Superhydrophilic polyelectrolyte brush layers with imparted anti-icing properties: effect of counter ions

Transformation of Fly Ash-Based Oxide Particles into a Functional Silica-Alumina Aerogel and Its Potential Application as an Anti-icing Surface

Lightweight, surface hydrophobic, highly insulating, and long-lasting aerogels are required for energy conservation and ice-repellent applications. Here, we present the conversion of fly ash to a silica-alumina aerogel (SAA) by utilizing its high silica content. The extracted silica component replaces expensive precursors typically used in conventional aerogel production. Ice adhesion performance was compared to that of polypropylene (PP), an insulating commodity polymer. First, we removed some salt impurities and heavy metals via water and alkaline washing protocols. Then, we produced SAA via the ambient pressure drying method by using trimethylchlorosilane (TMCS) as an adhesion promoter. The newly produced SAA has a surface area of 810 m2 g-1 and shows hydrophobic properties with a contact angle of 140 ± 5°. The thermal conductivity of SAA is 0.0238 W m-1 K-1 with C P = 1.1922 MJ m-3 K-1. The ice adhesion strength of the PP substrate was calculated as 188.30 ± 51.24 kPa, while the ice adhesion strength of the SAA was measured as 1.21 ± 0.40 kPa, which was about 150 times lower than that of PP. This indicated that SAA had icephobic properties since ice adhesion strength was less than 10 kPa. This study demonstrates that fly ash-based SAA can be utilized as an economical material with a large surface area and exceptional thermal insulation capacity and is free of harmful compounds (heavy metals), making it potentially suitable as an anti-ice thermal insulation material.

Transient Superhydrophilic Surface Modification of Polyimide by Metal Ion Beam Irradiation

Polyimide is commonly used as a substrate for flexible electronic devices because of its excellent thermal, physical, and electrical properties. To enhance the adhesion between substrates and electrodes, it is necessary to improve the hydrophilic properties of the polyimide. Various surface treatments, such as plasma treatment, laser ablation, and ultraviolet treatments, have been applied for this purpose. In this study, we demonstrated that Cu and Ti ion beam irradiation can temporarily create a superhydrophilic surface on polyimide after irradiation. When Cu or Ti ions bombarded the polyimide, the contact angle changed systematically with the beam current density and over time. We present atomic force microscopy (AFM) data for polyimide irradiated with Cu and Ti ions at different beam current densities and discuss the possible mechanisms behind the changes in the contact angle.

Bioinspired Zwitterionic Nanowires with Simultaneous Biofouling Reduction and Release

Marine biofouling is a complex and dynamic process that significantly increases the carbon emissions from the maritime industry by increasing drag losses. However, there are no existing non-toxic marine paints that can achieve both effective fouling reduction and efficient fouling release. Inspired by antifouling strategies in nature, herein, a superoleophobic zwitterionic nanowire coating with a nanostructured hydration layer is introduced, which exhibits simultaneous fouling reduction and release performance. The zwitterionic nanowires demonstrate >25% improvement in fouling reduction compared to state-of-the-art antifouling nanostructures, and four times higher fouling-release compared to conventional zwitterionic coatings. Fouling release is successfully achieved under a wall shear force that is four orders of magnitude lower than regular water jet cleaning. The mechanism of this simultaneous fouling reduction and release behavior is explored, and it is found that a combination of 1) a mechanical biocidal effect from the nanowire geometry, and 2) low interfacial adhesion resulting from the nanostructured hydration layer, are the major contributing factors. These findings provide insights into the design of nanostructured coatings with simultaneous fouling reduction and release. The newly established synthesis procedure for the zwitterionic nanowires opens new pathways for implementation as antifouling coatings in the maritime industry and biomedical devices.

Reducing Ice Adhesion to Polyelectrolyte Surfaces by Counterion-Mediated Nonfrozen Hydration Water

Hydrophilic anti-icing coatings can be energy-effective passive solutions for combating ice accretion and reducing ice adhesion. However, their underlying mechanisms of action remain inferential and are ill-defined from a molecular perspective. Here, we systematically investigate the influence of the counterion identity on the shear ice adhesion strength to cationic polymer coatings having quaternary alkyl ammonium moieties as chargeable groups. Temperature-dependent molecular information on the hydrated polymer films is obtained using total internal reflection (TIR) Raman spectroscopy, complemented with differential scanning calorimetry (DSC) and ellipsometry. Ice adhesion measurements show a pronounced counterion-specific behavior with a sharp increase in adhesion at temperatures that depend on the anion identity, following the order Cl- < F- < SCN- < Br- < I-. Linked to the freezing of hydration water, the specific ordering results from differences in ion pairing and the amount of water present within the polymer film. Moreover, similar effects can be promoted by varying the cross-linking density in the coating while keeping the anion identity fixed. These findings shed new light on low ice adhesion mechanisms and may inspire novel approaches for improved anti-icing coatings.

Recent progress in understanding the anti-icing behavior of materials

Despite the significant progress in fundamental research in the physics of atmospheric icing or the revolutionary changes in modern materials and coatings achieved due to the recent development of nanotechnology and synthetic chemistry, the problem of reliable protection against atmospheric icing remains a hot topic of surface science. In this paper, we present a brief analysis of the mechanisms of anti-icing behavior that attracted the greatest interest of the scientific community and approaches which realize these mechanisms. We also note the strengths and weaknesses of such approaches and discuss future studies and prospects for the practical application of developed coatings.

Swelling and Orientation Behavior of End-Grafted Polymer Chains by In Situ Attenuated Total Reflection Fourier Transform Infrared Spectroscopy Complementing In Situ Ellipsometry

The literature lacks established concrete parameters for assigning grafted chain regimes. In this context, dichroic in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and in situ ellipsometry were used complementarily, offering new opportunities for conformational analysis of end-grafted polymer chains. Especially polymer chain orientation was studied as a new parameter, among others, for proper chain regime assignment in this report. Alkyne-functionalized poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) with a molecular weight of 49.8 kg/mol and a contour length of around 80 nm was grafted to self-assembled monolayers bearing triazole end groups as reported. Different chain regimes were generated by using three different grafting densities. ATR-FTIR spectroscopy based on the ν(C═O) stretching vibration at around 1728 cm-1 provided a new direct approach to determine the GD of polymer chains. Significant shifts in the position of the ν(C═O) band comparing dry and wet states were observed, caused by increased hydrogen bonding interactions between PDMAEMA and water. Finally, the averaged orientation of PDMAEMA chains along the z-axis was determined using dichroic ATR-FTIR spectroscopy based on the dichroic ratios of the ν(C═O) band and molecular order parameters SZ,MOL calculated thereof. High SZ,MOL values were found for the wet state compared to the dry state, confirming that all GD PDMAEMA samples are in the brush regime in the swollen state.

Hydrogel Electrolyte with High Tolerance to a Wide Spectrum of pHs and Compressive Energy Storage Devices Based on It

Normally, hydrogel electrolytes widely used in flexible energy storage devices have limited tolerance to different pHs. Most gel electrolytes will lose their compressible capability when the adaptable pH is changed. Herein, a poly(acrylamide₃ -co-(sulfobetaine methacrylate)₁ )@polyacrylamide (P(A₃ -co-S₁ )@PAM) hydrogel electrolyte equipped with a dual crosslinking network (DN) is successfully fabricated, which exhibits excellent tolerance to any pHs, endowing various energy storage devices including batteries and supercapacitors with superior mechanical durability. The batteries with mild and alkaline P(A₃ -co-S₁ )@PAM electrolytes display superior stability (over 3000 cycles). Additionally, a Zn||MnO₂ battery based on the P(A₃ -co-S₁ )@PAM hydrogel electrolyte (mild) under 50% compression strain also shows excellent charge-discharge stability and high capacity at 152.4 mAh g^-1 after 600 cycles. The strong reversible hydrogen bonds and electrostatic forces originating from zwitterionic structures of poly(sulfobetaine methacrylate) play an important role in dissipating and dispersing energy imposed abruptly. Meanwhile, the zwitterionic structure and intermolecular NH⋯OC hydrogen bonds of the hydrogel lead to the property of acid resistance and alkali resistance. The tough and robust covalent crosslinking bonds and the tight arrangement of DN polymer chains enable the hydrogel electrolytes to recover their initial shape fast once unloading.

Reduced Ice Adhesion Using Amphiphilic Poly(Ionic Liquid)-Based Surfaces

Ice build-up on solid surfaces causes significant economic losses for a range of industries. One solution to this problem is the development of coatings with low ice adhesion strength. Amphiphilic poly(ionic liquid) (PIL)-based surfaces have been recently reported for antifogging/antifrosting applications. However, they have possible anti-icing properties through lowering the ice adhesion strength that have yet to be reported. Herein, we designed well-defined triblock copolymers composed of a polydimethylsiloxane component coupled with PIL segments of poly([2 (methacryloyloxy)ethyl] trimethylammonium chloride) (PMETAC), which were subsequently UV-cured with an oligo(ethylene glycol) dimethacrylate (OEGDMA) cross-linker. The structure-property relationships of the resultant semi-interpenetrating polymer networks (SIPNs) were investigated by varying the counterion (i.e., trimethylammonium bis(trifluoromethanesulfonyl)imide (TFSI^-)) and the content of the PIL segments and cross-linker. An ice adhesion strength as low as 13.3 ± 8.6 kPa was observed for the coating containing 12.5 wt % of PMETAC segment and 5 wt % of OEGDMA, which is one of the lowest values reported so far for the amphiphilic coatings. Characterization of the coatings in terms of surface features, wettability, and hydration states have enabled the elucidation of different deicing mechanisms. Self-lubrication due to the existence of nonfreezable bound water led to the obtained low ice adhesion strength. This work offers a new approach for the exploration of PIL-based icephobic coatings for practical applications.

Durably Self-Sustained Droplet on a Fully Miscible Liquid Film

Droplets impacting onto a solid or liquid surface inducing wetting, floatation, splash, coalescence, etc. is ubiquitous in nature and industrial processes. Here, we report that liquid droplets exhibit spherical caps upon contact with a fully miscible liquid film of lower surface tension, despite the spontaneous mixing of the two liquids. Such a spherical cap on a continuous liquid surface sustains a long lifespan up to minutes before ultimately merging into the film. Benefiting from large viscous forces in a thin film as a result of spatial confinement, the surface flow is substantially suppressed. Therefore, the surface tension gradient responsible for this phenomenon is maintained because the normal diffusion of film liquid into the droplet can timely dilute film liquid supplied by uphill Marangoni flow at the droplet surface. The present finding removes the conventional cognition that droplet coalescence is prompt on fully miscible continuous liquid surfaces, thus benefiting design of new types of microfluidic devices.

Ice-Shedding Polymer Coatings with High Hardness but Low Ice Adhesion

Ice readily sheds from weak oil-swollen polymer gels but tends to adhere to mechanically robust coatings. This paper reports bilayer coatings that simultaneously possess high bulk hardness but low ice adhesion. These coatings are prepared by cocuring a triisocyanate, P#'-g-PDMS [a methacrylate polyol bearing poly(dimethylsiloxane) (PDMS) side chains with # being 1, 2, or 3 and g denoting graft], and optionally a methacrylate polyol P#. The self-assembly of the system during coating formation yields a PDMS brush layer on the surface of the cross-linked polyurethane matrix. After the surface PDMS layer is lubricated with a silicone oil, this coating exhibits an ice adhesion τ that is 10 000-fold lower than that of a triisocyanate/P# coating. Ice slides under its own weight on such a coating at a tilt angle of 3°. Yet, the coating matrix is harder than poly(ethylene terephthalate), a widely used plastic. Additionally, such a coating maintains its low τ values for more than 10 consecutive icing/deicing cycles. Subsequent increases in τ are reversed by allowing time for the replenishment of the depleted surface lubricant with that released from the coating matrix. This design opens the door for effective yet hard ice-shedding polymer coatings.

Enhanced Surface Icephobicity on an Elastic Substrate

Ice accumulation on exposed surfaces is unavoidable as time elapses and the temperature decreases sufficiently. To mitigate icing problems, various types of icephobic substrates have been rationally designed, including superhydrophobic substrates (SHSs), aqueous lubricating layers, organic lubricating layers, organogels, polyelectrolyte brush layers, electrolyte-based hydrogels, elastic substrates, and multicrack initiator-promoted surfaces. Among these surfaces, elastic substrates show excellent enhanced surface icephobicity during dynamic processes (i.e., water-impacting and de-icing tests). Herein, we summarize recent progress in elastic icephobic substrates and discuss the reasons that surface icephobicity can be enhanced on elastic substrates in terms of enhanced water repellency and further lowering the ice adhesion strength. For enhanced water repellency, we focus on reducing the contact time of water impacting such that water droplets can be easily shed from an elastic substrate before ice occurs. Reducing the contact time of water impacting various substrates (i.e., micro/nanostructured rigid SHSs, macrotextured rigid SHSs, and elastic SHSs) is discussed, followed by exploring their mechanisms. We argue that the ice adhesion strength can be further lowered on an elastic substrate by rationally tuning the elastic modulus and surface textures (i.e., surface textured and hollow subsurface textured) and combining elastic substrate with other passive anti-icing strategies (or functioning passive icephobic substrates with an electrothermal or photothermal stimulus). In short, the introduction of an elastic substrate into a passive or active icephobicity surface opens an avenue toward designing a versatile icephobic surface, providing great potential for outdoor anti-icing applications.

Design of Icephobic Surfaces by Lowering Ice Adhesion Strength: A Mini Review

Ice accretion can lead to severe consequences in daily life and sometimes catastrophic events. To mitigate the hazard of icing, passive icephobic surfaces have drawn widespread attentions because of their abilities in repelling incoming water droplets, suppressing ice nucleation and/or lowering ice adhesion strength. As time elapses and temperature lowers sufficiently, ice accretion becomes inevitable, and a realistic roadmap to surface icephobicity for various outdoor anti-icing applications is to live with ice but with the lowest ice adhesion strength. In this review, surfaces with icephobicity are critically categorized into smooth surfaces, textured surfaces, slippery surfaces and sub-surface textured surfaces, and discussed in terms of theoretical limit, current status and perspectives. Particular attention is paid to multiple passive anti-icing strategies combined approaches as proposed on the basis of icephobic surfaces. Correlating the current strategies with one another will promote understanding of the key parameters in lowering ice adhesion strength. Finally, we provide remarks on the rational design of state-of-the-art icephobic surfaces with low ice adhesion strength.

Dynamic Anti-Icing Surfaces (DAIS)

Remarkable progress has been made in surface icephobicity in the recent years. The mainstream standpoint of the reported antiicing surfaces yet only considers the ice-substrate interface and its adjacent regions being of static nature. In reality, the local structures and the overall properties of ice-substrate interfaces evolve with time, temperature and various external stimuli. Understanding the dynamic properties of the icing interface is crucial for shedding new light on the design of new anti-icing surfaces to meet challenges of harsh conditions including extremely low temperature and/or long working time. This article surveys the state-of-the-art anti-icing surfaces and dissects their dynamic changes of the chemical/physical states at icing interface. According to the focused critical ice-substrate contacting locations, namely the most important ice-substrate interface and the adjacent regions in the substrate and in the ice, the available anti-icing surfaces are for the first time re-assessed by taking the dynamic evolution into account. Subsequently, the recent works in the preparation of dynamic anti-icing surfaces (DAIS) that consider time-evolving properties, with their potentials in practical applications, and the challenges confronted are summarized and discussed, aiming for providing a thorough review of the promising concept of DAIS for guiding the future icephobic materials designs.

Ultralow Icing Adhesion of a Superhydrophobic Coating Based on the Synergistic Effect of Soft and Stiff Particles

A novel superhydrophobic coating composed of soft polydimethylsiloxane microspheres and stiff SiO₂ nanoparticles was developed and prepared. This superhydrophobic coating showed excellent superhydrophobicity with a large water contact angle of 171.3° and also exhibited good anti-icing performance and ultralow icing adhesion of 1.53 kPa. Furthermore, the superhydrophobic coating displayed good icing/deicing cycle stability, in which the icing adhesion was still less than 10.0 kPa after 25 cycles. This excellent comprehensive performance is attributed to stress-localization between ice and the surface, resulting from the synergistic effect of soft and stiff particles. This work thus opens a new avenue to simultaneously optimize the anti-icing and icephobic performance of a superhydrophobic surface for various applications.

A New Measuring System for the Determination of the Ice Adhesion Strength on Smooth Surfaces

To gain knowledge about cause-effect relationships for the adhesion of ice on surfaces with different chemical groups, we wanted to study the effect of thin polymer layers on the ice adhesion strength. To minimize the effect of roughness, smooth substrates that have generally relatively low ice adhesion strengths were chosen. To be able to obtain highly reproducible values for the region of low ice adhesion and to measure small differences of ice adhesion at surfaces with different chemical compositions, a new measuring system for the determination of the ice adhesion strength which is based on a modified spin-coater was developed. We show its technical potential on the basis of first results on pure silicon wafers and selected hydrophilic polymer layers. Furthermore, we investigated the effect of the water quality on the ice adhesion strength. The obtained data are discussed in the context of physicochemical properties of the layers and of the chemical characteristics of the used polymers.

Choline supported poly(ionic liquid) graft copolymers as novel delivery systems of anionic pharmaceuticals for anti-inflammatory and anti-coagulant therapy

New type of carriers based on grafted poly(ionic liquid)s was designed for delivery of ionically attached salicylates (Sal). Choline derived ionic liquid monomeric units were successfully introduced with various content in the side chains by the controlled radical polymerization. Properly high amounts of ionic pharmaceutics in the polymer systems were achieved by the well-fitted length and grafting degree of the side chains. In aqueous solution the graft copolymers were self-assembled into the spherical superstructures with sizes up to 73 nm. Delivery studies showed "burst" release within 4 h, after that it was slower yielding ~70% of released drug within 80 h. Proposed nanocarriers supported low toxicity against human cells (NHDF and BEAS-2B), anti-inflammation activity evaluated with the use of pro-inflammatory interleukins (IL-6 and IL-8) and antibacterial activities towards E. coli. Adjustment of ionic drug content by structural parameters of graft copolymers, including grafting degree and graft length, are advantageous to tailor nanocarriers with self-assembly properties in aqueous media. Effective release process by ionic exchange and biological activity with low toxicity are promising for further development of this type of drug delivery (DDS).

Bioinspired Multifunctional Protein Coating for Antifogging, Self-Cleaning, and Antimicrobial Properties

A multifunctional coating with antifogging, self-cleaning, and antimicrobial properties has been prepared based on a mussel-inspired chimeric protein MP-KE, which is the first example that these proteins were successfully applied to fabricate antifogging surfaces. The coating exhibits super hydrophilic properties, as indicated by contact angles less than 5° and high light transmittance similar to bare glass substrates about 90%. The zwitterionic peptides of MP-KE empower water molecules to expand into thin hydrated films rapidly, providing the protein coating with diverse surface functions. Moreover, the coatings have excellent stability and a convenient preparation process because of the mussel adhesive motif of MP-KE which makes the coating anchor onto the surface strongly. As a protein material, this multifunctional coating possesses remarkable biocompatibility and has a potential application prospects in the biomedical and pharmaceutical fields.

Understanding the role of hollow sub-surface structures in reducing ice adhesion strength

The accretion of ice on exposed surfaces results in detrimental effects in many aspects of life and technology. Passive icephobic coatings, designed by strategies towards lowering ice adhesion to mitigate icing problems, have recently received great attention. In our previous studies, incorporation of hollow sub-surface structures which act as macro-scale crack initiators has been shown to drastically lower the ice adhesion on PDMS surfaces. In this study, the effects of hollow sub-surface structure geometry, such as the heights, shapes, and distributions, as well as the directions of the applied shear force, are experimentally investigated. Our results show that the number of potential macro-scale crack initiation sites dictates ice adhesion strength. The directions of the applied shear force also influence the ice adhesion strength when the potential crack length is dependent on the applied shear force direction. The inter-locking effect between ice and the coating, caused by the pre-deformation, needs to be considered if one of the dimensions of the hollow sub-surface structures approaches the millimeter scale. These results improve the understanding of the role of hollow sub-surface structures in reducing ice adhesion, providing new insights into the design principles for multi-scale crack initiator-promoted icephobic surfaces.

Highly Stable Amphiphilic Organogel with Exceptional Anti-icing Performance

Various organogel materials with either a liquid or solid surface layer have recently been designed and prepared. In this work, amphiphilic organogels (AmOG) are innovatively developed from copolymer P(PDMS-r-PEG-r-GMA) and 2,2'-diaminodiphenyldisulfide via epoxy group addition reaction and then infiltrated with amphiphilic lubricants instead of traditional hydrophilic or hydrophobic lubricants. Because of synergistic effects of hydrophilic and hydrophobic segments of amphiphilic lubricants, the AmOG surfaces showed high stability and excellent anti-icing performance. The delay in the freezing point of water was 1000 s on AmOG surfaces, which is 40 times longer as compared to the untreated hydrophilic glass surface. More importantly, low ice adhesion strength (15.1 kPa) was observed on AmOG which remained about 40 kPa even after 20 icing-deicing cycles. The novel amphiphilic organogels provide a new idea to prepare long-term anti-icing materials for practical applications.

Superhydrophilic Anti-Icing Coatings Based on Polyzwitterion Brushes

In this work, an anti-icing coating based on superhydrophilic polyzwitterion brushes is reported. Poly(sulfobetaine methacrylate) (PSBMA) brushes were synthesized by surface-initiated atom-transferred radical polymerization to create superhydrophilic films on silicon wafers. The thickness of the PSBMA brushes film increased linearly with the polymerization time, and the film remained superhydrophilic in a nonassociated state when the thickness was less than ∼100 nm. DSC and FTIR analyses revealed that PSBMA contains more nonfreezable bound water than typical polyelectrolytes such as poly(sodium styrenesulfonate) and poly(2-(methacryloyloxy)ethyltrimethylammonium chloride), leading to lower ice adhesion strength than the latter two. At -20 °C, the PSBMA brushes coating demonstrated low ice adhesion strength of 60 kPa, showing a significant reduction in ice adhesion by up to ∼75% compared to uncoated silicon wafer. The optimum PSBMA layer thickness for low ice adhesion was ∼100 nm. These findings suggest that polyzwitterions are excellent candidates for anti-icing coating application.

Icephobic Strategies and Materials with Superwettability: Design Principles and Mechanism

Ice formation and accretion on surfaces is a serious economic issue in energy supply and transportation. Recent strategies for developing icephobic surfaces are intimately associated with superwettability. Commonly, the superwettability of icephobic materials depends on their surface roughness and chemical composition. This article critically categorizes the possible strategies to mitigate icing problems from daily life. The wettability and classical nucleation theories are used to characterize the icephobic surfaces. Thermodynamically, the advantages/disadvantages of superhydrophobic surfaces are discussed to explain icephobic behavior. The importance of elasticity, slippery liquid-infused porous surfaces (SLIPSs), amphiphilicity, antifreezing protein, organogels, and stimuli-responsive materials has been highlighted to induce icephobic performance. In addition, the design principles and mechanism to fabricate icephobic surfaces with superwettability are explored and summarized.

Suppression of droplets freezing on glass surfaces on which antifreeze polypeptides are adhered by a silane coupling agent

The development of ice-phobic, glass-substrate surfaces is important for many reasons such as poor visibility through the ice-covered windshields of vehicles. The present authors have developed new glass surfaces coated with a silane coupling agent and polypeptides whose amino-acid sequence is identical to a partial sequence of winter flounder antifreeze protein. We have conducted experiments on the freezing of sessile water droplets on the glass surfaces, and measured the droplet temperature, contact angle, contact area and surface roughness. The results show that the supercooling temperature decreased noticeably in the case where a higher concentration solution of polypeptide was used for the coating. The adhesion strength of frozen droplets was lowest in the same case. In addition, we observed many nanoscale humps on the coated surface, which were formed by polypeptide aggregates in the solution. We argue that the combination of the hydrophilic humps and the hydrophobic base surfaces causes water molecules adjacent to the surfaces to have a variety of orientations in that plane, even after the ice layer started to grow. This then induces a misfit of water-molecule spacing in the ice layers and consequent formation of fragile polycrystalline structure. This explains the lower values of ice adhesion strength and supercooling enhancement in the cases of the polypeptide-coated glass plates.

Anti-icing Properties on Surfaces through a Functional Composite: Effect of Ionic Salts

This study reports the potential of a unique functional composite for anti-icing applications. To date, various ionic salt formulations have been applied to prevent ice accumulation on surfaces. However, salt can be removed by external factors and large amounts must be used to attain anti-icing properties. Incorporating hydrophilic salts into hydrophobic mediums and controlled release of specific agents can provide effective solution to reduce ice accumulation on surfaces. Here, we developed functional polymer composites with salt pockets of altered ionic salts consisting of potassium formate (KCOOH), sodium chloride (NaCl), or magnesium chloride (MgCl₂). We dissolved ionic salts in hydrophilic gel domains and dispersed in a hydrophobic styrene-butadiene-styrene polymer matrix. Na⁺ and Cl^- ions delayed ice formation by 42.6 min at -2 °C compared to that for unmodified surfaces. Functional composites prepared with the NaCl ionic salt exhibited better anti-icing behavior at -2 °C because of their high concentration compared to that of the composites prepared with KCOOH and MgCl₂ ionic salts. We also characterized the release of ionic salts from composite-modified hydrophobic medium separately up to 118 days. Furthermore, we monitored freezing of water on composite-incorporated or composite-coated hydrophobic surfaces in a camera-integrated cold chamber with a uniform temperature (-2 °C). The results demonstrated significant increases in the delay of freezing on composite-incorporated or composite-coated surfaces compared to that on controls. We observed altered effects of each ionic salt on the mechanical, morphological, and functional properties of the composite-incorporated or composite-coated hydrophobic surfaces. Our results suggested that the efficiency of a polymer composite to promote anti-icing behavior on a surface is directly related to the type and concentration of the particular ionic salt incorporation into the composite. This approach is promising and demonstrates significant potential of the ionic salt embedded within polymer composite-modified hydrophobic surfaces to attain delayed icing function.

Response of a Permanently Charged Polyelectrolyte Brush to External Ions: The Aspects of Structure and Dynamics

Structure and dynamics inside permanently charged polyelectrolyte brushes, sodium polystyrene sulfonate brushes, during their response to the introduction of external ions (NaCl) are investigated by neutron reflectivity and dielectric spectroscopy. Neutron reflectivity measurements show that the segmental density of the inner part of the brushes decreases and that of the outer part increases when the salt level is tuned from the salt-free condition to a moderate level (<10^-2 M)-the brushes swell further compared with the salt-free condition. This is attributed to the breakup of the multiplets formed by dipole-dipole pairs, and by this process, the previously constrained chain segments by the multiplets are released. Dielectric spectroscopy discovers a giant dipole by the charge separation of the adsorbed counterions and the PSS^- chains, induced by electric field. The dynamics of the induced giant dipole is accelerated with the increase of external salt, as a result of the charge regularization by elevated salt level. At high-enough salt level, the screening effect reduces the electrostatic repulsion between the neighboring chains and makes the brushes shrink.

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.

Polyelectrolyte Brush-Grafted Polydopamine-Based Catalysts with Enhanced Catalytic Activity and Stability

Three types of surface treatments, namely, polyethyleneimine (PEI) coating, short PEI (S-PEI) grafting, and long PEI (L-PEI) grafting, were performed on polydopamine (Pdop)-based catalysts to enhance their catalytic activity and stability. Brush-grafted catalysts were prepared by the stepwise synthesis of Au and short (or long) PEI brushes on Pdop particles (PdopP/Au/S- or L-PEI grafting). PEI-coated Pdop-based catalysts (PdopP/Au/PEI coating) were also prepared as non-brush-grafted catalysts. Among the surface-treated PdopP/Au catalysts, the brush-grafted catalysts (S-PEI and L-PEI grafting) exhibited excellent and stable catalytic performance because the brush grafting enabled the protection of the catalysts against harsh conditions, effective transfer of reactants to the catalysts, and confinement of reactants around the catalysts. The brush-grafted catalysts could also more effectively decompose larger dyes than the non-brush-grafted catalysts. The process-to-effectiveness of PEI coating is the best because the release of Pdop from PdopP/Au was moderately inhibited by the presence of only one layer of PEI coating on the PdopP/Au. Thus, this approach could be an alternative method to enhance the stability of PdopP/Au catalysts.

Bioinspired Surfaces with Superwettability for Anti-Icing and Ice-Phobic Application: Concept, Mechanism, and Design

Ice accumulation poses a series of severe issues in daily life. Inspired by the nature, superwettability surfaces have attracted great interests from fundamental research to anti-icing and ice-phobic applications. Here, recently published literature about the mechanism of ice prevention is reviewed, with a focus on the anti-icing and ice-phobic mechanisms, encompassing the behavior of condensate microdrops on the surface, wetting, ice nucleation, and freezing. Then, a detailed account of the innovative fabrication and fundamental research of anti-icing materials with special wettability is summarized with a focus on recent progresses including low-surface energy coatings and liquid-infused layered coatings. Finally, special attention is paid to a discussion about advantages and disadvantages of the technologies, as well as factors that affect the anti-icing and ice-phobic efficiency. Outlooks and the challenges for future development of the anti-icing and ice-phobic technology are presented and discussed.

Regulation Mechanism of Salt Ions for Superlubricity of Hydrophilic Polymer Cross-Linked Networks on Ti6Al4V

Poly(vinylphosphonic acid) (PVPA) cross-linked networks on Ti₆Al₄V show superlubricity behavior when sliding against polytetrafluoroethylene in water-based lubricants. The superlubricity can occur but only with the existence of salt ions in the polymer cross-linked networks. This is different from the phenomenon in most polymer brushes. An investigation into the mechanism revealed that cations and anions in the lubricants worked together to yield the superlubricity even under harsh conditions. It is proposed that the preferential interactions of cations with PVPA molecules rather than water molecules are the main reason for the superlubricity in water-based lubricants. The interaction of anions with water molecules regulates the properties of the tribological interfaces, which influences the magnitude of the friction coefficient. Owing to the novel cross-linked networks and the interactions between cations and polymer molecules, their superlubricity can be maintained even at a high salt ion concentration of 5 M. These excellent properties make PVPA-modified Ti₆Al₄V a potential candidate for application in artificial implants.

Icephobic Surfaces Induced by Interfacial Nonfrozen Water

It is known that smooth, hydrophobic solid surfaces exhibit low ice adhesion values, which have been shown to approach a lower ice adhesion strength limit (∼150 kPa) defined by the water receding contact angle. To overcome this limit, we have designed self-lubricating icephobic coatings by blending polydimethylsiloxane (PDMS)-poly(ethylene glycol) (PEG) amphiphilic copolymers into a polymer matrix. Such coatings provide low ice adhesion strength values (∼50 kPa) that can substantially reduce the lower bound of the ice adhesion strength achieved previously on smooth, hydrophobic solid surfaces. Different molecular mechanisms are responsible for the low ice adhesion strength attained by these two approaches. For the smooth hydrophobic surfaces, an increased water depletion layer thickness at the interface weakens the van der Waals' interactions between the ice and the polymeric substrate. For the self-lubricating icephobic coatings, the PEG component of the amphiphilic copolymer is capable of strongly hydrogen bonding with water molecules. The surface hydrogen-bonded water molecules do not freeze, even at substantial levels of subcooling, and therefore serve as a self-lubricating interfacial liquid-like layer that helps to reduce the adhesion strength of ice to the surface. The existence of nonfrozen water molecules at the ice-solid interface is confirmed by solid-state nuclear magnetic resonance (NMR) spectroscopy.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Glass surface modification via Cu(0)-mediated living radical polymerization of fluorinated and non-fluorinated acrylates

The glass surface was modified via Cu(0)-mediated living radical polymerization of butyl acrylate and trifluoroethyl methacrylate by using the grafting from and grafting to methodologies.

Cube sugar-like sponge/polymer brush composites for portable and user-friendly heavy metal ion adsorbents

Portable, non-toxic, and user-friendly sponge composites decorated with polyelectrolyte (PE) brushes were developed for the fast and efficient removal of heavy metal ions from waste water or drinking water. The polyacrylamide (PAM) and polyacrylic acid (PAA) brushes were grafted onto the sponge via "grafting-from" polymerization. For the polyethyleneimine (PEI) brush, "grafting-to" polymerization was used. A polydopamine (Pdop) layer was first coated on the sponge. Then, PEI was grafted onto the Pdop-coated sponge via a Michael addition reaction. The PEI-grafted sponge exhibited the best adsorption capacity and the fastest reaction rate of all the brushes due to the numerous adsorption sites of the PEI. The adsorption performance of two different PEI-grafted sponges depended on the molecular weight (MW) of the PEI. Simply by being dipped into a glass of water, non-toxic PEI-grafted sponge instantly removed the low concentration heavy metal ions, demonstrating a practical application for individual users.

Nanoscale deicing by molecular dynamics simulation

Deicing is important to human activities in low-temperature circumstances, and is critical for combating the damage caused by excessive accumulation of ice. The aim of creating anti-icing materials, surfaces and applications relies on the understanding of fundamental nanoscale ice adhesion mechanics. Here in this study, we employ all-atom modeling and molecular dynamics simulation to investigate ice adhesion. We apply force to detach and shear nano-sized ice cubes for probing the determinants of atomistic adhesion mechanics, and at the same time investigate the mechanical effect of a sandwiched aqueous water layer between ice and substrates. We observe that high interfacial energy restricts ice mobility and increases both ice detaching and shearing stresses. We quantify up to a 60% decrease in ice adhesion strength by an aqueous water layer, and provide atomistic details that support previous experimental studies. Our results contribute quantitative comparison of nanoscale adhesion strength of ice on hydrophobic and hydrophilic surfaces, and supply for the first time theoretical references for understanding the mechanics at the atomistic origins of macroscale ice adhesion.

Effect of Aluminum Substrate Surface Modification on Wettability and Freezing Delay of Water Droplet at Subzero Temperatures

In this study, we have investigated the freezing delay of a water droplet on precooled substrates of an aluminum alloy that is commonly used for heat-exchanger fins. The surfaces of the substrates were modified to obtain surfaces with different hydrophilicity/hydrophobicity and different surface chemistry but without significantly modifying the surface topography. The freezing delays and water contact angles were measured as a function of the substrate temperature and the results were compared to the predictions of the heterogeneous ice nucleation theory. Although the trends for each sample followed the trend in this theory, the differences in the extents of freezing delays were in apparent disagreement with the predictions. Concretely, a slightly hydrophilic substrate modified by (3-aminopropyl) triethoxysilane (APTES) showed longer freezing delays than both more hydrophilic and more hydrophobic substrates. We suggest that this is because this particular surface chemistry prevents ice formation at the interface of the substrate, prior to the deposition of the water droplet. On the basis of our results, we suggest that not only wettability and topography but also the concrete surface chemistry plays a significant role in the kinetics of the ice formation process when a water droplet is placed on a precooled substrate.

Temperature-Dependent Deicing Properties of Electrostatically Anchored Branched Brush Layers of Poly(ethylene oxide)

The hydration water of hydrophilic polymers freezes at subzero temperatures. The adsorption of such polymers will result in a hydrophilic surface layer that strongly binds water. Provided this interfacial hydration water remains liquidlike at subzero temperatures, its presence could possibly reduce ice adhesion, in particular, if the liquidlike layer is thicker than or comparable to the surface roughness. To explore this idea, a diblock copolymer, having one branched bottle-brush block of poly(ethylene oxide) and one linear cationic block, was electrostatically anchored on flat silica surfaces. The shear ice adhesion strength on such polymer-coated surfaces was investigated down to -25 °C using a homebuilt device. In addition, the temperature dependence of the ice adhesion on surfaces coated with only the cationic block, only the branched bottle-brush block, and with linear poly(ethylene oxide) was investigated. Significant ice adhesion reduction, in particular, at temperatures above -15 °C, was observed on silica surfaces coated with the electrostatically anchored diblock copolymer. Differential scanning calorimetry measurements on bulk polymer solutions demonstrate different thermal transitions of water interacting with branched and linear poly(ethylene oxide) (with hydration water melting points of about -18 and -10 °C, respectively). This difference is consistent with the low shear ice adhesion strength measured on surfaces carrying branched bottle-brush structured poly(ethylene oxide) at -10 °C, whereas no significant adhesion reduction was obtained with linear poly(ethylene oxide) at this temperature. We propose a lubrication effect of the hydration water bound to the branched bottle-brush structured poly(ethylene oxide), which, in the bulk, does not freeze until -18 °C.

Robust Thick Polymer Brushes Grafted from Gold Surfaces Using Bidentate Thiol-Based Atom-Transfer Radical Polymerization Initiators

A new bromoisobutyrate-terminated alkanethiol, 16-(3,5-bis(mercaptomethyl)phenoxy)hexadecyl 2-bromo-2-methylpropanoate (BMTBM), was designed as a bidentate adsorbate to form thermally stable bromoisobutyrate-terminated self-assembled monolayers (SAMs) on flat gold surfaces to conduct atom-transfer radical polymerizations (ATRPs). The monolayers derived from BMTBM were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) and compared to the monolayers formed from 16-mercaptohexadecyl 2-bromo-2-methylpropanoate (MBM), 16-(3-(mercaptomethyl)phenoxy)hexadecyl 2-bromo-2-methyl-propanoate (MTBM), and octadecanethiol (C18SH). In this study, although the monolayer derived from BMTBM was less densely packed than those derived from MBM and MTBM, the bidentate adsorbates demonstrated much higher thermal stability in solution-phase thermal desorption tests, owing to the "chelate effect". The enhanced stability of the BMTBM SAMs ensured the development of thick brushes of poly(methyl methacrylate) and polystyrene at elevated temperatures (60, 90, 105, and 120 °C). In contrast, SAMs derived from MBM and MTBM failed to grow polymer brushes at temperatures above 100 °C.

Strategies for anti-icing: low surface energy or liquid-infused?

Recent progress on the preparation and surface characteristics of polymeric anti-icing coatings from low surface energy or liquid-infused slippery surfaces is reviewed and illustrated.

Fabrication of a micro-nanostructured superhydrophobic aluminum surface with excellent corrosion resistance and anti-icing performance

Synergy is the key to realizing superhydrophobicity. The as-prepared superhydrophobic Al surface possesses both excellent corrosion resistance and anti-icing performance.

New insight into icing and de-icing properties of hydrophobic and hydrophilic structured surfaces based on core-shell particles

Icing is an important problem, which often leads to emergency situations in northern countries. The reduction of icing requires a detailed understanding of this process. In this work, we report on a systematic investigation of the effects of geometry and chemical properties of surfaces on the formation of an ice layer, its properties, and thawing. We compare in detail icing and ice thawing on flat and rough hydrophilic and hydrophobic surfaces. We also show advantages and disadvantages of the surfaces of each kind. We demonstrate that water condenses in a liquid form, leading to the formation of a thin continuous water layer on a hydrophilic surface. Meanwhile, separated rounded water droplets are formed on hydrophobic surfaces. As a result of slower heat exchange, the freezing of rounded water droplets on a hydrophobic surface occurs later than the freezing of the continuous water layer on a hydrophilic one. Moreover, growth of ice on hydrophobic surfaces is slower than on the hydrophilic ones, because ice grows due to the condensation of water vapor on already formed ice crystals, and not due to the condensation on the polymer surface. Rough hydrophobic surfaces also demonstrate a very low ice adhesion value, which is because of the reduced contact area with ice. The main disadvantage of hydrophobic and superhydrophobic surfaces is the pinning of water droplets on them after thawing. Flat hydrophilic poly(ethylene glycol)-modified surfaces also exhibit very low ice adhesion, which is due to the very low freezing point of the water-poly(ethylene glycol) mixtures. Water easily leaves from flat hydrophilic poly(ethylene glycol)-modified surfaces, and they quickly become dry. However, the ice growth rate on poly(ethylene glycol)-modified hydrophilic surfaces is the highest. These results indicate that neither purely (super)hydrophobic polymeric surfaces, nor "antifreeze" hydrophilic ones provide an ideal solution to the problem of icing.

Anti-icing potential of superhydrophobic Ti6Al4V surfaces: ice nucleation and growth

On the basis of the icing-delay performance and ice adhesion strength, the anti-icing potential of the superhydrophobic surface has been well-investigated in the past few years. The present work mainly emphasized the investigations of ice nucleation and growth to fully explore the anti-icing potential of the superhydrophobic surface. We took the various surfaces ranging from hydrophilic to superhydrophobic as the research objects and, combining the classical nucleation theory, discussed the ice nucleation behaviors of the water droplets on these sample surfaces under the condition of supercooling. Meanwhile, the macroscopical growth processes of ice on these surfaces were analyzed on the basis of the growth mechanism of the ice nucleus. It was found that the superhydrophobic surface could greatly reduce the solid-liquid interface nucleation rate, owing to the extremely low actual solid-liquid contact area caused by the composite micro-nanoscale hierarchical structures trapping air pockets, leading to the bulk nucleation dominating the entire ice nucleation at the lower temperatures. Furthermore, ice on the superhydrophobic surface possessed a lower macroscopical growth velocity as a result of the less ice nucleation rate and the insulating action of the trapped air pockets.

Superhydrophobic and anti-icing properties at overcooled temperature of a fluorinated hybrid surface prepared via a sol-gel process

A superhydrophobic surface with anti-icing performance has been the focus of research, but few studies have reported the effective and low cost strategy that met the requirements under overcooled conditions. In this article, the fluorinated sol-gel colloid coatings were simply prepared via hydrolytic condensation of nanosilica sol, methyltriethoxysilane (MTES) and 3-[(perfluorohexylsulfonyl)amino]propyltriethoxysilane (HFTES). The multi scale morphology and chemical composition of the artificial surfaces were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The influence of the surface roughness structure and fluorinated groups on the wettability and freezing delay time of the colloid surface under overcooled conditions were explored. As the HFTES content was higher than 6 wt%, the prepared colloid surface showed excellent superhydropobicity with a contact angle (CA) of about 166° at room temperature. The CA gradually reduced with the decrease of the temperature. Only the samples with high HFTES contents (above 30 wt%) exhibited special superhydrophobic and anti-icing properties under freeze temperature. Besides the surface roughness structure, the high fluoride enrichment on the surface plays a major role in the superhydrophobic and anti-icing properties under overcooled conditions.

Physics of icing and rational design of surfaces with extraordinary icephobicity

Icing of surfaces is commonplace in nature and technology, affecting everyday life and sometimes causing catastrophic events. Understanding (and counteracting) surface icing brings with it significant scientific challenges that requires interdisciplinary knowledge from diverse scientific fields such as nucleation thermodynamics and heat transfer, fluid dynamics, surface chemistry, and surface nanoengineering. Here we discuss key aspects and findings related to the physics of ice formation on surfaces and show how such knowledge could be employed to rationally develop surfaces with extreme resistance to icing (extraordinary icephobicity). Although superhydrophobic surfaces with micro-, nano-, or (often biomimetic) hierarchical roughnesses have shown in laboratory settings (under certain conditions) excellent repellency and low adhesion to water down to temperatures near or below the freezing point, extreme icephobicity necessitates additional important functionalities. Other approaches, such as lubricant-impregnated surfaces, exhibit both advantages and serious limitations with respect to icing. In all, a clear path toward passive surfaces with extreme resistance to ice formation remains a challenge, but it is one well worth undertaking. Equally important to potential applications is scalable surface manufacturing and the ability of icephobic surfaces to perform reliably and sustainably outside the laboratory under adverse conditions. Surfaces should possess mechanical and chemical stability, and they should be thermally resilient. Such issues and related research directions are also addressed in this article.

Counterion-activated nanoactuator: reversibly switchable killing/releasing bacteria on polycation brushes

A strategy to release attached bacteria from surface-grafted bactericidal poly((trimethylamino)ethyl methacrylate chloride) (pTMAEMA) brushes has been proposed. The pTMAEMA brushes were fabricated via the surface-initiated atom transfer radical polymerization for contact killing of bacteria, including Escherichia coli, Staphylococcus epidermidis and Stenotrophomonas maltophilia. The bacteria-conditioning surfaces, afterward, were washed with electrolyte solutions containing anions with different lipophilic characteristic, charge density, polarity and adsorbility to quaternary ammonium groups in polymers. Because of the special ion-pairing interactions, the interfacial properties, including wettability and ζ-potential, can be manipulated in a controlled manner. Therefore, the counterion-assisted modulation of pTMAEMA brushes facilitates the bacterial release and regeneration of antimicrobial polymer films. The physicochemical properties of polymer brushes and their interactions with counterions were characterized using an ellipsometer, contact angle goniometer, X-ray photoelectron spectroscopy and an electrokinetic analyzer. The repetitive killing and releasing actions of pTMAEMA through unlocking and locking counterions were demonstrated, showing the robust effectiveness of the pTMAEMA-based nanoactuator in controlling the physical action by the chemical stimuli. The real-world implementation of the nanoactuator was demonstrated with a surgical scalpel by repelling killed bacteria and retaining reusability.