Superhydrophobic Surface-Assisted Preparation of Microspheres and Supraparticles and Their Applications

Superhydrophobic surface(SHS) has been well developed, as SHS renders the property of minimizing the water/solid contact interface. Water droplets deposited onto SHS with contact angles exceeding 150°, allow them to retain spherical shapes, and the low adhesion of SHS facilitates easy droplet collection when tilting the substrate. These characteristics make SHS suitable for a wide range of applications. One particularly promising application is the fabrication of microsphere and supraparticle materials. SHS offers a distinct advantage as a universal platform capable of providing customized services for a variety of microspheres and supraparticles. In this review, an overview of the strategies for fabricating microspheres and supraparticles with the aid of SHS, including cross-linking process, polymer melting,and droplet template evaporation methods, is first presented. Then, the applications of microspheres and supraparticles formed onto SHS are discussed in detail, for example, fabricating photonic devices with controllable structures and tunable structural colors, acting as catalysts with emerging or synergetic properties, being integrated into the biomedical field to construct the devices with different medicinal purposes, being utilized for inducing protein crystallization and detecting trace amounts of analytes. Finally,the perspective on future developments involved with this research field is given, along with some obstacles and opportunities.

Recent advances in preparation of metallic superhydrophobic surface by chemical etching and its applications

In the past few decades,inspired by the superhydrophobic surfaces(SHPS)of animals and plants such as lotus leaves,rose petals,legs of water striders,and wings of butterflies,preparing metal materials with metallic SHPS(MSHPS)have attracted great research interest,due to the great prospect in practical applications.To obtain SHPS on conventional metal materials,it is necessary to construct rough surface,followed by modification with low surface energy substances.In this paper,the action mechanism and the current research status of MSHPS were reviewed through the following aspects.Firstly,the model of wetting theory was presented,and then the progress in MSHPS preparation through chemical etching method was discussed.Secondly,the applications of MSHPS in self-cleaning,anti-icing,corrosion resistance,drag reduction,oil-water separation,and other aspects were introduced.Finally,the challenges encountered in the present application of MSHPS were summarized,and the future research interests were discussed.

Superhydrophobic surface of Mg alloys:A review

In the present review,the formation of superhydrophobic(SHP)structures on the surface of Mg alloys was investigated.Different methods including hydrothermal technique,chemical and electrochemical deposition,conversion and polymer coating,and etching routes were discussed.The superhydrophobicity could form on the surface of Mg alloys by the application of different chemical,electrochemical,and physical methods followed by the immersion of these alloys in the solution containing modifying agents including fatty acids or long-chain molecules.The formed morphology,composition,and contact angle were reported and the effect of synthesis route on these characteristics was reviewed.

Synthesis of Biomimetic Superhydrophobic Surface through Electrochemical Deposition on Porous Alumina

The superhydrophobicity of plant leaves is a benefit of the hierarchical structures of their surfaces. These structures have been imitated in the creation of synthetic surfaces. In this paper, a novel process for fabrication of biomimetic hierarchical structures by electrochemical deposition of a metal on porous alumina is described. An aluminum specimen was anodically oxidized to obtain a porous alumina template, which was used as an electrode to fabricate a surface with micro structures through electrochemical deposition of a metal such as nickel and copper after the enlargement of pores. Astonishingly, a hier- archical structure with nanometer pillars and micrometer clusters was synthesized in the pores of the template. The nanometer pillars were determined by the nanometer pores. The lbrmation of micrometer clusters was related to the thin walls of the pores and the crystallization of the metal on a flat surface. From the as-prepared biomimetic surfaces, lotus-leaf-like superhydrophobic surfaces with nickel and copper deposition were achieved.

Simple Fabrication of Hierarchical Micro/Nanostructure Superhydrophobic Surface with Stable and Superior Anticorrosion Silicon Steel via Laser Marking Treatment

To improve the weak corrosion resistance of silicon steel to acid solution and alkaline solution with high temperature,a stable hierarchical micro/nanostructure superhydrophobic surface with myriad irregular micro-scale hump and sheet-like nanostructure was successfully prepared on silicon steel by a simple,efficient and facile operation in large-area laser marking treatment.The morphology,composition,wettability of the as-prepared surface were studied.The superhydrophobic performance of the surface was investigated as well.Additionally,the corrosion resistance of the superhydrophobic surface to acidic solutions at room temperature and alkaline solutions at high temperature (80 ℃) was carefully explored.The corrosion resistance mechanism was clarified.Moreover,considering the practical application of the surface in the future,the hardness of the hierarchical micro/nanostructure superhydrophobic surface was studied.The experimental results indicate that the hierarchical micro/nanostructure surface with texture spacing of 100 μm treated at laser scanning speed of 100 mms/ presents superior superhydrophobicity after decreasing surface energy.The contact angle can be as high as 156.6°.Additionally,the superhydrophobic surface provide superior and stable anticorrosive protection for silicon steel in various corrosive environments.More importantly,the prepared structure of the surface shows high hardness,which ensures that the surface of the superhydrophobic surface cannot be destroyed easily.The surface is able to maintain great superhydrophobic performance when it suffers from slight impacting and abrasion.

Application of Superhydrophobic Surface on Boiling Heat Transfer Characteristics of Nanofluids

Boiling heat transfer is a mode using the phase change of working medium to strengthen the heat exchange due to its good heat exchange capability,and it is widely used in heat exchange engineering.Nanofluids have been used in the direction of enhanced heat transfer for their superior thermophysical property.The wetting,spreading and ripple phenomena of superhydrophobic surfaces widely exist in nature and daily life.It has great application value for engineering technology.In this article,the boiling heat exchange characteristics of nanofluids on superhydrophobic surface are numerically studied.It was found that with the increase of superheating degree,the steam volume ratio of unmodified heated surface increases to saturation,while the steam volume and evaporation ratio of modified superhydrophobic surface increase firstly and then decrease.At the same time,bubbles are generated and accumulated more fully on superhydrophobic surface.It was also found that nanofluids with low viscosity are more affected by superhydrophobic surface characteristics,and the increase is more significant with high superheating degree,and the superhydrophobic surface is beneficial to enhancing boiling heat exchange.Compared with the simulation results,it could be concluded that the boiling heat exchange performance of CuO-water nano-fluids on the modified superhydrophobic surface is better than that of CuO-ethylene glycol nanofluids under high superheating degree.

Fabrication of Superhydrophobic Surfaces on Aluminum Alloy Via Electrodeposition of Copper Followed by Electrochemical Modification

Superhydrophobic aluminum surfaces have been prepared by means of electrodeposition of copper on aluminum surfaces, followed by electrochemical modification using stearic acid organic molecules. Scanning electron microscopy(SEM) images show that the electrodeposited copper films follow "island growth mode" in the form of microdots and their number densities increase with the rise of the negative deposition potentials. At an electrodeposition potential of-0.2 V the number density of the copper microdots are found to be 4.5×104cm^(-2)that are increased to 2.9×105cm^(-2)at a potential of-0.8 V. Systematically, the distances between the microdots are found to be reduced from 26.6 μm to 11.03 μm with the increase of negative electrochemical potential from-0.2 V to-0.8 V. X-ray diffraction(XRD) analyses have confirmed the formation of copper stearate on the stearic acid modified copper films. The roughness of the stearic acid modified electrodeposited copper films is found to increase with the increase in the density of the copper microdots. A critical copper deposition potential of-0.6 V in conjunction with the stearic acid modification provides a surface roughness of 6.2 μm with a water contact angle of 157?, resulting in superhydrophobic properties on the aluminum substrates.

Triboelectric‘electrostatic tweezers'for manipulating droplets on lubricated slippery surfaces prepared by femtosecond laser processing

The use of‘Electrostatic tweezers'is a promising tool for droplet manipulation,but it faces many limitations in manipulating droplets on superhydrophobic surfaces.Here,we achieve noncontact and multifunctional droplet manipulation on Nepenthes-inspired lubricated slippery surfaces via triboelectric electrostatic tweezers(TETs).The TET manipulation of droplets on a slippery surface has many advantages over electrostatic droplet manipulation on a superhydrophobic surface.The electrostatic field induces the redistribution of the charges inside the neutral droplet,which causes the triboelectric charged rod to drive the droplet to move forward under the electrostatic force.Positively or negatively charged droplets can also be driven by TET based on electrostatic attraction and repulsion.TET enables us to manipulate droplets under diverse conditions,including anti-gravity climb,suspended droplets,corrosive liquids,low-surface-tension liquids(e.g.ethanol with a surface tension of 22.3 mN·m^(-1)),different droplet volumes(from 100 nl to 0.5 ml),passing through narrow slits,sliding over damaged areas,on various solid substrates,and even droplets in an enclosed system.Various droplet-related applications,such as motion guidance,motion switching,droplet-based microreactions,surface cleaning,surface defogging,liquid sorting,and cell labeling,can be easily achieved with TETs.

An experimental investigation into the icing and melting process of a water droplet impinging onto a superhydrophobic surface

The freezing and melting process of a small water droplet on a superhydrophobic cold surface was investigated using the Laser Induced Fluorescence(LIF)technique.The superhydrophobic surface was prepared using a sol-gel method on a red copper test plate.From the obtained fluorescence images,the phase transition characteristics during the freezing and melting process of a water droplet were clearly observed.It was found that,at the beginning of the droplet freezing process,liquid water turned into ice at a very fast rate.Such phase transition process decreased gradually with time and the volume of frozen ice approached a constant value at the end of the icing process.In addition,the freezing time was found to reduce with the decrease of the test plate temperature.Besides,when the test plate temperature is relatively high,the effect of droplet volume on the freezing time is very significant.Over all,we provide some tentative insights into the microphysical process related to the icing and melting process of water droplets.

One-step Condensation/copolymerization of VTES and DVB for Self-assembly Bionic Superhydrophobic Surface Coating and Study on Oil-water Separation

Mimicry of nature drives the development of bionic materials.Bionic superhydrophobic materials are a kind of high-efficiency materials to handle oil spills and water pollution.However,the stability of surface coatings of the superhydrophobic materials remains a challenge.Herein,a new category of self-assembly bionic superhydrophobic surface coating was prepared via one-step condensation/copolymerization of vinyltriethoxysilane(VTES)and divinylbenzene(DVB),which realized the close combination of covalent bonds between organic(e.g.DVB)and inorganic matter(e.g.VTES),and avoided the swelling of polydivinylbenzene(PDVB)in the process of collection of oil from water.This organic-inorganic hybrid polymer could self-assembly deposit on the surface of sponge even other substrates.For example,P(VTES-DVB)-Si0_(2)/MS obtained by assembling P(VTES-DVB)-Si0_(2)on the surface of Melamine Sponge(MS)exhibited superhydrophobicity with a Water Contact Angle(WCA)of 157.3,the optimal adsorption capacity of 77 g g 136 g g-1 for diverse oils,and an excellent separation efficiency of 99.3%.Besides,the excellent acid and alkali resistance of P(VTES-DVB)-Si0_(2)/MS suggested the potential value in practical oil-water separation.P(VTES-DVB)-Si0_(2)showed the outstanding hydrophobic performance by using as coating on different substrates.This work provided a new idea about the stable combination of organic and inorganic matter in the surface modification.

An easy-to-implement method for fabricating superhydrophobic surfaces inspired by taro leaf

An easy-to-implement method by which to fabricate superhydrophobic surfaces inspired taro leaf was successfully applied on316 L stainless steel via combining nanosecond laser(NL)processing and spin-coating techniques.The laser-textured surface composed of microscale frameworks and central bumps was fabricated by NL processing based on properly designed biomimetic patterns,and a layer of nanoscale carbon black/polydimethylsiloxane(CB/PDMS)particles was covered on it by spin-coating.The effect of pattern parameters(i.e.,the inscribed circle radius of framework and the radius of central bump)on wettability of biomimetic surface was investigated.All as-prepared biomimetic surfaces with micro-nano hierarchical structures showed excellent superhydrophobicity with the water contact angle of~155°and contact angle hysteresis of~2°.By comparing the untreated surface,the wetting behavior and evaporation mode of the biomimetic surface occurred an obvious transformation.Meanwhile,experiments indicated that the biomimetic surface not only had liquid-repelling and self-cleaning functions,but also maintained remarkable mechanical robustness and superhydrophobic durability.The method is efficient for fabricating biomimetic superhydrophobic surfaces applied to liquid-repelling,evaporation-transforming and self-cleaning fields.

ELECTROSPRAYING/ELECTROSPINNING OF POLY(γ-STEARYL-LGLUTAMATE):FORMATION OF SURFACES WITH SUPERHYDROPHOBICITY

Electrospraying/electrospinning of poly(γ-stearyl-L-glutamate) (PSLG) was investigated on a series solutions with different concentrations in chloroform.Field emission scanning electron microscopy (FESEM) and attenuated Iotal reflectance Fourier transform infrared spectroscopy (FT-IR/ATR) were used to characterize the morphology and structure of the electrosprayed/electrospun polypeptide mats.It was found that electrospraying of PSLG with concentrations lower than 16 wt% afforded beads,while microfibers cou...

Electrospinning in membrane contactor:manufacturing Elec-PVDF/SiO2 superhydrophobic surface for efficient flue gas desulphurization applications

In membrane contactors,maintaining a high SO_(2)absorption flux and an excellent wetting resistance are crucial for hazardous gas removal.In this study,we adopted an electrospinning strategy to fabricate highly robust superhydrophobic dual-layer Elec-PVDF/SiO_(2)composite membrane contactors used for flue gas desulfurization.The composite membrane contactor consisted of a durable and ultrathin three-dimensional(3D)superhydrophobic surface and a porous supporting layer,where the formulation was optimized by regulating the PVDF concentration,solvent ratio and SiO_(2)particles content in electrospinning solution.The scanning electronic microscopy(SEM),EDS-mapping,water contact angle(WCA)and surface roughness of as-prepared Elec-PVDF/SiO_(2)composite membrane contactors were conducted to explore the physical and chemical structure.The SiO_(2)nanoparticles were uniformly loaded in ElecPVDF/SiO_(2)composite membrane contactor,and constructed micro-nano dual-coarse lotus-leaf-like morphology,which noticeably elevated surface roughness(Ra).The SiO_(2)nanoparticles also functioned as hydrophobic modifiers,which boosted the WAC up to 155.The SO_(2)absorption fluxes and SO_(2)removal efficiencies were investigated.In particular,the membrane contactor doped with 20 wt%SiO_(2)nanoparticles significantly elevated the stability of desulfurization performance.Besides,the membrane mass transfer coefficient(Km)and corresponding membrane mass transfer resistance(H/Km)were explored.

Flexible Broadband Light Absorbers with a Superhydrophobic Surface Fabricated by Ultraviolet-assisted Nanoimprint Lithography

We present a simple approach to fabricate a kind of composite films with a superhydrophobic and broadband light absorbing surface by ultraviolet-assisted nanoimprinting over a gradiently deposited composite matrix.The wettability and optical property of the resultant surfaces are tunable by the deposition time before polymerization(T_(s))and mold’s topography.Mechanically robust and elastomeric films exhibiting high sunlight absorptivity up to 98.13%and contact angle of their surfaces up to 150°are prepared under optimized conditions,as using a mold with a small pattern size(hexagonal periodic mold with cylinder diameter of ca.37μm)under T_(s)=10 min for imprinting the crosslinked poly[di(ethylene glycol)ethyl ether acrylate]and poly(isobornyl acrylate)in the presence of polypyrrole(PPy)nanoparticles.Such dual functions are found related to the hierarchical architecture of the surface,arising from the synergetic effects of the periodical patterned polymer substrate and spontaneously assembled PPy microstructures on the patterns.The current strategy based on the combination of ultraviolet-assisted nanoimprint lithography and hierarchical assembly of gradiently deposited black nano-fillers offers a new insight into the design of robust superhydrophobic and black surfaces,which is helpful to deepen our understanding of the relationship between liquid/light manipulation and micro/nanostructured surfaces.

Fabrication of Stable Superhydrophobic Cupric Hydroxide Surface with Hierarchical Structure on Copper Substrate

Cupric hydroxide films with a new hierarchical architecture consisting of beautiful nanotubes and nanoflowers were directly fabricated on copper substrate via a solution-immersion process at a constant temperature of 23 ℃. Stable superhydrophobic Cu（OH）2 surface was obtained after Cu（OH）2 films were modified with hydrolyzed 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane （CsH4CI3F13Si, FOTMS）. The surface morphology and composition of the film were studied using scanning electron microscopy （SEM）, X-ray diffraction （XRD）, and X-ray photoelectron spectroscopy （XPS）, respectively. Result shows that the surface of Cu（OH）2 films directly grown on copper substrate was hydrophilic, whereas the modified Cu（OH）2 films exhibited the superhydrophobicity with a water contact angle （CA） of about 160.8°, as well as a small sliding angle （SA） of about 1°. The special hierarchical structure, along with the slow surface energy leads to the high superhydrophobicity of the surface.

Fabrication of the Superhydrophobic Surface on Magnesium Alloy and Its Corrosion Resistance

The superhydrophobic surface was fabricated on the AZ31 alloy by the combination of the hydrotherreal treatment method and post modification with stearic acid. The superhydrophobic surface showed a static water contact angle of 157.6°. The characteristics of the coatings were characterized by scanning electron microscopy （SEM）, X-ray diffraction （XRD） and Fourier transform infrared spectroscopy （FTIR）. The corrosion resistance of the superhydrophobic coatings was investigated by potentiodynamic polarization test and electrochemical impedance spectroscopy （EIS）. The results revealed that the superhydrophobic coatings, characterized by petal-like structure significantly improved the corrosion resistance of the AZ31 ahoy.

Recent Progress in Superhydrophobic Coatings Using Molecular Dynamics Simulations and Experimental Techniques

Superhydrophobic(SH)coatings are intended to resist a surface from cor­rosion and thereby increases the product life duration.It is also a promising solution to save cleaning costs and time by providing self-clean nature to the surface.This review article provides the most recent updates in design­ing SH surfaces and their characterizations adopted both in experimental and computational techniques.To gain a comprehensive perspective,the SH surfaces present in nature those are inspiring human beings to mimic such surfaces are introduced at the beginning of this article.Subsequently,different fabrication techniques undertaken recently to design artificial SH surfaces are briefly discussed.Recent progress in computations employed in the development of SH surfaces is then discussed.Next,the limitations in SH surfaces are addressed.Finally,perceptiveness of different strategies and their limitations are presented in the concluding remarks and outlook.Overall,this mini review article brings together and highlights the significant advancements in fabrication of superhydrophobic surfaces which may surely help the early-stage researchers/scientists to plan their work accordingly.

Modeling Superhydrophobic Contact Angles and Wetting Transition

It is well known that surface roughness has a very important effect on superhydrophobicity.The Wenzel and Cassie-Baxter models,which correspond to the homogeneous and heterogeneous wetting respectively,are currently primary instructions for designing superhydrophobic surfaces.However,the particular drop shape that a drop exhibits might depend on how it is formed. A water drop can occupy multiple equilibrium states,which relate to different local minimal energy.In some cases,both equilibrium states can even co-exist on a same substrate.Thus the apparent contact angles may vary and have different values.We discuss how the Wenzel and Cassie-Baxter equations determine the homogeneous and heterogeneous wetting theoretically. Contact angle analysis on hierarchical surface structure and contact angle hysteresis has been put specific attention.In particular, we study the energy barrier of transition from Cassie-Baxter state to Wenzel state,based on existing achievement by previous researchers,to determine the possibility of the transition and how it can be interpreted.It has been demonstrated that surface roughness and geometry will influence the energy required for a drop to get into equilibrium,no matter it is homogeneous or heterogeneous wetting.

Anti⁃icing/De⁃icing Mechanism and Application Progress of Bio⁃inspired Surface for Aircraft

Icing on the surface of aircraft will not only aggravate its quality and affect flight control,but even cause safety accidents,which is one of the important factors restricting all-weather flight.Bio-inspired anti-icing surfaces have gained great attention recently due to their low-hysteresis,non-stick properties,slow nucleation rate and low ice adhesion strength.These bio-inspired anti-icing surfaces,such as superhydrophobic surfaces,slippery liquid-infused porous surfaces and quasi-liquid film surfaces,have realized excellent anti-icing performance at various stages of icing.However,for harsh environment,there are still many problems and challenges.From the perspective of bioinspiration,the mechanism of icing nucleation,liquid bounce and ice adhesion has been reviewed together with the application progress and bottleneck issues about anti-icing in view of the process of icing.Subsequently,the reliability and development prospect of active,passive and active-passive integrated anti-icing technology are discussed,respectively.

Impact of Drops on Non-wetting Biomimetic Surfaces

We have carried out an experimental study of liquid drop impact on superhydrophobic substrates covered by a carpet of chemically coated nano-wires.The micro-structure of the surface is similar to some biological ones(Lotus leaf for example).In this situation the contact angle can then be considered as equal to 180 degrees,with no hysteresis.Due to its initial inertia,the drop experiences a flattening phase after it hits the surface,taking the shape of a pancake.Once it reaches its maximal lateral extension,the drop begins to retract and bounces back.We have extracted the lateral extension of the drop,and we propose a model that explains the trend.We find a limit initial velocity beyond which the drop protrudes into the nano-wire carpet.We discuss the relevance of practical issues in terms of self-cleaning surfaces or spray-cooling.