A Simple Nanocellulose Coating for Self-Cleaning upon Water Action: Molecular Design of Stable Surface Hydrophilicity

Coating solid surfaces with cellulose nanofibril (CNF) monolayers via physical deposition was found to keep the surfaces free of a variety of oils, ranging from viscous engine oil to polar n-butanol, upon water action. The self-cleaning function was well correlated with the unique molecular structure of the CNF, in which abundant surface carboxyl and hydroxy groups are uniformly, densely, and symmetrically arranged to form a polar corona on a crystalline nanocellulose strand. This isotropic core–corona configuration offers new and easily adoptable guidance to design self-cleaning surfaces at the molecular level. Thanks to its excellent self-cleaning behavior, the CNF coating converted conventional meshes into highly effective membranes for oil–water separation with no prior surface treatment required.     

Counterion‐Dictated Self‐Cleaning Behavior of Polycation Coating upon Water Action: Macroscopic Dissection of Hydration of Anions

The counterions of polydiallyldimethylammonium (PDADMA) coatings were altered by incubation in aqueous solutions of different electrolytes. Oil de‐wetting on the resulting polycationic surfaces upon water action exhibited a straightforward connection with the Jones–Dole viscosity B‐coefficient (Bη) sign of surface counteranions. Upon water action, surface counteranions with negative Bη render PDADMA coatings oil‐adhering, but those with positive Bη furnish PDADMA coatings with excellent self‐cleaning. The oil‐adhering PDADMA surfaces can become self‐cleaning upon water action in response to the Bη of surface counteranions sign‐switching with increasing water temperature. Courtesy of surface counter‐anions with Bη>0, self‐cleaning PDADMA coatings enable not only conversion of conventional meshes into self‐cleaning membranes for oil/water separation, but also regioselective maneuver of oil flow on polycationic surfaces according to the Bη sign of surface counteranions patterned atop.     

Peristome‐Mimetic Curved Surface for Spontaneous and Directional Separation of Micro Water‐in‐Oil Drops

Separation of micro‐scaled water‐in‐oil droplets is important in environmental protection, bioassays, and saving functional inks. So far, bulk oil–water separation has been achieved by membrane separation and sponge absorption, but micro‐drop separation still remains a challenge. Herein we report that instead of the “plug‐and‐go” separation model, tiny water‐in‐oil droplets can be separated into pure water and oil droplets through “go‐in‐opposite ways” on curved peristome‐mimetic surfaces, in milliseconds, without energy input. More importantly, this overflow controlled method can be applied to handle oil‐in‐oil droplets with surface tension differences as low as 14.7 mN m⁻¹ and viscous liquids with viscosities as high as hundreds centipoises, which markedly increases the range of applicable liquids for micro‐scaled separation. Furthermore, the curved peristome‐mimetic surface guides the separated drops in different directions with high efficiency.     

Wrinkled Graphene Monoliths as Superabsorbing Building Blocks for Superhydrophobic and Superhydrophilic Surfaces

Superhydrophobic and superhydrophilic surfaces are of great interest because of a large range of applications, for example, as antifogging and self‐cleaning coatings, as antibiofouling paints for boats, in metal refining, and for water–oil separation. An aqueous ink based on three‐dimensional graphene monoliths (Gr) can be used for constructing both superhydrophobic and superhydrophilic surfaces on arbitrary substrates with different surficial structures from the meso‐ to the macroscale. The surface wettability of a Gr‐coated surface mainly depends on which additional layers (air for a superhydrophobic surface and water for a superhydrophilic surface) are adsorbed on the surface of the graphene sheets. Switching a Gr‐coated surface between being superhydrophobic and superhydrophilic can thus be easily achieved by drying and prewetting with ethanol. The Gr‐based superhydrophobic membranes or films should have great potential as efficient separators for fast and gravity‐driven oil–water separation.     

A New Cyanofluorene–Triphenylamine Copolymer: Synthesis and Photoinduced Intramolecular Electron Transfer Processes

A new π‐conjugated copolymer, namely, poly{cyanofluore‐alt‐[5‐(N,N′‐diphenylamino)phenylenevinylene]} ((CNF–TPA)_n), was synthesized by condensation polymerization of 2,2′‐(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)diacetonitrile and 5‐(N,N′‐diphenylamino)benzene‐1,3‐dicarbaldehyde by using the Knoevenagel reaction. By design, diphenylamine, alkylfluorene and poly(p‐phenylenevinylene) linkages were combined to form a (CNF–TPA)_n copolymer which exhibits high thermal stability and glass‐transition temperature. Photodynamic measurements in polar benzonitrile indicate fast and efficient photoinduced electron transfer (≈10¹¹ s^?1) from triphenylamine (TPA) to cyanofluorene (CNF) to produce the long‐lived charge‐separated state (90 μs). The finding that the charge‐recombination process of (CNF^.?–TPA^.+)_n is much slower than the charge separation in polar benzonitrile suggests a potential application in molecular‐level electronic and optoelectronic devices.     

Surprising Stability of Larger Criegee Intermediates on Aqueous Interfaces

Criegee intermediates have implications as key intermediates in atmospheric, organic, and enzymatic reactions. However, their chemistry in aqueous environments is relatively unexplored. Herein, Born–Oppenheimer molecular dynamics (BOMD) simulations examine the dynamic behavior of syn ‐ and anti ‐CH₃CHOO at the air–water interface. They show that unlike the simplest Criegee intermediate (CH₂OO), both syn ‐ and anti ‐CH₃CHOO remain inert towards reaction with water. The unexpected high stability of C₂ Criegee intermediates is due to the presence of a hydrophobic methyl substituent on the Criegee carbon that lowers the proton transfer ability and inhibits the formation of a pre‐reaction complex for the Criegee–water reaction. The simulation of the larger Criegee intermediates, (CH₃)₂COO, syn ‐ and anti ‐CH₂C(CH₃)C(H)OO on the water droplet surface suggests that strongly hydrophobic substituents determine the reactivity of Criegee intermediates at the air–water interface.     

Reinigen mit Licht und Regen

After the discovery of the lotus‐effect in the 1970s the attention concerning the field of self‐cleaning surfaces increased rapidly. Besides hydrophobic surfaces which are designed following the example of the lotus flower, superhydrophilic and photocatalytic active coatings play an important role. These coatings are using sunlight to oxidize pollutants and remove residues by rinsing with water. Coatings based on the lotus‐effect suffer from their weak mechanical durability, but are able to work without light. On the other hand, photocatalytic layers are mechanically stable, but they need to be illuminated to become self‐cleaning.     

Existence of Two‐Dimensional Physical Gels even at Zero Surface Pressure at the Air/Water Interface: Rheology of Self‐Assembled Domains of Small Molecules

Films of mesoscopic domains self‐assembled from fluorocarbon/hydrocarbon diblock copolymers (FnHm ) at the air/water interface were found to display highly elastic behavior. We determined the interfacial viscoelasticity of domain‐patterned FnHm Langmuir monolayers by applying periodic shear stresses. Remarkably, we found the formation of two‐dimensional gels even at zero surface pressure. These monolayers are predominantly elastic, which is unprecedented for surfactants, exhibiting gelation only at high surface pressures. Systematic variation of the hydrocarbon (n =8; m =14, 16, 18, 20) and fluorocarbon (n =8, 10, 12; m =16) block lengths demonstrated that subtle changes in the block length ratio significantly alter the mechanics of two‐dimensional gels across one order of magnitude. These findings open perspectives for the fabrication of two‐dimensional gels with tuneable viscoelasticity via self‐assembly of mesoscale, low‐molecular‐weight materials.     

Lotus bioinspired superhydrophobic,self‐cleaning surfaces from hierarchically assembled templates

The super hydrophobic, self‐cleaning properties of natural species derive from the fine hierarchical topography evolved on their surfaces. Hierarchical architectures which are function‐mimetic of the lotus leaf are here described and created from multi‐scale hierarchical assembled templates. The first level of hierarchy was a micromachined dome structure template and the second level of hierarchy was added by layering a thin nanoporous membrane such as porous anodized alumina or an ion track etch membrane. The assembled templates were nanoimprinted by a single step process on thermoplastic films. The wetting angle of the surfaces reached a value of 160° and the self‐cleaning behavior was observed. The superhydrophobic behavior remained over 1 year after fabrication, which demonstrates the stability of these polymeric self‐cleaning topographies. © 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B. Polym. Phys. 2014 , 52, 603–609     

Designing superhydrophobic surface based on fluoropolymer–silica nanocomposite via RAFT‐mediated polymerization‐induced self‐assembly

Superhydrophobic surfaces (SHS) find versatile applications as coatings due to their very high water‐repellency, self‐cleaning, and anti‐icing properties. This investigation describes the preparation of a SHS from surfactant‐free hybrid fluoropolymer latex. In this case, reversible addition‐fragmentation chain transfer (RAFT) polymerization was adopted to prepare a copolymer of 4‐vinyl pyridine (4VP) and vinyl triethoxysilane (VTES), where the pyridine units were quaternized to make the copolymer soluble in water. The copolymer was further used as a macro‐RAFT agent to polymerize 2,2,2‐trifluoroethyl methacrylate (TFEMA) in a surfactant‐free emulsion via polymerization‐induced self‐assembly (PISA). The macro‐RAFT agent contained a small amount of VTES as co‐monomer which was utilized to graft silica nanoparticles (SNPs) onto the P(TFEMA) spheres. The film prepared using the nanocomposite latex exhibited a nano‐structured surface as observed by SEM and AFM analyses. Surface modification of the film with fluorinated trichlorosilane produced an SHS with a water contact angle (WCA) of 151.5°. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 266–275     

Durable ultra‐hydrophobic surfaces for self‐cleaning applications

Self‐cleaning surfaces have received a great deal of attention, both in research studies and commercial applications. Both transparent and non‐transparent self‐cleaning surfaces are highly desirable as they offer many advantages, and their potential applications are endless. The self‐cleaning mechanism can be seen in nature. The Lotus flower, a symbol of purity in Asian cultures, grows in muddy waters, but it stays clean and untouched by dirt, organisms, and pollutants. The Lotus leaf self‐cleaning surface is hydrophobic and rough, showing a multi‐layer morphology of nanoscaled roughness. While hydrophobicity produces a high contact angle, the surface morphology reduces the adhesion of water drops to the surface, which slides easily across the leaf surface carrying the dirt particles with them. Different ultra‐hydrophobic, non‐transparent, and transparent coatings, for potential self‐cleaning applications, were produced on polycarbonate (PC) substrates, using hydrophobic chemistry and different configurations of roughening micro‐ and nano‐particles. However, in most cases, these coatings present low adhesion and durability. The stability and durability of the ultra‐hydrophobic surfaces is of key importance for potential, commercially viable, self‐cleaning applications thus durability and stability enhancement of such coatings was attempted by different methods, evaluated, and eventually improved using a solvent‐bonding technique. Copyright © 2008 John Wiley & Sons, Ltd.     

Literature review on superhydrophobic self‐cleaning surfaces produced by electrospinning

Self‐cleaning surface is potentially a very useful addition for many commercial products due to economic, aesthetic, and environmental reasons. Super‐hydrophobic self‐cleaning, also called Lotus effect, utilizes right combination of surface chemistry and roughness to force water droplets to form high contact angle on a surface, easily roll off a surface and pick up dirt particles on its way. Electrospinning is a promising technique for creation of superhydrophobic self‐cleaning surfaces owing to a wide set of parameters that allow effectively controlling roughness of resulted webs. This article gives a brief introduction to the theory of super‐hydrophobic self‐cleaning and basic principles of the electrospinning process and reviews the scientific literature where electrospinning was used to create superhydrophobic surfaces. The article reviewed are categorized into several groups and their results are compared in terms of superhydrophobic properties. Several issues with current state of the art and highlights of important areas for future research are discussed in the conclusion. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Electrospun N‐Substituted Polyurethane Membranes with Self‐Healing Ability for Self‐Cleaning and Oil/Water Separation

Membranes with special functionalities, such as self‐cleaning, especially those for oil/water separation, have attracted much attention due to their wide applications. However, they are difficult to recycle and reuse after being damaged. Herein, we put forward a new N‐substituted polyurethane membrane concept with self‐healing ability to address this challenge. The membrane obtained by electrospinning has a self‐cleaning surface with an excellent self‐healing ability. Importantly, by tuning the membrane composition, the membrane exhibits different wettability for effective separation of oil/water mixtures and water‐in‐oil emulsions, whilst still displaying a self‐healing ability and durability against damage. To the best of our knowledge, this is the first report to demonstrate a self‐healing membrane for oil/water separation, which provides the fundamental research for the development of advanced oil/water separation materials.     

Hydrophilic Oligo(lactic acid)s Captured by a Hydrophobic Polyaromatic Cavity in Water

Biologically relevant hydrophilic molecules rarely interact with hydrophobic compounds and surfaces in water owing to effective hydration. Nevertheless, herein we report that the hydrophobic cavity of a polyaromatic capsule, formed through coordination‐driven self‐assembly, can encapsulate hydrophilic oligo(lactic acid)s in water with relatively high binding constants (up to K_a=3×10⁵ m ⁻¹). X‐ray crystallographic and ITC analyses revealed that the unusual host–guest behavior is caused by enthalpic stabilization through multiple CH–π and hydrogen‐bonding interactions. The polyaromatic cavity stabilizes hydrolyzable cyclic di(lactic acid) and captures tetra(lactic acid) preferentially from a mixture of oligo(lactic acid)s even in water.     

Colloidal Synthesis and Charge‐Carrier Dynamics of Cs2AgSb1−yBiyX6 (X: Br,Cl; 0 ≤y ≤1) Double Perovskite Nanocrystals

A series of lead‐free double perovskite nanocrystals (NCs) Cs₂AgSb_1?yBi_yX₆ (X: Br, Cl; 0≤y≤1) is synthesized. In particular, the Cs₂AgSbBr₆ NCs is a new double perovskite material that has not been reported for the bulk form. Mixed Ag–Sb/Bi NCs exhibit enhanced stability in colloidal solution compared to Ag–Bi or Ag–Sb NCs. Femtosecond transient absorption studies indicate the presence of two prominent fast trapping processes in the charge‐carrier relaxation. The two fast trapping processes are dominated by intrinsic self‐trapping (ca. 1–2 ps) arising from giant exciton–phonon coupling and surface‐defect trapping (ca. 50–100 ps). Slow hot‐carrier relaxation is observed at high pump fluence, and the possible mechanisms for the slow hot‐carrier relaxation are also discussed.     

Growth of Stable Surface Oxides on Pt(111) at Near‐Ambient Pressures

Detailed knowledge of the structure and degree of oxidation of platinum surfaces under operando conditions is essential for understanding catalytic performance. However, experimental investigations of platinum surface oxides have been hampered by technical limitations, preventing in situ investigations at relevant pressures. As a result, the time‐dependent evolution of oxide formation has only received superficial treatment. In addition, the amorphous structures of many surface oxides have hindered realistic theoretical studies. Using near‐ambient pressure X‐ray photoelectron spectroscopy (NAP‐XPS) we show that a time scale of hours (t ≥4 h) is required for the formation of platinum surface oxides. These experimental observations are consistent with ReaxFF grand canonical Monte Carlo (ReaxFF‐GCMC) calculations, predicting the structures and coverages of stable, amorphous surface oxides at temperatures between 430–680 K and an O₂ partial pressure of 1 mbar.     

A Shifted Double‐Diamond Titania Scaffold

Photonic crystals are expected to be metamaterials because of their potential to control the propagation of light in the linear and nonlinear regimes. Biological single‐network, triply periodic constant mean curvature surface structures are considered excellent candidates owing to their large complete band gap. However, the chemical construction of these relevant structures is rare and developing new structures from thermodynamically stable double‐network self‐organizing systems is challenging. Herein, we reveal that the shifted double‐diamond titania scaffold can achieve a complete band gap. The largest (7.71 %) band gap is theoretically obtained by shifting 0.332 c with the dielectric contrast of titania (6.25). A titania scaffold with similar shifted double‐diamond structure was fabricated using a reverse core–shell microphase‐templating system with an amphiphilic diblock copolymer and a titania source in a mixture of tetrahydrofuran and water, which could result in a 2.05–3.78 % gap.     

Motion Control of Polymeric Nanomotors Based on Host–Guest Interactions

Controlling the motion of artificial self‐propelled micro‐ and nanomotors independent of the fuel concentration is still a great challenge. Here we describe the first report of speed manipulation of supramolecular nanomotors via blue light‐responsive valves, which can regulate the access of hydrogen peroxide fuel into the motors. Light‐sensitive polymeric nanomotors are built up via the self‐assembly of functional block copolymers, followed by bowl‐shaped stomatocyte formation and incorporation of platinum nanoparticles. Subsequent addition of β‐cyclodextrin (β‐CD) leads to the formation of inclusion complexes with the trans‐isomers of the azobenzene derivatives grafted from the surfaces of the stomatocytes. β‐CDs attachment decreases the diffusion rate of hydrogen peroxide into the cavities of the motors because of partly blocking of the openings of the stomatocyte. This results in a lowering of the speed of the nanomotors. Upon blue light irradiation, the trans‐azobenzene moieties isomerize to the cis‐form, which lead to the detachment of the β‐CDs due to their inability to form complexes with the cis‐isomer. As a result, the speed of the nanomotors increases accordingly. Such a conformational change provides us with the unique possibility to control the speed of the supramolecular nanomotor via light‐responsive host–guest complexation. We envision that such artificial responsive nano‐systems with controlled motion could have potential applications in drug delivery.     

pH responsive surfaces with nanoscale topography

We report the preparation of nanostructured adaptive polymer surfaces by diffusion of an amphihilic block copolymer toward the interface. The surface segregation of a diblock copolymer, polystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA), occurred when blended with high molecular weight polystyrene employed as a matrix. On annealing, the polymer surfaces changed both the chemical composition and the hydrophilicity depending on the environment and pH, respectively. By exposure to either water vapor or air, the surface wettability varied between hydrophilic and hydrophobic. In addition, surface enrichment on diblock copolymer by water vapor annealing led to self‐assembly occurring at the interface. Hence, nanostructured domains can be observed by AFM in liquid media. Moreover, the PAA segments placed at the interface respond to pH and can switch from an extended hydrophilic state at basic pH values to a collapsed hydrophobic state in acidic media. Accordingly, the surface morphology changed from swelled micelles to nanometer size holes. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2982–2990, 2010     

Preparation and Characterisation of Super‐Hydrophobic Surfaces

The interest in highly water‐repellent surfaces has grown in recent years due to the desire for self‐cleaning surfaces. A super‐hydrophobic surface is one that achieves a water contact angle of 150° or greater. This article explores the different approaches used to construct super‐hydrophobic surfaces and identifies the key properties of each surface that contribute to its hydrophobicity. The models used to describe surface interaction with water are considered, with attention directed to the methods of contact angle analysis. A summary describing the different routes to hydrophobicity is also given.