Both Petal and Lotus Effects on a Single Silicon Substrate by Tuning the Wettability of Nanostructured Surfaces

Abstract

We describe a new method of fabricating large-area, highly scalable, "hybrid" superhydrophobic surfaces on silicon (Si) substrates with tunable, spatially selective adhesion behavior by controlling the morphologies of Si nanowire arrays. Gold (Au) nanoparticles were deposited on Si by glancing-angle deposition, followed by metal-assisted chemical etching of Si to form Si nanowire arrays. These surfaces were chemically modified and rendered hydrophobic by fluorosilane deposition. Au nanoparticles with different size distributions resulted in the synthesis of Si nanowires with very different morphologies (i.e., clumped and straight nanowire surfaces). The difference in nanowire morphology is attributed to capillary force-induced nanocohesion, which is due to the difference in nanowire porosity. The clumped nanowire surface demonstrated the lotus effect, and the straighter nanowires demonstrated the ability to pin water droplets while maintaining large contact angles (i.e., the petal effect). The high contact angles in both cases are explained by invoking the Cassie-Baxter wetting state. The high adhesion behavior of the straight nanowire surface may be explained by a combination of attractive van der Waals forces and capillary adhesion. We demonstrate the spatial patterning of both low- and high-adhesion superhydrophobicity on the same substrate by the simultaneous synthesis of clumped and straight silicon nanowires. The demonstration of hybrid superhydrophobic surfaces with spatially selective, tunable adhesion behavior on single substrates paves the way for future applications in microfluidic channels, substrates for biologically and chemically based analysis and detection where it is necessary to analyze a particular droplet in a defined location on a surface, and as a platform to study in situ chemical mixing and interfacial reactions of liquid pearls.

Perspectives on oblique angle deposition of thin films: From fundamentals to devices

“…The high versatility of this technique in obtaining different nanostructures (metals, organic compounds, oxides, hybrids and other complex heterostructures) is an additional feature that supports its use for wetting applications. Important achievements have been made in the last few years through the OAD fabrication of surfaces possessing singular adhesive properties [555], hydrophobicity [556][557][558][559], superhydrophobicity [186][187][188]557,[560][561][562][563], superhydrophilicity [366] or superolephobicity [564,565]. Initial approaches to the development of highly hydrophobic surfaces by OAD combined the surface nanostructuration capability of this technique with the chemical modification of the surface composition by different methods.…”

“…More recently, a similar approach involving the molecular vapor deposition of silane onto metal OAD nanocolumns has been proposed as a way of fabricating anti-icing surfaces [558]. Moving a step forward, Choi et al [561,562] have produced superhydrophobic surfaces with a dual-scale roughness that mimics lotus petal effects (i.e., the ability to pin water droplets while maintaining a large contact angle) [567]. This was achieved through the fluorosilanization of Si nanowires arranged at a micron-level via OAD onto a pre-patterned substrate decorated with gold nanoparticles.…”

Wear-resistant rose petal-effect surfaces with superhydrophobicity and high droplet adhesion using hydrophobic and hydrophilic nanoparticles

“…There also exist surfaces, such as the red rose petal, that exhibit superhydrophobicity with high droplet adhesion; the droplet remains pinned even when the petal is inverted. However, while there has been much research in superhydrophobic surfaces with low droplet adhesion (the so-called ''Lotus effect''), relatively fewer studies have been published on superhydrophobic surfaces with high droplet adhesion (the socalled ''petal effect'') [4][5][6][7][8][9][10]. These surfaces have potential for applications including single-molecule spectroscopy [11] and controlled transport of small volumes of liquid in open microfluidic devices [12].…”

Effect of ammonia on morphology, wettability and photoresponse of ZnO nanorods grown by hydrothermal method

“…The visible emission band was caused by the point defects, such as oxygen vacancies [22], zinc interstitials [23], interstitial oxygen [24], and perhaps had much to do with the ZnO structure [25]. That is to say, water drop do not penetrate into grooves of the ZnO nanorods, but stand on a "pincushion" [26]. The wetting behavior is described by the Cassie-Baxter model as [27]:…”