Hydrogel Inverse Replicas of Breath Figures Exhibit Superoleophobicity Due to Patterned Surface Roughness

Exploiting breath figure reversibility for in situ pattern modulation and hierarchical design

The breath figure (BF) method employs condensation droplets as dynamic templates for patterning polymer films. In the classical approach, dropwise condensation and film solidification are simultaneously induced through solvent evaporation, leading to empirically derived patterns with limited predictability of the final design. Here we use the temporally arrested BF methodology, controlling condensation and polymerisation independently to create diverse BF patterns with varied pore size, arrangement and distribution. External temperature control enables us to further investigate and exploit the inherent reversibility of the phase change process that governs the pattern formation. We modulate the level of subcooling and superheating to achieve subsequent regimes of condensation and evaporation, permitting in situ regulation of the droplet growth and shrinkage kinetics. The full reversibility of the phase change processes joined with active photopolymerisation in the current approach thus allows arresting of predictable BF kinetics at intermediate stages, thereby accessing patterns with varied pore size and spacing for unchanged material properties and environmental conditions. This simultaneous active control over both the kinetics of phase change and polymer solidification offers affordable routes for the fabrication of diverse predictable porous surfaces; manufacture of monolithic hierarchical BF patterns are ultimately facilitated through the advanced control of the BF assembly using the method presented here.

Tailoring Materials with Specific Wettability in Biomedical Engineering

As a fundamental feature of solid surfaces, wettability is playing an increasingly important role in our daily life. Benefitting from the inspiration of biological paradigms and the development in manufacturing technology, numerous wettability materials with elaborately designed surface topology and chemical compositions have been fabricated. Based on these advances, wettability materials have found broad technological implications in various fields ranging from academy, industry, agriculture to biomedical engineering. Among them, the practical applications of wettability materials in biomedical-related fields are receiving remarkable researches during the past decades because of the increasing attention to healthcare. In this review, the research progress of materials with specific wettability is discussed. After briefly introducing the underlying mechanisms, the fabrication strategies of artificial materials with specific wettability are described. The emphasis is put on the application progress of wettability biomaterials in biomedical engineering. The prospects for the future trend of wettability materials are also presented.

Recent Advances in Bioinspired Gel Surfaces with Superwettability and Special Adhesion

Engineering surface wettability is of great importance in academic research and practical applications. The exploration of hydrogel-based natural surfaces with superior properties has revealed new design principles of surface superwettability. Gels are composed of a cross-linked polymer network that traps numerous solvents through weak interactions. The natural fluidity of the trapped solvents confers the liquid-like property to gel surfaces, making them significantly different from solid surfaces. Bioinspired gel surfaces have shown promising applications in diverse fields. This work aims to summarize the fundamental understanding and emerging applications of bioinspired gel surfaces with superwettability and special adhesion. First, several typical hydrogel-based natural surfaces with superwettability and special adhesion are briefly introduced, followed by highlighting the unique properties and design principles of gel-based surfaces. Then, the superwettability and emerging applications of bioinspired gel surfaces, including liquid/liquid separation, antiadhesion of organisms and solids, and fabrication of thin polymer films, are presented in detail. Finally, an outlook on the future development of these novel gel surfaces is also provided.

SEM analysis of the tunable honeycomb structure of irradiated poly(vinyl chloride) films doped with polyphosphate

The fabrication of tunable poly(vinyl chloride) porous films containing polyphosphate as an additive was successful. Irradiation of poly(vinyl chloride) films containing polyphosphate at a low concentration (0.5% by weight) with an ultraviolet light (λmax = 313 nm) for 300 h leads to the formation of a honeycomb like structure. The scanning electron microscopy images, at different magnification power, confirmed the production of the PVC honeycomb-like structure. The morphological images of the polymeric film showed a rough surface and a large number of regularly distributed hexagonal pores. The number of pores increased upon irradiation time and it was maximum after 300 h. The honeycomb structure formation could be due to the regular aggregation of polyphosphate among the polymeric chains, the increase in solution intrinsic viscosity and evaluation of hydrogen chloride gas through dehydrochlorination process.

Molecular Understanding on the Underwater Oleophobicity of Self-Assembled Monolayers: Zwitterionic versus Nonionic

Molecular dynamics simulations are conducted to investigate the underwater oleophobicity of self-assembled monolayers (SAMs) with different head groups. Simulation results show that the order of underwater oleophobicity of SAMs is methyl < amide < oligo(ethylene glycol) (OEG) < ethanolamine (ETA) < hydroxyl < mixed-charged zwitterionic. The underwater-oil contact angles (OCAs) are <133° for all nonionic hydrophilic SAMs, while the mixed-charged zwitterionic SAMs are underwater superoleophobic (OCA can reach 180°). It appears that surfaces with stronger underwater oleophobicity have better antifouling performance. Further study on the effect of different alkyl ammonium ions on mixed-charged SAMs reveals that the underwater OCAs are >143.6° for all SAMs; mixed-charged SAMs containing primary alkyl ammonium ion are likely to possess the best underwater oleophobicity for its strong hydration capacity. It seems that alkyl sulfonate anion (SO₃^-) is more hydrophilic than alkyl trimethylammonium ion (NC₃⁺) for the hydrophobic methyl groups on nitrogen atoms and that the hydration of SO₃^- in mixed-charged SAMs can be seriously blocked by NC₃⁺. The monomer of SO₃^- should be slightly longer than that of NC₃⁺ to obtain better underwater oleophobicity in NC₃⁺-/SO₃^--SAMs. In addition, the underwater oleophobicity of SAMs might become worse at low grafting densities. This work systematically proves that a zwitterionic surface is more underwater oleophobic than a nonionic surface. These results will help for the design and development of superoleophobic surfaces.

Regulating Underwater Superoleophobicity to Superoleophilicity on Hierarchical Structured Copper Substrates through Assembling n-Alkanoic Acids

In this paper, we report a simple method based on assembling n-alkanoic acids on hierarchical structured copper toward preparing surfaces with tunable oil wetting performance in water. Surface wettability from superoleophobicity to superoleophilicity in water can be regulated through tuning the chain length of n-alkanoic acids. Importantly, even in strongly acid and basic water, such phenomena can still be observed. The cooperation between the hierarchical structures and the surface chemical composition variation is responsible for the controllability. Meanwhile, the tunable ability is universal and the controllability is suitable for various oils including silicon oil, n-hexane, and chloroform. Moreover, the method was also used on copper mesh substrates, and we reported the related application of selective oil/water separation. This paper provides a flexible strategy toward preparing surfaces with tunable oil wetting performances, which can also be suitable for other materials, and offers some fresh ideas in manipulating underwater oil wetting performances on surfaces.

Hierarchical patterning of hydrogels by replica molding of impregnated breath figures leads to superoleophobicity

The surface chemistry and topography govern the spreading of liquids on a solid. When an oil drop makes a contact angle, θ > 90° on a solid surface, the solid is termed as oleophobic. Adding roughness to an inherently oleophobic surface enhances its oil dewetting and can lead to superoleophobicity when θ > 150°. In this study, we introduce the concept of a two-tier hierarchical roughness on the surface of soft materials such as hydrogels by forming the patterned inverse replica of breath figure polymer films impregnated with nanoparticles. The directed deposition of nanoparticles in the breath figure pores is accomplished by an aerosol assisted technique that exclusively leads to deposition within the pores and filling of the pores. The inverse replica of such impregnated films exhibits a close packed hexagonally structured second tier of surface roughness which directly leads to a superoleophobic surface. Since these structures have well defined geometries, it is possible to estimate the contact angle by assuming a partial wetting of the oil drop in a 'fakir' state on the rough surface. The estimation is in good agreement with the experimental contact angle value. While the work demonstrates a facile method to impart superoleophobicity to a hydrogel surface, it also demonstrates new methods to imbue breath figure pores with functional materials that can be easily transferred to the pores of the inverse replica.