Impact of surface forces on wetting of hierarchical surfaces and contact angle hysteresis

Free-energy landscapes of intrusion and extrusion of liquid in truncated and inverted truncated conical pores: Implications for the Cassie-Baxter to Wenzel transition

As the simplest model of transition between the superhydrophobic Cassie-Baxter (CB) and Wenzel (W) states of a macroscopic droplet sitting on a microscopically rough or corrugated substrate, a substrate whose surface is covered by identical truncated or inverted truncated conical pores is considered. The free-energy landscapes of the intrusion and extrusion processes of a liquid into single pore are analyzed when the liquid is compressed or stretched so that the liquid phase is either stable or metastable relative to the vapor phase. Therefore, this model is also relevant to the stability of the superhydrophobic submerged substrates. In this study, the macroscopic classical capillary theory is adopted. Even within this simplified model, two simple geometries of truncated and inverted truncated cones lead to completely different free-energy landscapes. A simple criterion for the stability of the CB state based on Laplace pressure is shown not to be sufficient to understand the destruction and recovery of the CB state. The free-energy landscapes indicate that a gradual and an abrupt destruction of CB state is possible, which depends on the orientation of the conical pore and whether the liquid is compressed or stretched. The extensions of these theoretical results to more complex geometries are briefly discussed.

Deposition of nanoparticles from a volatile carrier liquid

HYPOTHESIS

Traversing length scales in a volatile suspension alters the various contributions to particle deposition from conjoining and disjoining surface forces and from convective and liquid evaporative effects, which is apparent in the deposit morphology.

EXPERIMENT

We investigate the particulate structures to result from the self-assembly of nanoparticles following the evaporation of a volatile carrier liquid from the level of the single particle and up to a level which is apparent to the naked eye, while quantifying the contributions of the main mechanisms that are involved in the deposition process.

FINDINGS

We show that from the level of the nanoparticles in our experiment and up to a length scale of approximately 10 μm, the morphology of the deposit is particularly sensitive to particle adhesion to the substrate and to liquid evaporation. At greater length scales, the morphology of the deposit is well correlated with the finite volume of particles and with particle convection effects. The particulate structures are in the form of detached particles and particle islands, stripes, and continuous coating, which may vary at different length scales of the same deposit.

A Comparative Study of Multifunctional Coatings Based on Electrospun Fibers with Incorporated ZnO Nanoparticles

In this work, polymeric fibers of polystyrene (PS) with incorporated ZnO nanoparticles have been deposited onto an aluminum alloy substrate (6061T6) by using the electrospinning technique. In order to optimize the deposition process, the applied voltage and flow rate have been evaluated in order to obtain micrometric electrospun fibers with a high average roughness and superhydrophobic behavior. Thermogravimetric analysis (TGA) has also been employed in order to corroborate the amount of ZnO incorporated into the electrospun fibers, whereas differential scanning calorimetry (DSC) has been performed in order to determine the glass transition temperature (Tg) of the polymeric electrospun fibers. In addition, a specific thermal treatment (Tg + 20 °C) of the synthesized electrospun fibers has been evaluated in the resultant corrosion resistance. A comparative study with previously reported results corresponding to polyvinyl chloride (PVC) fibers is carried out along this paper to show the changes in behavior due to the different compositions and fiber diameters. The coating has produced an important reduction of the corrosion current of the aluminum substrate in two orders of magnitude, showing also an important enhancement against pitting corrosion resistance. Finally, this deposition technique can be used as an innovative way for the design of both superhydrophobic and anticorrosive surfaces in one unique step over metallic substrates with arbitrary geometry.

Designing Carbon Nanotube-Based Oil Absorbing Membranes from Gamma Irradiated and Electrospun Polystyrene Nanocomposites

Carbon-based materials are outstanding candidates for oil spill clean-ups due to their superhydrophobicity, high surface area, chemical inertness, low density, recyclability, and selectivity. The current work deals with the fabrication of membrane oil absorbents based on carbon nanotube (CNT) reinforced polystyrene (PS) nanocomposites by electrospinning technique. The spun membranes are also irradiated with the gamma radiation to induce enough crosslinks and thus good polymer-filler interactions. The structural, morphological, and surface properties in addition to the oil/water separation efficiency were investigated by varying the concentration of CNT and the dose of γ-irradiation. Fabricated nanofiber membranes show superior hydrophobicity and selective oil absorption at 0.5 wt.% of CNT concentration. The best mechanical properties are also obtained at this particular concentration and at 15 KGy optimum γ-irradiation dosage. The gamma irradiated PS/0.5 wt.% CNT membrane also exhibits good antibacterial effects against the bacteria, Escherichia coli, in the form of bacterial inhibition rings around the membranes. The present study thus shows the environmental applicability of the fabricated PS/CNT membranes in treating oil-contaminated water.

Mimicking from Rose Petal to Lotus Leaf: Biomimetic Multiscale Hierarchical Particles with Tunable Water Adhesion

Water-droplet adhesions of the coatings constructed by all-polymer multiscale hierarchical particles (MHPs) were finely adjusted within the range from highly adhesive to self-cleanable. The MHPs were synthesized via thermal-induced polymerization of the reactants absorbed into self-made hollow reactors and in situ capping of nanocomplexes onto the reactors' shell simultaneously. The dynamic wettability of the prepared MHPs was tuned between water-droplet sliding and water-droplet adhering by simply controlling the type of capped nanocomplexes. Water-adhesive force changed in the range from 31.28 to 89.34 μN. In addition, the raspberry-like particles (MHPs without nanocomplex capping) were used to construct superhydrophobic rose-petal-like surface with a high water-adhesive force, which can be applied in microdroplet transportation without loss. The MHPs with appropriate nanocomplex capping were used to fabricate superhydrophobic lotus-leaf-like fabric, exhibiting excellent antifouling property and superior mechanical stability. We believe that the prepared superhydrophobic MHPs with diverse water-adhesive forces are promising in potential academic research and industrial applications.

Numerical Calculation Method of Apparent Contact Angles on Heterogeneous Double-Roughness Surfaces

Double-roughness surfaces can be used to mimic lotus surfaces. The apparent contact angles (ACAs) of droplets on these surfaces were first calculated by Herminghaus. Then Patankar utilized the pillar model to improve the Herminghaus approach and put forward the formulas for ACAs calculation of the homogeneous double-roughness surfaces where the dual-scale structures and the bases were the same wettable materials. In this paper, we propose a numerical calculation method of ACAs on the heterogeneous double-roughness surfaces where the dual-scale structures and the bases are made of different wettable materials. This numerical calculation method has successfully enhanced the Herminghaus approach. It is promising to become a novel design approach of heterogeneous superhydrophobic surfaces, which are frequently applied in technical fields of self-cleaning, anti-icing, antifogging, and enhancing condensation heat transfer.

A Facile All-Solution-Processed Surface with High Water Contact Angle and High Water Adhesive Force

A series of sticky superhydrophobicity surfaces with high water contact angle and high water adhesive force is facilely prepared via an all-solution-processed method based on polymerization-induced phase separation between liquid crystals (LCs) and epoxy resin, which produces layers of epoxy microspheres (EMSs) with nanofolds on the surface of a substrate. The morphologies and size distributions of EMSs are confirmed by scanning electron microscopy. Results reveal that the obtained EMS coated-surface exhibits high apparent contact angle of 152.0° and high water adhesive force up to 117.6 μN. By varying the composition of the sample or preparing conditions, the sizes of the produced EMSs can be artificially regulated and, thus, control the wetting properties and water adhesive behaviors. Also, the sticky superhydrophobic surface exhibits excellent chemical stability, as well as long-term durability. Water droplet transportation experiments further prove that the as-made surface can be effectively used as a mechanical hand for water transportation applications. Based on this, it is believed that the simple method proposed in this paper will pave a new way for producing a sticky superhydrophobic surface and obtain a wide range of use.

Dynamic effects and adhesion of water droplet impact on hydrophobic surfaces: bouncing or sticking

This work reported the dynamic effects of water droplet impact on flat, porous and pincushion structure films of star shaped polyhedral oligomeric silsesquioxane (POSS) fluorinated acrylates, POSS-poly(trifluoroethyl methacrylate)₈ (POSS-(PTFEMA)₈) and POSS-(poly(trifluoroethyl methacrylate)-b-poly(methyl methacrylate))₈ (POSS-(PTFEMA-b-PMMA)₈), using the breath figure method. The porous and pincushion structure films with different surface chemical compositions were obtained by controlling the copolymer structure and temperature and by stripping of the surface. The water contact angles on the different films were measured, and the water droplets on the pincushion structure films when reversed at 45°, 90°, 135° and 180° were also studied. It was found that the pincushion structure films revealed a water adhesion ability. Furthermore, the water droplet impact behavior on these films was investigated. The morphology variations of water droplets, spreading diameter of the droplets, energy conversion, restitution coefficient and adhesion force were examined. Finally, the schematic illustration of water droplets under the static and dynamic states in contact with the pincushion and porous structure surfaces was proposed. It is critical to materialize various applications such as microdroplet transportation, soil erosion, spray painting, anti-icing surface and antifouling agents for textiles.

Hexagonal Arrays of Cylindrical Nickel Microstructures for Improved Oxygen Evolution Reaction

Fuel-cell systems are of interest for a wide range of applications, in part for their utility in power generation from nonfossil-fuel sources. However, the generation of these alternative fuels, through electrochemical means, is a relatively inefficient process due to gas passivation of the electrode surfaces. Uniform microstructured nickel surfaces were prepared by photolithographic techniques as a systematic approach to correlating surface morphologies to their performance in the electrochemically driven oxygen evolution reaction (OER) in alkaline media. Hexagonal arrays of microstructured Ni cylinders were prepared with features of proportional dimensions to the oxygen bubbles generated during the OER process. Recessed and pillared features were investigated relative to planar Ni electrodes for their influence on OER performance and, potentially, bubble release. The arrays of cylindrical recesses were found to exhibit an enhanced OER efficiency relative to planar nickel electrodes. These microstructured electrodes had twice the current density of the planar electrodes at an overpotential of 100 mV. The results of these studies have important implications to guide the preparation of more-efficient fuel generation by water electrolysis and related processes.

A robust duplex Cu/PDMS-coated mesh with superhydrophobic surface for applications in cleaning of spilled oil

In this study, we created a robust nanostructure coated on the surface of a mesh via the electrodeposition and chemical modification to enhance the mechanical durability of the mesh surface with superhydrophobic performance.

Why re-entrant surface topography is needed for robust oleophobicity

Surface patterns affect wetting properties of solid materials allowing manipulation of the phase state of an adjacent fluid. The best known example of this effect is the superhydrophobic composite (Cassie-Baxter) interface with vapour/air pockets between the solid and liquid. Mathematically, the effect of surface micropatterns can be studied by an averaging technique similarly to the method of separation of motions in dynamics. However, averaged parameters are insufficient for robust superhydrophobic and superoleophobic surfaces because additional topography features are important: hierarchical organization and re-entrant roughness. The latter is crucial for the oleophobicity because it enhances the stability of a composite interface. The re-entrant topography can be achieved by various methods. Understanding the role of re-entrant surface topography gives us new insights on the multitude of wetting scenarios beyond the standard Wenzel and Cassie-Baxter models.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'.

Superhydrophobic surfaces fabricated by femtosecond laser with tunable water adhesion: from lotus leaf to rose petal

Superhydrophobic surfaces with tunable water adhesion have attracted much interest in fundamental research and practical applications. In this paper, we used a simple method to fabricate superhydrophobic surfaces with tunable water adhesion. Periodic microstructures with different topographies were fabricated on copper surface via femtosecond (fs) laser irradiation. The topography of these microstructures can be controlled by simply changing the scanning speed of the laser beam. After surface chemical modification, these as-prepared surfaces showed superhydrophobicity combined with different adhesion to water. Surfaces with deep microstructures showed self-cleaning properties with extremely low water adhesion, and the water adhesion increased when the surface microstructures became flat. The changes in surface water adhesion are attributed to the transition from Cassie state to Wenzel state. We also demonstrated that these superhydrophobic surfaces with different adhesion can be used for transferring small water droplets without any loss. We demonstrate that our approach provides a novel but simple way to tune the surface adhesion of superhydrophobic metallic surfaces for good potential applications in related areas.

Superhydrophobic Ti6Al4V surfaces with regular array patterns for anti-icing applications

We present a route to fabricate a robust anti-icing superhydrophobic surface containing the hierarchical structures of microscale array patterns (built by micromachining) and nanohairs (prepared via hydrothermal growth) on a Ti₆Al₄V substrate.

Sticky superhydrophobic hard nanofibers from soft matter

Superhydrophobic soft and hard nanofibers with various water adhesions are obtained by electrodeposition of poly(3,4-propylenedioxythiophene) (ProDOT) derivatives containing two branched alkyl chains. In the case of the hard nanofibers, the fibers are vertically aligned to the substrate and their characteristics can be easily controlled but always with high water adhesion.