Characterization Of Superhydrophobic Surfaces Research Articles

Nanotechnology to Improve the Performances of Hydrodynamic Surfaces

Nature continues to inspire scientists to adapt solutions in order to satisfy human needs, mainly in the maritime domain with metallic surface corrosion and its mechanical friction. In this research, the source of innovation comes from the lotus leaf and its well-known super-hydrophobicity. In this study, we have investigated the lotus leaf as a model for a super-hydrophobic maritime surface. The hydrothermal technique, which is considered to be a simple, low-cost, and scalable coating method, is applied to create zinc oxide (ZnO) nanorods (NRs), and an evaporation method is used to apply octadecyltrimethoxysilane (ODS). We apply such eco-green coatings onto commercial epoxy paints. Superhydrophobic surfaces (SHS) are obtained on maritime aluminum substrates. The characterization of SHS indicates improved behavior of water droplets on the treated surface: higher water static contact angles (WCA) from 98° to more than 152° and reduced sliding angles (SA) from 46° to 7°. Sliding speeds (SS) have been largely raised from 54 in the epoxy case to 1300 mm·s−1 after treatment. These results clearly demonstrate the real opportunity to apply ZnO-based nanomaterials onto existing commercial maritime coatings.

Preparation and characterization of superhydrophobic surface based on polydimethylsiloxane (PDMS)

Biomimetic superhydrophobic surfaces exhibit excellent self-cleaning properties due to their special micro/nano-scale binary structures. In order to prepare the superhydrophobic surface of the polydimethylsiloxane (PDMS), a facile fabrication method for replicating micro/nano-scale binary aluminium structures into PDMS is presented. The microscopic morphology, composition, surface roughness (Ra) and wettability of the sample surface were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, roughness measurement equipment and contact angle meter respectively. Based on the measurements of the contact angles of deionized water (DI water) and ethanediol, surface free energies of the coatings were estimated according to the Owens two-liquid method. The superhydrophobic PDMS exhibited lower surface free energy than flat PDMS with a DI water contact angle (WCA) of 165°. The surface roughness (Ra) increased with the increasing of etching time in the range 0–80 min, and then decreased with the change of etching time, similar to the variation of contact angle with etching time. Moreover, the prepared surface had different micro-morphologies and its wettability was changed by regulating the chemical etching time. In addition, the superhydrophobic PDMS also showed good self-cleaning properties and the bouncing effect of the water droplets.

Fabrication and characterization of superhydrophobic surfaces on aluminum alloy substrates

Superhydrophobic surfaces have potential anti-icing applications in industries and daily life. In the present study, we combine the methods of chemical etching and surface modification with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) which has very low surface energy to simplify the fabrication procedures for superhydrophobic surfaces on aluminum alloy substrates. The results show that the contact angle (CA), rolling angle (RA) and contact angle hysteresis (CAH) of superhydrophobic surfaces etched with 8.0wt% HCl aqueous solutions are 162.5°, 1.9° and 1.1°, respectively; the apparent surface free energies (ASFEs) of superhydrophobic surfaces increase with the decrease in surface temperature; the freezing time of water droplets on superhydrophobic surfaces is retarded by 1568s, and the temperature drops to as low as −11.9̊C. The results indicate that superhydrophobic surfaces exhibit excellent anti-icing properties.

Reliable Measurement of the Receding Contact Angle

Surface wettability is usually evaluated by the contact angle between the perimeter of a water drop and the surface. However, this single measurement is not enough for proper characterization, and the so-called advancing and receding contact angles also need to be measured. Measuring the receding contact angle can be challenging, especially for extremely hydrophobic surfaces. We demonstrate a reliable procedure by using the common needle-in-the-sessile-drop method. Generally, the contact line movement needs to be followed, and true receding movement has to be distinguished from "pseudo-movement" occurring before the receding angle is reached. Depending on the contact angle hysteresis, the initial size of the drop may need to be surprisingly large to achieve a reliable result. Although our motivation for this work was the characterization of superhydrophobic surfaces, we also show that this method works universally ranging from hydrophilic to superhydrophobic surfaces.

How superhydrophobicity breaks down.

A droplet deposited or impacting on a superhydrophobic surface rolls off easily, leaving the surface dry and clean. This remarkable property is due to a surface structure that favors the entrainment of air cushions beneath the drop, leading to the so-called Cassie state. The Cassie state competes with the Wenzel (impaled) state, in which the liquid fully wets the substrate. To use superhydrophobicity, impalement of the drop into the surface structure needs to be prevented. To understand the underlying processes, we image the impalement dynamics in three dimensions by confocal microscopy. While the drop evaporates from a pillar array, its rim recedes via stepwise depinning from the edge of the pillars. Before depinning, finger-like necks form due to adhesion of the drop at the pillar's circumference. Once the pressure becomes too high, or the drop too small, the drop slowly impales the texture. The thickness of the air cushion decreases gradually. As soon as the water-air interface touches the substrate, complete wetting proceeds within milliseconds. This visualization of the impalement dynamics will facilitate the development and characterization of superhydrophobic surfaces.

Wetting Transitions on Hierarchical Surfaces

The current study reports the fabrication and characterization of superhydrophobic surfaces with increasing nanoroughness by decreasing silica nanoparticle size in a sol–gel matrix. Using small-angle X-ray scattering (SAXS) measurements allowed for the direct quantification of air entrapped at the interface, revealing for the first time that significant air remains on hierarchical surfaces despite observed droplet pinning through hysteresis measurements. Combining contact angle hysteresis and SAXS measurements of the surfaces immersed in sodium dodecylsulfate (SDS) solutions with Cassie and Tadmor’s model, a series of predicted contact angles were generated, comparing wetting transition mechanisms based on wetting line advance, droplet adhesion/pinning, and interfacial air entrapment. The study provided confirmation of key theoretical assumptions on wetting of hierarchical surfaces: (i) Cassie wetting of the nanofeatures is the preferred wetting progression on hierarchical surfaces; and (ii) the presence of an intermediate petal state is dependent on the level of nanoroughness as compared to the microroughness.

Fabrication and characterization of superhydrophobic surface by using water vapor impingement method

In this research, superhydrophobic coating with average static contact angle of 166° has been successfully prepared by using a new surface roughening method. Discussion on the applicability of Cassie and Wenzel equations used to determine surface parameters from experimental data is being included as well. This new surface roughening method uses accelerated water vapor to impinge wet film forming crater-like structure surface; analogous to meteor striking earth forming crater. Wet film is comprised of aluminium tri-sec butoxide, propan-2-ol, and ethyl acetoacetate according to molar ratio of 1:10:1. Craters with diameter distribution of 1–20μm were formed on the roughened surface. It was then subjected to 10min of immersion in boiling water, 20min of immersion in fluoroalkylsilane solution, and 10min of 100°C hot air treatment producing superhydrophobic coating. This superhydrophobic crater-like structure surface has approximately 8% solid-to-liquid area fraction and high roughness value of 6.4. These values were obtained from both Cassie and Wenzel equations but with some minor modification in order to fit them into experimental data. Average sliding angle of 2.6° and hysteresis value of 10° were recorded.

Fabrication and characterization of superhydrophobic surface by electroplating regular rough micro-structures of metal nickel

The superhydrophobic behavior has attracted many researchers' attention. The conventional method of obtaining superhydrophobicity involves the combination of the coating of low-surface-energy material and constructing roughness on smooth surface. According to Cassie-Baxter law, this paper proposes a method that the superhydrophobicity could be obtained on structured hydrophilic surface without any coating of low-surface-energy material. Based on the surface micromachining technology, the superhydrophobic behavior has been achieved by the fabrication of micro-nickel cylinder array. It has been proved by the experiments that the micro-cylinder arrays exhibit superhydrophobic behavior; the maximum contact angle (CA) between the array and the water droplet can be up to 155^o, while the intrinsic CA of smooth nickel surface is 82^o. By controlling the parameters of cylinders (diameter/height/spacing), we investigated the relationship between the hydrophobicity and the above parameters. It proved that the spacing height ratio contributed to the transition from Cassie's model to Wenzel's, which was the main cause of disappearance of superhydrophobicity.

Superhydrophobic poly(L-lactic acid) surface as potential bacterial colonization substrate

Hydrophobicity is a very important surface property and there is a growing interest in the production and characterization of superhydrophobic surfaces. Accordingly, it was recently shown how to obtain a superhydrophobic surface using a simple and cost-effective method on a polymer named poly(L-lactic acid) (PLLA). To evaluate the ability of such material as a substrate for bacterial colonization, this work assessed the capability of different bacteria to colonize a biomimetic rough superhydrophobic (SH) PLLA surface and also a smooth hydrophobic (H) one. The interaction between these surfaces and bacteria with different morphologies and cell walls was studied using one strain of Staphylococcus aureus and one of Pseudomonas aeruginosa. Results showed that both bacterial strains colonized the surfaces tested, although significantly higher numbers of S. aureus cells were found on SH surfaces comparing to H ones. Moreover, scanning electron microscopy images showed an extracellular matrix produced by P. aeruginosa on SH PLLA surfaces, indicating that this bacterium is able to form a biofilm on such substratum. Bacterial removal through lotus leaf effect was also tested, being more efficient on H coupons than on SH PLLA ones. Overall, the results showed that SH PLLA surfaces can be used as a substrate for bacterial colonization and, thus, have an exceptional potential for biotechnology applications.

Fabrication and Characterization of Superhydrophobic Surfaces with Dynamic Stability

AbstractSuperhydrophobic surfaces of dynamic stability are crucial for applications in water‐repellent materials. In this work, a hierarchical structure composed of a dendritic microporous surface with nanostructured porosity is demonstrated that shows robust superhydrophobicity with dynamic stability. The hierarchical structures are obtained on both copper foils and wires by a dynamic gas‐bubble template‐assisted electrochemical deposition method. The substrates can then be modified with alkyl thiols to obtain the surface superhydrophobicity. A new kind of testing, mechanical monitor‐assisted continuous water surface strokes, is developed to reveal the dynamic stability of the as‐prepared superhydrophobic copper wires. The as‐prepared superhydrophobic copper wires can exert a high propulsive force, and particularly, show little adhesive force in the process of continuous strokes on the water surface, exhibiting robust superhydrophobicity with dynamic stability. The approach allows a strategy for the fabrication of superhydrophobic surfaces with dynamic stability, and suggests a new method to evaluate the dynamic stability of superhydrophobic surfaces.

Some thoughts on superhydrophobic wetting

Surface roughness has a profound influence on the wetting properties of a material. This fact is especially true with respect to the wetting of superhydrophobic surfaces. As a result of a special surface structure, a drop of water brought into contact with such a material forms an almost perfect sphere and even a very slight tilting of the superhydrophobic object is sufficient to cause the drop to roll off. For the description of the behavior of drops on rough surfaces two theories, namely those of Cassie and Wenzel, are often employed. However, it is currently becoming well established that both models explain the wetting behavior more from a qualitative or practical point of view rather than giving a quantitative description. For a prediction of actual contact angles, a more complex description is required that, next to thermodynamics, takes into account the kinetics of wetting. In this article, we focus on a few key aspects of superhydrophobic wetting, namely the characterization of superhydrophobic surfaces, models for the movement of drops, transitions between the Cassie and Wenzel states, and the behavior of superhydrophobic materials under condensation.