High Apparent Contact Angle Research Articles

Cooperative construction of oil/water separator using renewable lignin and PDMS

Super-hydrophobic materials have played a significant role in the oil/water separation field. The traditional super-hydrophobic materials are prepared by fluorine-containing materials. However, although fluorine-containing materials have lower surface energy, they are expensive and difficult to degrade. Therefore, it's a great challenge for the designed and prepared the super-hydrophobic porous materials with super-oleophilicity to explore an environmentally friendly, easy-to-operate, low-cost, and renewable separator for oil/water mixtures separation. In this work, taking advantage of the microscale of renewable lignin and the low surface energy of polydimethylsiloxane （PDMS）, the porous super-hydrophobic sponge material with super-oleophilicity was prepared by the synergy of polydimethylsiloxane (PDMS) and lignin. After soaking and curing, the porous super-hydrophobic sponge showed a high apparent water contact angle which is up to 160°. The designed porous sponge materials can effectively absorb heavy oil in oil-water mixtures separation. Moreover, the porous sponge materials possess temperature stability, acid and alkali stability, and are easy easily recyclable. As high-efficiency oil-water separation material prepared by the synergy between renewable natural polymers and fluorine-free materials, this technique is not only conducive to remediate environmental problems but also helpful to explore the potentiality of lignin that seemingly bio-waste. At the same time, valuable oil can also be recycled.

A bioinspired approach to fabricate fluorescent nanotubes with strong water adhesion by soft template electropolymerization and post-grafting

HypothesisIn this original work, we aim to control both the surface wetting and fluorescence properties of extremely ordered and porous conducting polymer nanotubes prepared by soft template electropolymerization and post-grafting. For reaching this aim, various substituents of different hydrophobicity and fluorescence were post-grafted and the post-grafting yields were evaluated by surface analyses. We show that the used polymer is already fluorescent before post-grafting while the post-grafting yield and as a consequence the surface hydrophobicity highly depend on the substituent. ExperimentsHere, we have chosen to chemically grafting various fluorinated and aromatic substituents using a post-grafting in order to keep the same surface topography. Flat conducting polymer surfaces with similar properties have been also prepared for determining the surface energy with the Owens-Wendt equation and estimating the post-grafting yield by X-ray Photoemission Spectroscopy (XPS) and Time of Flight Secondary Emission Spectrometry (ToF-SIMS). For example, using fluorinated chains of various length (C4F9, C6F13 and C8F17), it is demonstrated that the surface hydrophobicity and oleophobicity do not increase with the fluorinated chain length due to the different post-grafting yields and because of the presence of nanoroughness after post-grafting. FindingsThese surfaces have high apparent water contact angle up to 130.5° but also strong water adhesion, comparable to rose petal effect even if there are no nanotubes on petal surface. XPS and ToF-SIMS analyses provided a detailed characterisation of the surface chemistry with a qualitative classification of the grafted surfaces (F6 > F4 > F8). SEM analysis shows that grafting does not alter the surface morphology. Finally, fluorescence analyses show that the polymer surfaces before post-treatment are already nicely fluorescent. Although the main goal of this paper was and is to understand the role of surface chemistry in tailoring the wetting properties of these surfaces rather than provide specific application examples, we believe that the obtained results can help the development of specific nanostructured materials for potential applications in liquid transport, or in stimuli responsive antimicrobial surfaces.

Spontaneous Adsorption-Induced Salvinia-like Micropillars with High Adhesion.

Superhydrophobic surfaces with high adhesion provide high potential for underwater applications. Inspired by Salvinia leaf, here, we have reported a simple method for fabricating adhesive Salvinia-like micropillars via photolithography and spontaneous adsorption of organic molecules from the atmosphere. With continuous hydrocarbon adsorption on sputtered cerium dioxide (CeO2) films, the surface gradually evolved and eventually became chemically heterogeneous. Huge wetting contrast from superhydrophilic to superhydrophobic over exposure time was observed; meanwhile, the wetting mode changed from the Wenzel (W) state to Cassie-Baxter (C-B) state. As a result, hydrophobic hydrocarbons (C-C/C-H) and trapped air between adjacent pillars contributed to the high apparent contact angle (CA), while the hydrophilic domains of C-O/O═C-O and CeO2 on the top layer made the surface highly adhesive with water droplets. In comparison with traditional fluorinated superhydrophobic surfaces, CeO2-coated surfaces showed high adhesive force with water droplets and can be used as a "mechanical hand" for water droplet transport. The adsorption-induced Salvinia-like micropillars with high adhesion may find many other droplet-based applications in microfluidic fields.

Robust icephobic coating based on the spiky fluorinated Al2O3 particles

Omniphobic and icephobic twin-scale surfaces based on the “urchin”-like fluorinated Al2O3 particles are presented. Combined effect of hierarchical topography and fluorination supplied to the surfaces omniphobic and icephobic properties. The study of the stability of the Cassie wetting state is reported. High apparent contact angles were accompanied with the low contact angle hysteresis and high stability of the Cassie air trapping wetting state. Time delay of the ice crystallization as high as 88pm 5 min was established when compared to the ice formation on flat aluminum and non-fluorinated “urchin”-like surfaces. Crystallized water droplets formed on the reported nano-structured surfaces were easily blown out by the air jet with the velocity of v=3.0pm 1.0 m/s, (which is markedly lower than that common for exploitation of aircrafts and turbines). Heated “urchin”-like surfaces completely restored their omniphobic and icephobic surfaces after thawing. Qualitative analysis of water freezing is supplied.

Macroporous Superhydrophobic Coatings with Switchable Wettability Enabled by Smart Shape Memory Polymers

AbstractHere, a facile bottom‐up nanofabrication technology for making smart superhydrophobic coatings with switchable wettability and tunable optical properties by integrating shape memory polymers (SMPs) with templated macroporous photonic crystals is reported. Thermoresponsive shape memory efforts exhibited by a polyurethane‐based shape memory copolymer enable reversible microstructural transitions between a “memorized” permanent configuration composing of highly ordered arrays of macropores and a temporary deformed structure with collapsed macropores. The unique macroporous SMP photonic crystal arrays with large surface roughness can entrap a large portion of air in the interconnecting macropores, resulting in superior superhydrophobic properties including high apparent water contact angle (CA > 160°) and low CA hysteresis. Importantly, the shape memory‐enabled microstructural transitions lead to drastic wettability switching from superhydrophobic (CA > 160°) to hydrophobic (CA ≈ 110°) or from oleophobic (CA ≈139° for hexadecane) to oleophilic (CA ≈ 80°). The large CA tunability (>50°) coupled with the simultaneous color change from shining iridescence to colorless associated with the same microstructural transition can not only provide a noninvasive means for visually indicating the surface wetting states, this favorable coupling between the wetting and optical behaviors of macroporous SMP photonic crystal membranes can also pave the way for developing new tunable nanooptical devices.

Controlling water adhesion on superhydrophobic surfaces with bi-functional polymers

With an aim to control the surface hydrophobicity and water adhesion, as observed on various natural surfaces, novel 3,4-propylenedioxythiophene (ProDOT) monomers having one and two 3-trimethylsilylpropyl (TMS) groups were synthesized and subjected to electropolymerization to form surface coatings. The monomer ProDOT is employed owing to its tendency to form fibrous structures by electropolymerization [T. Darmanin, F. Guittard, Mater. Chem. Phys. 146 (2014) 6–11], whereas the TMS groups generate very low surface energies comparable to short chain fluorocarbons [N. M. Kovalchuk, et al., Colloids Surfaces A 604 (2020) 125277.]. It is shown that even though these two types of monomer lead to fibrous structures, the dimensions of the fibers as well as the wetting properties are different. The monomer with only a single TMS group (ProDOTSiMe3) generates extremely long nanofibers with only low surface roughness. The resulting surfaces have extremely high apparent contact angles (θw) up to 141.7° and strong water adhesion, similar to rose petals or gecko feet. On the other hand, the analogue with two TMS groups (ProDOT(SiMe3)2) forms short nanofibers but with extremely high surface roughness. The resulting surfaces are superhydrophobic with θw > 160° and ultra-low water adhesion (hysteresis and sliding angles < 1°), similar to lotus leaves. These results point to interesting applications offering control over water adhesion whilst maintaining high hydrophobicity.

The concept, deposition routes, and applications of superhydrophobic surfaces – Review

The superhydrophobic surface (SHS) is the surface with a high apparent contact angle (> 150 °), low hysteresis in the contact angle (< 10 °), and low sliding angle. Owing to many applications of SHS (such as water-resistant surfaces, antifogging surfaces, anti-icing surfaces, anticorrosion surfaces, etc.), the artificial SHS could be prepared. This review article focuses on the deposition, behavior and application of the SHS. It includes an introduction, fundamental principles of superhydrophobicity, main factors for SHS fabrications, deposition methods used to prepare the SHS, and factors affecting the electrochemical synthesis of electroconducting polymers. Moreover, the mechanism of the electrochemical synthesis of polymers, applications of SHS in different fields, and the stability of SHS have been presented. Finally, the challenges of SHS have been discussed.

Durable superamphiphobic coatings with high static and dynamic repellency towards liquids with low surface tension and high viscosity

Superamphiphobic coatings have attracted great attention in academia and industry. However, the fabrication of durable superamphiphobic coatings with high repellency towards liquids with low surface tension and high viscosity is still very challenging. Here, we report a simple method for the fabrication of durable superamphiphobic coatings with high static and dynamic repellency to such liquids. First, a homogeneous suspension was fabricated by hydrolytic condensation of silanes in the presence of the chain-like aggregates of SiO2 nanoparticles. Subsequently, the superamphiphobic coatings were fabricated by spray-coating the polyurethane adhesive onto the substrates followed by spray-coating the as-prepared suspension onto the polyurethane adhesive layer. The coatings show high repellency to liquids with low surface tension and high viscosity, e.g., high apparent contact angles, low sliding angles, high impact resistance, low horizontal deformation and fast rolling velocity. Furthermore, the coatings show excellent mechanical durability against intensive sandpaper abrasion (60 m of abrasion under 10.5 kPa), water impact (60 min under 100 kPa) and tape peeling (200 cycles). The coatings also exhibit good aging stability. Based on these merits, the superamphiphobic coatings may have great potentials in various applications, e.g., handling with natural and synthetic polymer solutions.

A bioinspired strategy for poly(3,4-ethylenedioxypyrrole) films with strong water adhesion

Abstract Using a bioinspired approach, we prepare poly(3,4-ethylenedioxypyrrole) (PEDOP) films with parahydrophobic properties, characterized by high apparent water contact angle and strong water adhesion. The films are made by electropolymerization and the influence of substitution by an alkyl chain of various length (from C4H9 to C14H29) on the 3,4-ethylenedioxy-bridge is reported. More precisely, the best properties are obtained from a length of C12H25 due to the formation of spherical nanoparticles.

Droplet Impact on the Super-Hydrophobic Surface with Micro-Pillar Arrays Fabricated by Hybrid Laser Ablation and Silanization Process.

A super-hydrophobic aluminum alloy surface with decorated pillar arrays was obtained by hybrid laser ablation and further silanization process. The as-prepared surface showed a high apparent contact angle of 158.2 ± 2.0° and low sliding angle of 3 ± 1°. Surface morphologies and surface chemistry were explored to obtain insights into the generation process of super-hydrophobicity. The main objective of this current work is to investigate the maximum spreading factor of water droplets impacting on the pillar-patterned super-hydrophobic surface based on the energy conservation concept. Although many previous studies have investigated the droplet impacting behavior on flat solid surfaces, the empirical models were proposed based on a few parameters including the Reynolds number (Re), Weber number (We), as well as the Ohnesorge number (Oh). This resulted in limitations for the super-hydrophobic surfaces due to the ignorance of the geometrical parameters of the pillars and viscous energy dissipation for liquid flow within the pillar arrays. In this paper, the maximum spreading factor was deduced from the perspective of energy balance, and the predicted results were in good agreement with our experimental results with a mean error of 4.99% and standard deviation of 0.10.

Replicating and resolving wetting and adhesion characteristics of a Rose petal

A Rose petal is well known for a unique form of hydrophobicity, exhibiting simultaneously high apparent equilibrium contact angle (θ*) as well as high adhesion that hinders the rolling off of droplets. This behaviour is fundamentally different than that commonly encountered in other bio-mimetic superhydrophobic surfaces (like a lotus leaf) that have air entrapped in the solid-liquid contact zone (Cassie Baxter wetting state), thereby exhibiting low adhesion. Based on laser scanning confocal microscopy as well as underwater in-situ atomic force microscopy, we obtain high resolution image of the water-substrate contact region (∼0.9–20 μm) which is significantly higher than that reported so far in the literature. We clearly demonstrate that this unique "Petal effect" can indeed be attributed to the Cassie impregnating wetting state, thereby resolving the prevailing ambiguity on this topic. We have also replicated the structure of the actual rose petals using soft lithography on cross-linked polydimethylsiloxane (PDMS, Sylgard 184) and show that both θ* as well as the adhesive properties of the replicated surface to be nearly identical to that of an actual petal. This implies that the force of adhesion depends on the wetting state and the area of contact. Incidentally, the negative replica of the petal, which is obtained as an intermediate during the replication process, exhibits slightly higher adhesion and identical θ*, as compared to the actual rose petal and its positive replica. However, the prevalent wetting state on the negative replica turns out to be Cassie like due to the presence of entrapped air. Both the negative and the positive replica of the Rose petal can potentially be used as biomimetically fabricated sticky hydrophobic surface.

Droplet adhesion to hydrophobic fibrous surfaces

A water droplet can exhibit a high apparent contact angle on a hydrophobic fibrous surface. However, a high contact angle does not guarantee droplet mobility on the surface. The reasons behind droplet adhesion to a hydrophobic fibrous surface has not yet been analyzed or formulated. In this work, the force required to detach a droplet from a hydrophobic fibrous surface is investigated experimentally and computationally. Electrospun Polystyrene mats are considered for this study as they exhibit high contact angles coupled with poor droplet mobility. To better isolate the effects of microstructural properties of the mats and study their effects on droplet detachment, randomness of the fiber orientation is minimized by producing highly oriented fibers in orthogonal layers. As the earth gravity is not strong enough to detach small droplets from such surfaces, aqueous ferrofluid droplets are used in a controllable magnetic field to enhance the effect of gravity. The detachment process is recorded via a high-speed camera and the images are used to detect the moment of detachment and to analyze droplet shape before and during detachment. Numerical simulations are also conducted to provide additional insight into the physics of droplet detachment, and more importantly, to develop an equation for estimating droplet detachment force from a fibrous surface. In this paper, we discuss the effects of fiber properties, e.g., Young-Laplace contact angle or fiber spacing, on the force needed to detach a droplet from a fibrous surface.

An Environmentally Friendly Approach for the Fabrication of Conductive Superhydrophobic Coatings with Sandwich-Like Structures.

A large amount of research has been devoted to developing novel superhydrophobic coatings. However, it is still a great challenge to pursuean environmentally friendly method that leads to superhydrophobic coatings. Herein, we demonstrate for the first time, an environmentally friendly method for the preparation of conductive superhydrophobic coatings with sandwich-like structures by using aminoethylaminopropyl polydimethylsiloxane modified waterborne polyurethane (SiWPU) and N-octadecylamine functionalized multi-wall carbon nanotubes. These environmentally friendly coatings with the sheet resistance of 1.1 ± 0.1 kΩ/sq exhibit a high apparent contact angle of 158.1° ± 2° and a low sliding angle below 1°. The influence of the surface texture before and after heat treatment on the wetting properties is discussed. In addition, the coatings can be electrically heated by 3~113 °C with a voltage of 12~72 V, and thus, can be used for deicing. Furthermore, the resulting coatings demonstrate good performance of wear resistance and ultraviolet resistance, which will have broad application potential in harsh environments.

Superhydrophobic surfaces: a review on fundamentals, applications, and challenges

Superhydrophobicity is the tendency of a surface to repel water drops. A surface is qualified as a superhydrophobic surface only if the surface possesses a high apparent contact angle (>150°), low contact angle hysteresis (<10°), low sliding angle (<5°) and high stability of Cassie model state. Efforts have been made to mimic the superhydrophobicity found in nature (for example, lotus leaf), so that artificial superhydrophobic surfaces could be prepared for a variety of applications. Due to their versatile use in many applications, such as water-resistant surfaces, antifogging surfaces, anti-icing surfaces, anticorrosion surfaces etc., many methods have been developed to fabricate them. In this article, the fundamental principles of superhydrophobicity, some of the recent works in the preparation of superhydrophobic surfaces, their potential applications, and the challenges confronted in their new applications are reviewed and discussed.

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.

Wettability and Contact Time on a Biomimetic Superhydrophobic Surface.

Inspired by the array microstructure of natural superhydrophobic surfaces (lotus leaf and cicada wing), an array microstructure was successfully constructed by high speed wire electrical discharge machining (HS-WEDM) on the surfaces of a 7075 aluminum alloy without any chemical treatment. The artificial surfaces had a high apparent contact angle of 153° ± 1° with a contact angle hysteresis less than 5° and showed a good superhydrophobic property. Wettability, contact time, and the corresponding superhydrophobic mechanism of artificial superhydrophobic surface were investigated. The results indicated that the micro-scale array microstructure was an important factor for the superhydrophobic surface, while different array microstructures exhibited different effects on the wettability and contact time of the artificial superhydrophobic surface. The length (L), interval (S), and height (H) of the array microstructure are the main influential factors on the wettability and contact time. The order of importance of these factors is H > S > L for increasing the apparent contact angle and reducing the contact time. The method, using HS-WEDM to fabricate superhydrophobic surface, is simple, low-cost, and environmentally friendly and can easily control the wettability and contact time on the artificial surfaces by changing the array microstructure.

From Initial Nucleation to Cassie-Baxter State of Condensed Droplets on Nanotextured Superhydrophobic Surfaces

Understanding how droplet condensation happens plays an essential role for our fundamental insights of wetting behaviors in nature and numerous applications. Since there is a lack of study of the initial formation and growing processes of condensed droplets down to nano-/submicroscale, relevant underlying mechanisms remain to be explored. We report an in situ observation of vapor condensation on nano-/microtextured superhydrophobic surfaces using optical microscopy. An interesting picture of the vapor condensation, from the initial appearance of individual small droplets (≤1 μm) to a Cassie-Baxter wetting state (>30 μm), are exhibited. It is found that individual droplets preferentially nucleate at the top and the edge of single micropillars with very high apparent contact angles on the nanotextures. Scenarios of two distinguished growing modes are reported statistically and the underlying mechanisms are discussed in the view of thermodynamics. We particularly reveal that the formation of the Cassie-Baxter wetting state is a result of a continuous coalescence of individual small droplets, in which the nanotexture-enhanced superhydrophobicity plays a crucial role. We envision that these fundamental findings can deepen our understanding of the nucleation and development of condensed droplets in nanoscale, so as to optimize design strategies of superhydrophobic materials for a broad range of water-harvesting and heat-transfer systems.

Bioinspired Rose-Petal-Like Substrates Generated by Electropolymerization on Micropatterned Gold Substrates.

Invited for this month's cover are the collaborating groups of Dr. Thierry Darmanin at Université Côte d'Azur, France and Dr. François Rossi at JRC European Commission, Italy. The cover picture shows a novel strategy for preparing substrates having a rose-petal effect (high water adhesion). The micropatterning specifications (pillar diameter and pitch) and the electropolymerization parameters are key to obtaining both high water apparent contact angles and a high hysteresis. Read the full text of the article at 10.1002/cplu.201600387.

Controllable wetting state high adhesion hydrophobic surface

High adhesion hydrophobic surfaces with high apparent contact angles were achieved by a convenient approach.

Influence of the monomer structure and electrochemical parameters on the formation of nanotubes with parahydrophobic properties (high water adhesion) by a templateless electropolymerization process

Controlling the formation of surface nanostructures and nanotubes in particular is extremely important for various applications in electronic devices for energy systems, biosensing but also for the control of water adhesion. Here, we use a direct (without template) electropolymerization process to produce vertically aligned nanotubes. Different monomers are tested as well as different solvents, electrolytes and electrodeposition methods. We show that naphtho[2,3-b]thieno[3,4-e][1,4]dioxine (NaphDOT) is the best monomer to obtain these nanotubes while dichloromethane has to be used as solvent for their formation. The surfaces with nanotubes display both extremely high apparent contact angles (θw=142.7°) and high water adhesion even if the nanotubes are made of intrinsically hydrophilic polymers and are not densely packed.