Transparent surface with reversibly switchable wettability between superhydrophobicity and superhydrophilicity

Smart Surfaces with pH-Responsiveness Enhanced by Multiscale Hierarchical Structures Fabricated by Laser Direct Writing

In contemporary applications, smart surfaces capable of altering their properties in response to external stimuli have garnered significant attention. Nonetheless, the efficient creation of smart surfaces exhibiting robust and rapid responsiveness and meticulous controllability on a large scale remains a challenge. This paper introduces an innovative approach to fabricate smart surfaces with strong pH-responsiveness, combining femtosecond laser direct writing (LDW) processing technology with stimulus-responsive polymer grafting. The proposed model involves the grafting of poly(2-diethylaminoethyl methacrylate) (PDEAEMA) onto rough and patterned Au/polystyrene (PS) bilayer surfaces through Au-SH bonding. The incorporation of LDW processing technology extends the choice of microstructures and roughness achievable on material surfaces, while PDEAEMA imparts pH responsiveness. Our findings revealed that the difference in contact angle between acidic and basic droplets on the rough PDEAEMA-g-Au surface (∼118°) greatly surpasses that on the flat PDEAEMA-g-Au surface (∼72°). Next, by leveraging the precision control over surface microstructures enabled by the LDW processing technique, this difference was further augmented to ∼127° on the optimized patterned PDEAEMA-g-Au surface. Further, we created two distinct combined smart surfaces with varying wettability profiles on which the hydrophilic-hydrophobic boundaries exhibit reliable asymmetric wettability for acidic and basic droplets. Additionally, we prepared a separator, realizing a better visual distinction between acid and base and collecting them separately. Given the effective abilities found in this study, we postulate that our smart surfaces hold substantial potential across diverse applications, encompassing microfluidic devices, intelligent sensors, and biomedicine.

Silicone Nanofilament Coatings as Flexible Catalyst Supports for a Knoevenagel Condensation Reaction in Batch and Flow Systems

In this work, silicone nanofilament (SNF) coatings were prepared via a droplet-assisted growth and shaping (DAGS) approach, where the preparation of the coatings is allowed under ambient conditions. The application of SNF coatings as catalyst supports for amino moieties from (3-aminopropyl)triethoxysilane (APTES) was investigated. With the optimized coating conditions identified, the Brunauer-Emmett-Teller surface areas of a bare glass filter substrate and bare glass beads after the coating have increased by 5-fold and 16-fold, respectively. The SNF-coated filters were readily functionalized with amino groups via a liquid-phase deposition process, and their catalytic activities for a Knoevenagel reaction were evaluated using a batch reactor and a packed bed reactor. In both reactors, the as-prepared filters demonstrated superior catalytic performance over the functionalized filters without SNF coatings. Notably, the unique flexibility of the SNF coatings allowed the facile preparation of a packed bed reactor and a scalable catalytic system. It is expected that the packed bed system established in this study will support the development and the use of various SNF-supported organocatalysts and catalytic materials.

Enhanced Interfacial Affinity of the Supercapacitor Electrode with a Hydrogel Electrolyte by a Preadsorbed Polyzwitterion Layer

Polymer hydrogel-based solid-state supercapacitors exhibit great potential applications in flexible devices. Nevertheless, the poor electrode-electrolyte interfacial properties restrict their advances. Herein, by taking the well-developed polyvinyl alcohol (PVA)/H₂SO₄ gel electrolyte and the graphene film electrode as the prototype, a very simple strategy is demonstrated to improve the interfacial affinity between the electrode and the hydrogel electrolyte by a preadsorbed highly hydrophilic polyzwitterion layer of poly(propylsulfonate dimethylammonium propylmethacrylamide) (PPDP) on the electrode surface. Electrochemical measurements confirm that the charge-transfer resistance on the interface is effectively reduced after modification with PPDP. Consequently, the obtained areal capacitance experiences a 3-fold increase compared to the unmodified ones. Results from electrochemical quartz crystal microbalance with dissipation demonstrate that more ions can be reversibly transferred on the modified interface during the change-discharge cycles, suggesting that the accessible surface area on the electrode is also increased. The hydrophilic PVA layer shows a similar function but with a much smaller efficiency. The strategy depicted here is highly universalizable and can be generalized to different electrode/electrolyte systems or other electrochemical energy storage devices.

Exchange Counterion in Polycationic Hydrogels: Tunability of Hydrophobicity, Water State, and Floating Capability for a Floating pH Device

Smart gel materials are capable of controlling and switching swelling, water state, and wettability properties triggered by external stimuli. In this study, we fabricated a series of polyelectrolyte hydrogels bearing a 3-trimethylammoniumpropyl pendant to a methacrylamide-based backbone and examined the switchability with hydrophobic-like counteranions. The exchange between the initial chloride and camphor sulfate (CaS), dodecyl sulfate (DS), and perfluorooctanoate (PFO) counterions was investigated. The kinetics of the exchange showed that the fast exchange (within 4 h) of PFO allowed for a favorable coordination for ion pairing, resulting in a decrease in hydration. The reversibility of the exchange to the Cl⁻ ion was only enabled for the CaS ion due to its bulkiness, while the PFO and DS hydrogels were unable to exchange, even by using tetrabutylammonium chloride, which is a structurally similar reagent, due to aggregation or the coagulates in the collapsed state of the linear tails of the counterions. The hydrogels exhibited a modulable water state and water swelling. Moreover, the hydrogels containing DS and PFO, as counterions, showed surface hydrophobic (contact angle 90°) and high hydrophobic (110°) behavior, respectively. The Raman spectrometry fluorescence with a pyrene probe indicated an increase in strong hydrogen-bonded water molecules, water confinement, and hydrophobic domains in the PFO hydrogel. Moreover, the PFO-modified hydrogel demonstrated a free-floating ability on the water surface, with a strong water repellency, showing that it has the potential to be applied in a floating pH detection device to distinguish between volatile and nonvolatile bases in a controlled manner.

A Stable NanoPAA-ZnO/ZnCl2 Composite with Variable 3D Structured Morphology and Sustained Superhydrophilicity

A ZnO/ZnCl₂ composite with stable 3D structural morphologies and long lasting superhydrophilicity was synthesized on the top surface of a nano porous anodic alumina (nanoPAA) substrate. The wettability of a nanoPAA-ZnO/ZnCl₂ was systematically characterized and the experimental data indicated that the water contact angle (WCA) of 0° could be achieved as well as maintained over 7 days and still remained at 4.36° after 50 days, and its 3D structural morphology had no clearly observable change during this period. The mechanism for the superhydrophilicity of the composites was interpreted in terms of the inherent hydrophilicity of ZnO/ZnCl₂ nanofilm, the three-dimensional structures of wrinkled nanoflakes, the nanogaps between neighbor nanoflakes, the difference of structual morphologies (i.e., size, shape, and upright posture of nanoflakes), and the measured True Volume of voids in the nanocomposite. The structural morphologies were mainly determined by the parameters such as the original concentration of precursor ZnCl₂ and the pore diameter of nanoPAA substrate. The study proposes a promising superhydrophilic nanomaterial and a cost-effective synthesis method, which will play a practical role in the fields of biomedical molecular sensors and micro/nanofluidic chips.

Ionic effects on synthetic polymers: from solutions to brushes and gels

The ionic effects on synthetic polymers have attracted extensive attention due to the crucial role of ions in the determination of the properties of synthetic polymers. This review places the focus on specific ion effects, multivalent ion effects, and ionic hydrophilicity/hydrophobicity effects in synthetic polymer systems from solutions to brushes and gels. The specific ion effects on neutral polymers are determined by both the direct and indirect specific ion-polymer interactions, whereas the ion specificities of charged polymers are mainly dominated by the specific ion-pairing interactions. The ionic cross-linking effect exerted by the multivalent ions is widely used to tune the properties of polyelectrolytes, while the reentrant behavior of polyelectrolytes in the presence of multivalent ions still remains poorly understood. The ionic hydrophilicity/hydrophobicity effects not only can be applied to make strong polyelectrolytes thermosensitive, but also can be used to prepare polymeric nano-objects and to control the wettability of polyelectrolyte brush-modified surfaces. The not well-studied ionic hydrogen bond effects are also discussed in the last section of this review.

Tuning the Properties of Charged Polymers at the Solid/Liquid Interface with Ions

In conventional theories, where ions are treated as point charges, the properties of charged polymers can be tuned using ions via the ionic strength. However, this article will show that the properties of charged polymers at the solid/liquid interface, including charged polymer brushes and polyelectrolyte multilayers, can be tuned by ions beyond ionic strength effects. Ion specificity, multivalency, ionic hydrogen bonding, and ionic hydrophobicity/hydrophilicity are used to tune a range of properties of charged polymers at the solid/liquid interface such as hydration, conformation, stiffness, surface wettability, lubricity, adhesion, and protein adsorption. The ionic effects demonstrated here greatly broaden our understanding of the use of ions to tune the interfacial properties of charged polymers. It is anticipated that these ionic effects can be further expanded by incorporating new types of important ion-charged polymer interactions and can also be extended to neutral polymer systems.

Water drop-surface interactions as the basis for the design of anti-fogging surfaces: Theory, practice, and applications trends

Glass- and polymer-based materials have become essential in the fabrication of a multitude of elements, including eyeglasses, automobile windshields, bathroom mirrors, greenhouses, and food packages, which unfortunately mist up under typical operating conditions. Far from being an innocuous phenomenon, the formation of minute water drops on the surface is detrimental to their optical properties (e.g., light-transmitting capability) and, in many cases, results in esthetical, hygienic, and safety concerns. In this context, it is therefore not surprising that research in the field of fog-resistant surfaces is gaining in popularity, particularly in recent years, in view of the growing number of studies focusing on this topic. This review addresses the most relevant advances released thus far on anti-fogging surfaces, with a particular focus on coating deposition, surface micro/nanostructuring, and surface functionalization. A brief explanation of how surfaces fog up and the main issues of interest linked to fogging phenomenon, including common problems, anti-fogging strategies, and wetting states are first presented. Anti-fogging mechanisms are then discussed in terms of the morphology of water drops, continuing with a description of the main fabrication techniques toward anti-fogging property. This review concludes with the current and the future perspectives on the utility of anti-fogging surfaces for several applications and some remaining challenges in this field.

Gel-like ionic complexes for antimicrobial, hemostatic and adhesive properties

Ion-specific effects offer a great opportunity to construct intelligent macromolecular systems with diverse architectures, on-demand controlled release behaviors and interfacial responsiveness.

Wettability alteration in a functional capillary tube for visual quantitative point of care testing

Capillarity is an extremely common physical-chemical phenomenon related to wettability in nature, which has wide theoretical and practical interest. Herein, we reported a facile sensing device based on capillary force change in a vertical capillary tube. In this height-based capillary sensor (HCS), the inner surface of the capillary tube was modified with a layer of molecules with wetting responsibility based on the well-known simple surface chemistry. With targets in different concentrations, the wettability of the surface modified with responsive molecules would produce different changes. The responsive surfaces would change the capillary force of the vertical capillary tube, and result in different column heights. Like a thermometer, H⁺ and phenol have been quantified visually based on the height of the liquid inside the capillary tube.

Counterion Specificity of Polyelectrolyte Brushes: Role of Specific Ion-Pairing Interactions

We demonstrate here that the properties of poly (2-(methacryloyloxy) ethyl trimethylammonium chloride) brushes can be tuned by counterion species. When the brushes are exposed to external chloride (Cl^- ) counterions, obvious dehydration and collapse are only observed at high salt concentrations. In the presence of very strongly chaotropic perchlorate (ClO₄^- ), the brushes strongly dehydrate and collapse at a very low salt concentration. For the strongly chaotropic thiocyanate ion (SCN^- ), the changes in hydration and conformation of the brushes are similar to those observed for ClO₄^- but at a smaller extent at very low salt concentrations. With the addition of kosmotropic acetate (Ac^- ), hydration of the brushes increases, accompanied by a swelling of the brushes in the low-salt-concentration regime. In contrast, the brushes dehydrate and collapse with increasing concentration of Ac^- in the high-salt-concentration regime. The counterion specificity of the brushes demonstrated here is determined by specific ion-pairing interactions through modulating the osmotic pressure within the brushes and the hydrophobicity of the ion pairs.

Flourishing Bioinspired Antifogging Materials with Superwettability: Progresses and Challenges

Antifogging (AF) structure materials found in nature have great potential for enabling novel and emerging products and technologies to facilitate the daily life of human societies, attracting enormous research interests owing to their potential applications in display devices, traffics, agricultural greenhouse, food packaging, solar products, and other fields. The outstanding performance of biological AF surfaces encourages the rapid development and wide application of new AF materials. In fact, AF properties are inextricably associated with their surface superwettability. Generally, the superwettability of AF materials depends on a combination of their surface geometrical structures and surface chemical compositions. To explore their general design principles, recent progresses in the investigation of bioinspired AF materials are summarized herein. Recent developments of the mechanism, fabrication, and applications of bioinspired AF materials with superwettability are also a focus. This includes information on constructing superwetting AF materials based on designing the topographical structure and regulating the surface chemical composition. Finally, the remaining challenges and promising breakthroughs in this field are also briefly discussed.

Polymers and biopolymers at interfaces

This review updates recent progress in the understanding of the behaviour of polymers at surfaces and interfaces, highlighting examples in the areas of wetting, dewetting, crystallization, and 'smart' materials. Recent developments in analysis tools have yielded a large increase in the study of biological systems, and some of these will also be discussed, focussing on areas where surfaces are important. These areas include molecular binding events and protein adsorption as well as the mapping of the surfaces of cells. Important techniques commonly used for the analysis of surfaces and interfaces are discussed separately to aid the understanding of their application.

Simple, Reversible, and Fast Modulation in Superwettability, Gradient, and Adsorption by Counterion Exchange on Self-Assembled Monolayer

A simple fabrication method for preparation of surfaces able to switch from superhydrophobic to superhydrophilic state in a reversible and fast way is described. A self-assembled monolayer (SAM) consisting of quaternary ammonium group with aliphatic tail bearing terminal thiol functionality was created on gold nano/microstructured and gold planar surfaces, respectively. A rough nano/microstructured surface was prepared by galvanic reaction on a silicon wafer. The reversible counterion exchange on the rough surface resulted in a switchable contact angle between <5° and 151°. The prewetted rough surface with Cl(-) as a counterion possesses a superoleophobic underwater character. The kinetics of counterion exchanges suggests a long hydration process and strong electron ion pairing between quaternary ammonium group and perfluorooctanoate counterion. Moreover, a wettability gradient from superhydrophobic to superhydrophilic can be formed on the modified rough gold surface in a robust and simple way by passive incubation of the substrate in a counterion solution and controlled by ionic strength. Furthermore, adsorption of gold nanoparticles to modified plain gold surface can be controlled to a high extent by counterions present on the SAM layer.

Inspired smart materials with external stimuli responsive wettability: a review

Recent progress in smart surfaces with responsive wettability upon external stimuli is reviewed and some of the barriers and potentially promising breakthroughs in this field are also briefly discussed.

Fabrication of Robust and Antifouling Superhydrophobic Surfaces via Surface-Initiated Atom Transfer Radical Polymerization

Superhydrophobic surfaces were fabricated via surface-initiated atom transfer radical polymerization of fluorinated methacrylates on poly(ethylene terephthalate) (PET) fabrics. The hydrophobicity of the PET fabric was systematically tunable by controlling the polymerization time. The obtained superhydrophobic fabrics showed excellent chemical robustness even after exposure to different chemicals, such as acid, base, salt, acetone, and toluene. Importantly, the fabrics maintained superhydrophobicity after 2500 abrasion cycles, 100 laundering cycles, and long time exposure to UV irradiation. Also, the surface of the superhydrophobic fabrics showed excellent antifouling properties.

One-dimensional silicone nanofilaments

A decade ago one-dimensional silicone nanofilaments (1D-SNF) such as fibres and wires were described for the first time. Since then, the exploration of 1D-SNF has led to remarkable advancements with respect to material science and surface science: one-, two- and three-dimensional nanostructures of silicone were unknown before. The discovery of silicone nanostructures marks a turning point in the research on the silicone material at the nanoscale. Coatings made of 1D-SNF are among the most superhydrophobic surfaces known today. They are free of fluorine, can be applied to a large range of technologically important materials and their properties can be modified chemically. This opens the way to many interesting applications such as water harvesting, superoleophobicity, separation of oil and water, patterned wettability and storage and manipulation of data on a surface. Because of their high surface area, coatings consisting of 1D-SNF are used for protein adsorption experiments and as carrier systems for catalytically active nanoparticles. This paper reviews the current knowledge relating to the broad development of 1D-SNF technologies. Common preparation and coating techniques are presented along with a comparison and discussion of the published coating parameters to provide an insight on how these affect the topography of the 1D-SNF or coating. The proposed mechanisms of growth are presented, and their potentials and shortcomings are discussed. We introduce all explored applications and finally identify future prospects and potentials of 1D-SNF with respect to applications in material science and surface science.

Porous polymer-based monolithic layers enabling pH triggered switching between superhydrophobic and superhydrophilic properties

Porous polymer monolithic layers that enable reversible pH-responsive switching between superhydrophilicity and superhydrophobicity have been prepared via a “thiol-ene” click reaction.