Film-air Interface Research Articles

Ion-Fluid Transport-Control Feedback along Nanopore Networks.

Biological signaling correlates with the interrelation between ion and nanofluidic transportation pathways. However, artificial embodies with reconfigurable ion-fluid transport interaction aspects remain largely elusive. Herein, we unveiled an intimate interplay between nanopore-driven advancing flow and ion carriage for the spontaneous imbibition of aqueous solutions at the nanoporous thin film level. Ionic factors dominate transport phenomena processing and integration (ions influence fluid motion, which in turn governs the self-regulated ion traveling). We show an ion-induced translation effect that finely converts a chemical input, the nature of ions, into a related fluidic output: modulation of the extent of imbibition. We further find complex imbibition dynamics induced by the ion type and population. We peculiarly pinpoint a stop-and-go effective transport process with a programmable delay time triggered by selective guest-host interactions. The ion-fluid transport interplay is captured by a simple model that considers the counterbalance between the capillary infiltration and solution concentration, owing to water loss at the nanoporous film-air interface. Our results demonstrate that nanopore networks present fresh scenarios for understanding and controlling autonomous macroscopic liquid locomotion and offer a distinctive working principle for smart ion operation.

Synthesis of Ag@SiO2 core-shell nanoparticles for antibacterial application in water-based acrylic polymer coatings

In this study, Ag@SiO2 core-shell nanoparticles were synthesized using the Stöber method and then physically mixing with MPS functionalized poly (styrene-co-acrylate) (PSA) emulsion to obtain a novel antibacterial composite material (PSA/Ag@SiO2). The dispersion of composite latexes was improved by the formation of Si-O-Si covalent bonds, and the migration of AgNPs on the surface of composite films enhanced their antimicrobial activity. Analysis of UV-Vis absorption spectra, TEM images, XPS and XRD data confirmed the core-shell structure of Ag@SiO2 and the successful coating of AgNPs by SiO2. TEM micrographs showed that Ag@SiO2 core-shell nanoparticles were uniformly dispersed in the composite latexes, with an average particle size of 198 nm for PSA/Ag@SiO2 composite latexes, larger than that of PSA latexes (162 nm). FTIR spectra also indicated the presence of Si-O-Si covalent bonds resulting from the sol-gel reaction between MPS functionalized PSA with Ag@SiO2. XPS and WCA characterization revealed the disruption of the core-shell structure of Ag@SiO2, leading to the migration of AgNPs to the air-film interface. The results of antibacterial activity demonstrated that PSA/Ag@SiO2 composite films displayed antibacterial activity against both E. coli and S. aureus, and the former displayer a slightly better antibacterial activity than the latter. The water-based acrylic polymer coatings with antibacterial properties explored in current work have various applications in wood paints, varnishes, metal protection, decorative systems and other related fields.

On icicle ripples

Natural icicles have an overall conical shape modulated by surface ripples. It has been noted from many observations of icicles formed in nature and in the laboratory that the wavelength of the ripples has a very narrow spectrum between about 8 and 12 mm and that, as time evolves, the phase of the ripples migrates upwards. In this pedagogical review, I explore some of the physical mechanisms that can cause and mediate the formation and migration of ripples on icicles using simple mathematical models. To keep the mathematics more straightforward and transparent, I confine attention to two dimensions. A key physical parameter is the surface tension between the film of water that coats an icicle and the air that surrounds it, which causes a phase shift between the film–air interface and the ice–film interface. I show that the wavelength of ripples is dominantly proportional to the cube root of the square of the gravity-capillary length times the thickness of the water film. At high film-flow rates, advection-dominated heat transfer coupled with the interfacial phase shift becomes the dominant driver of instability. Gibbs–Thomson undercooling provides an unexpectedly large stabilisation of small wavelengths at these large flow rates, sufficient to maintain wavelength selection at millimetre scales.

A high-efficient and salt-rejecting 2D film for photothermal evaporation

The solar-driven desalination is seen as a sustainable way to combat water scarcity. However, the solar steam generation efficiency has long been restricted by the high vaporization enthalpy of water and low energy density of natural sunlight. We introduced graphene oxide (GO) cross-linked with polyethyleneimine (PEI) as the photothermal material, with the enriched ammonic functional groups in modified GO membrane (GPM) activating water molecules to evaporate with much lower energy consumption. The vaporization enthalpy at the air-film interface is reduced up to 42% in GPM film by tuning the thermodynamic states of water. Consequently, GPM film enables a high evaporation rate of 2.48kg m-2 h-1 with 95.7% energy conversion efficiency under 1 sun. With the aid of positive charges introduced by hydrolysis of PEI, the GPM exhibits excellent salt resistance and delivers an evaporation rate around 1.8kg m-2 h-1 when treating 20 wt% NaCl solution.

Perfluoropolyether methyl propionate polysiloxane coating with high hydrophobicity and low dielectric property

AbstractDeveloping materials with a combination of superior properties such as hydrophobicity and low dielectric is very challenging. This work proposes a design strategy based on the synergistic effect of silicon fluorine to fabricate fluorous silicon materials with excellent hydrophobicity and dielectric properties. Perfluoropolyethyl propionate polysiloxane (PEPS) with different grafting ratios was synthesized by the reaction of polymethylhydrosiloxane (PMHS) and perfluoropolyethyl methacrylate (PPM) hydrosilane. The self‐crosslinking perfluoropolyether methyl propionate polysiloxane (PFSP) coating with good hydrophobicity and low dielectric was prepared by further introducing triethoxyvinylsilane (TOVS) into the PEPS and condensing. The fluorine content of two interfaces (film PET and film air) of PFSP‐4 film was analyzed by XPS, and it was found that the fluorine content of the film air interface was 46.19% and that of the film PET interface was 17.3%. With the increase of perfluoropolyether content in PFSP, fluorine‐containing groups migrate to the coating surface, which is more conducive to enhancing the hydrophobicity of the coating. The results indicate that the film has excellent hydrophobicity with a high water contact angle of up to 117° due to the introduction of PPM. In addition, the dipole moment of perfluoropolyether methyl methacrylate is 2.27, which is lower than that of methyl. Therefore, the introduction of a large number of perfluoropolyether bonds effectively reduces the polarity of the material, which is beneficial to its low dielectric properties. The dielectric constant of PFSP‐4 is 2.4, which is 33% lower than that of a fluorine‐free silicone coating. The results show that the self‐crosslinking perfluoropolyether methacrylate polysiloxane coating has excellent hydrophobicity and low dielectric properties, so it has a feasible application prospect in the electronic industry.

Resonance damping of gravity–capillary waves on water covered with a visco-elastic film of finite thickness: A reappraisal

A new approach to the problem of damping of gravity–capillary waves (GCW) on water covered with a layer of viscous liquid (a film) of finite thickness with two elastic boundaries is developed. It is shown that the rotational component of GCW can be described formally as a “forced” longitudinal or Marangoni wave (MW), and the potential component of GCW plays a role of the “external force.” The resonance-like excitation of the forced MW is demonstrated when the GCW and MW frequencies and wave numbers are approximately close to each other. For a film that is thinner than the viscous boundary layers in film, a single forced MW exists that is located within the boundary layer beneath the water surface. For a thick film, the forced MW is characterized by the existence of two spatially separated MW modes: one is localized in the boundary layer below the upper, air–film interface and another within the boundary layers in the vicinity of the water–film interface. Then, at different elasticities of the interfaces, a double peak dependence of the GCW damping coefficient on wave number can occur due to the resonance with the two forced MW modes. The dependence of the damping coefficient on film thickness is characterized by a strong maximum appearing when the film and boundary layer thickness values are comparable to each other. The developed theory is consistent with existing numerical studies and experiment.

Design and synthesis of waterborne light-responsive cellulose nanocrystal/fluorinated polyacrylate films toward oil/water repellent and self-healing properties

The development of environmentally friendly waterborne polymeric materials is becoming critical today. In this study, a kind of self-healing cellulose nanocrystal/fluorinated polyacrylate based on the reversible photo-induced dimerization of coumarin groups was prepared via RAFT-mediated Pickering emulsion polymerization using cellulose nanocrystal (CNC) particles modified with light-responsive amphiphilic copolymers as Pickering stabilizers. Emulsion polymerization and latex film properties were significantly varied in dependence on the amount of 7-(2-methacryloyloxyethoxy)-4-methylcoumarin (CMA) monomer. Hydrophobic, oleophobic and mechanical properties of the latex film were dynamic in response to UV light. The maximum water contact angle and CH2I2 contact angle increased by 6.1° and 2.3°on the irradiation of UV light at 365 nm, respectively. The tensile strength of latex film increased initially with increasing amount of CMA increased, from 0 up to 4 wt%, and then decreased. The maximum tensile value increased from 3.0 MPa to 6.4 MPa after the dimerization reaction activated by 365 nm UV light. Results from atomic force microscope (AFM) and energy dispersive X-ray spectrometer (EDX) revealed that latex film possessed a roughness structure and the fluorine had a remarkable enrichment on the film-air interface. Moreover, latex films showed an excellent surface scratch self-healing ability owing to the reversible dimerization reaction of the coumarin groups, and the reversible dimerization reaction of coumarin groups within cellulose nanocrystal/fluorinated polyacrylate polymer was extensively investigated via FT-IR spectrometer and differential scanning calorimetry (DSC). The fruitful outcomes indicated that light-responsive cellulose nanocrystal/fluorinated polyacrylate exhibited a greatly potential application as a smart material.

Influence of polymerizable surfactants on the adhesion performance and water resistance of water-based acrylic pressure-sensitive adhesives (PSAs)

The behavior of different polymerizable surfactants was investigated to improve the water-resistance of pressure-sensitive adhesives (PSAs) based on acrylic acid (AA) and n-butyl acrylate (n-BA). Differences in the adhesion properties were found according to the chemical structure and the critical micelle concentration of the polymerizable surfactant, especially when compared to the non-polymerizable reference surfactant (Dowfax 2A1). The polymerizable surfactants with a branched structure resulted in more gel content and higher glass transition temperature in the adhesive. In general, the peel resistance and tack properties decreased with the use of polymerizable surfactants in the PSA formula. However, increased shear resistance was observed with respect to the PSA adhesive synthesized with the reference surfactant. Furthermore, the adhesives made with higher molecular weight polymerizable surfactants resulted in higher shear elastic modulus, shear strength, and deformation energy than the rest of the PSA adhesives. On the other hand, as expected, all the polymerizable surfactants studied contributed to the improved water resistance of the PSA compared to the reference PSA prepared with a conventional surfactant. The covalent bond formed between the polymer and the surfactant limited the migration of the surfactant towards the air-film interface thus reducing the formation of water entry pathways. The polymerizable surfactants with linear molecular structure showed the best water resistance performance.

Vertically Heterogeneous 2D Semi-Interpenetrating Networks Based on Cellulose Acetate and Cross-Linked Polybutadiene.

This work reports the feasibility of polybutadiene (PB) cross-linking under UV irradiation in the presence of a linear polymer, cellulose acetate (CA), to form semi-interpenetrating polymer networks at the air-water interface. The thermodynamic properties and the morphology of two-dimensional (2D) CA/PB blends are investigated after UV irradiation and for a wide range of CA volume fractions. A contraction of the mixed Langmuir films is observed independent of the composition, in agreement with that recorded for the individual PB monolayer after cross-linking. The PB network formation is demonstrated by in situ sum-frequency generation spectroscopy on the equivolumic CA/PB mixed film. From Brewster angle microscopy observations, the PB network synthesis does not induce any morphology change at the mesoscopic scale, and all of the mixed films remain homogeneous laterally. In situ neutron reflectometry is used to probe the effect of PB cross-linking on the vertical structure of CA/PB mixed films. For all studied compositions, significant thickening of the films is evidenced, consistent with their contraction ratio. This thickening is accompanied by a partial expulsion of the PB toward the film-air interface, which is attributed to the hydrophobic character of the PB. This phenomenon is stronger for films rich in PB. In particular, the structure of the PB-rich film undergoes a transition from vertically homogeneous to inhomogeneous along the depth. 2D semi-interpenetrating polymer networks can thus be synthesized at the air-water interface with a morphology that is strongly influenced by the polymer-polymer and polymer-environment interactions.

Aligned polymer dispersed liquid crystal film for light enhancement of quantum dot backlight

Quantum dots (QDs) have been used to make backlight, which provides a superior color gamut, for liquid crystal flat panel displays. In the backlight system, quantum dots, embedded in a polymer film and illuminated by blue light, emit red and green light with narrow bandwidths. There is, however, a problem with the system in that the quantum dots emit light in all directions, and most of the emitted light is in directions with large incident angles and cannot exit the film due to the total internal reflection at the film-air interface and is wasted. We propose to use an aligned polymer dispersed liquid crystal (APDLC) film to reduce the total internal reflection in the QD backlight and thus improve the light efficiency. A regular PDLC film, where the embedded liquid crystal droplets are randomly oriented, exhibits isotropic scattering and is not a good candidate for the enhancement of light efficiency of QD backlight. Through a two-step polymerization, we successfully developed an aligned polymer dispersed liquid crystal (APDLC) film where the liquid crystal droplets are permanently unidirectionally aligned in the film’s normal direction. It exhibits selective scattering: it scatters light with large incident angles but not light with small incident angles. When the APDLC film is laminated on the QD backlight film, a significant enhancement of the light efficiency of the QD backlight is achieved. The APDLC film can also be used to increase the light efficiency of other flat panel displays, such as organic light emitting diode (OLED) display and micro-light emitting diode (MLED) display.

Surface Pressure-Induced Interdiffused Structure Evidenced by Neutron Reflectometry in Cellulose Acetate/Polybutadiene Langmuir Films.

Binary blends of water-insoluble polymers are a versatile strategy to obtain nanostructured films at the air-water interface. However, there are few reported structural studies of such systems in the literature. Depending on the compatibility of the polymers and the role of the air-water interface, one can expect various morphologies. In that context, we probed Langmuir monolayers of cellulose acetate (CA), of deuterated and postoxidized polybutadiene (PBd) and three mixtures of CA/PBd at various concentrations by coupling surface pressure-area isotherms, Brewster angle microscopy (BAM), and neutron reflectometry at the air-water interface to determine their thermodynamic and structural properties. The homogeneity of the films in the vertical direction, averaged laterally over the spatial coherence length of the neutron beam (∼5 μm), was assessed by neutron reflectometry measurements using D2O/H2O subphases contrast-matched to the mixed films. At 5 mN/m, the whole mixed films can be described by a single slightly hydrated thin layer. However, at 15 mN/m, the fit of the reflectivity curves requires a two-layer model consisting of a CA/PBd blend layer in contact with the water, interdiffused with a PBd layer at the interface with air. At intermediate surface pressure (10 mN/m), the determined structure was between those obtained at 5 and 15 mN/m depending on film composition. This PBd enrichment at the air-film interface at high surface pressure, which leads to the PBd depletion in the blend monolayer at the water surface, is attributed to the hydrophobic character of this polymer compared with the predominantly hydrophilic CA.

Engineering fluorinated-cation containing inverted perovskite solar cells with an efficiency of >21% and improved stability towards humidity

Efficient and stable perovskite solar cells with a simple active layer are desirable for manufacturing. Three-dimensional perovskite solar cells are most efficient but need to have improved environmental stability. Inclusion of larger ammonium salts has led to a trade-off between improved stability and efficiency, which is attributed to the perovskite films containing a two-dimensional component. Here, we show that addition of 0.3 mole percent of a fluorinated lead salt into the three-dimensional methylammonium lead iodide perovskite enables low temperature fabrication of simple inverted solar cells with a maximum power conversion efficiency of 21.1%. The perovskite layer has no detectable two-dimensional component at salt concentrations of up to 5 mole percent. The high concentration of fluorinated material found at the film-air interface provides greater hydrophobicity, increased size and orientation of the surface perovskite crystals, and unencapsulated devices with increased stability to high humidity.

Investigation of the migration ability of silicone and properties of waterborne polyurethanes

A series of silicone–waterborne polyurethanes (WPU) are prepared by a pre–polymer method based on poly(tetramethylene ether glycol), isophorone diisocyanate, and polydimethylsiloxane (PDMS). The effect of the molecular weight of PDMS and film–forming temperature on the properties of WPU are investigated. The hydrophobicity of the films is characterized by X–ray photoelectron spectroscopy, contact angle and water absorbed measurements. With increasing molecular weight of PDMS in the WPU films, the contact angles increase, surface free energies decrease, and a substantial enrichment of silicon moieties at the film–air interface is observed. As the film–forming temperature increases, more silicone migrates to the film–air interface, which improves the hydrophobicity of the film bulk. The extent of variable phase separation caused by the mobility and incompatibility between silicone and the WPU segment is observed in the matrix of the WPU films by scanning electron microscopy. The degree of phase separation increases with the molecular weight of PDMS and film–forming temperature.

Synthesis of cellulose nanocrystals-armored fluorinated polyacrylate latexes via Pickering emulsion polymerization and their film properties

Cellulose nanocrystals-armored fluorinated polyacrylate latex particles with the core-shell spherical shape were prepared by a facile and efficient strategy based on Pickering emulsion polymerization using the modified cellulose nanocrystals, poly(2-(dimethylamino) ethyl methacrylate)-g-cellulose nanocrystal-g-poly(2,2,3,4,4,4-hexafluorobutyl acrylate) (PDMAEMA-g-CNC-g-PHFBA) as a sole Pickering stabilizer. The TEM result indicated that PDMAEMA-g-CNC-g-PHFBA effectively anchored on the outside of latex particles and improved the performance of emulsions, demonstrating the important role of the PDMAEMA-g-CNC-g-PHFBA in the Pickering stabilization process. The influences of the type of Pickering stabilizers and the PDMAEMA-g-CNC-g-PHFBA amount on the emulsion polymerization were investigated. The size of latex particles could be adjusted by PDMAEMA-g-CNC-g- PHFBA amount. XPS result showed that the fluorine-containing groups tended to enrich at the air-film interface of latex film during the heat process, leading to the decrease of surface free energy. The water-oil repellent and mechanical properties of the film could be improved as the content of PDMAEMA-g-CNC-g-PHFBA increased. In addition, thermal analysis showed an enhancement of the thermal properties of latex film upon increasing the PDMAEMA-g-CNC-g-PHFBA content.

Tunable structure and connectivity of organosilica hybrid films by using different epoxy based precursors in atmospheric plasma deposition

Organosilica hybrid films and fluorinated organosilica hybrid films have been successfully deposited using atmospheric plasma deposition. Different precursors, which incorporates epoxy groups in their molecules, were used to obtain connected structures with tunable chemistry and connectivity. The study demonstrated the capability of tuning the chemical composition, connectivity and mechanical properties of organosilica films by selecting the precursors and operational parameters of the process. The use of (3-glycidiloxypropyl) trimethoxysilane (GPTMS) and 2-(3,4-epoxy-cyclohexylethyl) trimethoxysilane (TRIMO) as precursors made possible the deposition of hybrid films with high organic character, increasing the relative C content up to 20 at% and 40at%, respectively, due to the lower fragmentation caused during the deposition by atmospheric plasma. The obtained structures had higher concentration of symmetric cages than those obtained from precursors without epoxy groups, which lead to a more compact structure and higher stiffness than conventional organic films. The combination with a fluorinated molecule also had a strong influence in the resultant structure caused by the preferential orientation of fluorinated molecules due to the low surface energy of FDTS that causes migration of fluorinated tails to the film-air interface during the film growth.

Computational Image Analysis of Guided Acoustic Waves Enables Rheological Assessment of Sub-nanoliter Volumes.

We present a method for the computational image analysis of high frequency guided sound waves based upon the measurement of optical interference fringes, produced at the air interface of a thin film of liquid. These acoustic actuations induce an affine deformation of the liquid, creating a lensing effect that can be readily observed using a simple imaging system. We exploit this effect to measure and analyze the spatiotemporal behavior of the thin liquid film as the acoustic wave interacts with it. We also show that, by investigating the dynamics of the relaxation processes of these deformations when actuation ceases, we are able to determine the liquid’s viscosity using just a lens-free imaging system and a simple disposable biochip. Contrary to all other acoustic-based techniques in rheology, our measurements do not require monitoring of the wave parameters to obtain quantitative values for fluid viscosities, for sample volumes as low as 200 pL. We envisage that the proposed methods could enable high throughput, chip-based, reagent-free rheological studies within very small samples.

Controllable microstructure of polymer-small molecule blend thin films for high-performance organic field-effect transistors

The carrier mobility is an important figure of merit and is sensitively influenced by the microstructure of semiconducting films such as crystallization and morphology. Here, a method of blending semiconducting small molecule 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) and insulating polymer binder is reported to well control the microstructures of blend semiconducting films. It is found that the insulating polymer in the blends prevents the dewetting of C8-BTBT from the SiO2 substrate and provides a platform for C8-BTBT molecules to segregate and crystallize at the air-film interface. By controlling the spinning speed, polymer type and ratio of the blend solution, it is possible to regulate the domain size and the surface roughness of C8-BTBT films, which in turn determines the performances of OTFTs. The bottom gate transistors based on C8-BTBT/polystyrene(PS) blend demonstrate excellent stability and a mobility up to 6.80 cm2/Vs due to the large domain size, smooth grain boundary and the phase-separated interface, which shows the potential applications in high-performance flexible and printed electronics.

Faraday rotation in iron garnet films beyond elemental substitutions

In previous decades significant efforts have been devoted to increasing the magneto-optical efficiency of iron garnet materials for the miniaturization of nonreciprocal devices such as isolators and circulators. Elemental substitutions or proper nano-structuring to benefit from optical resonances have been pursued. However, all these approaches still require film thicknesses of at least several tens of microns to deliver useful device applications, and suffer from narrow bandwidths in the case of optical resonance effects. This Letter reports on a newly discovered enhancement of the Faraday Effect observed experimentally in nanoscale bismuth-substituted iron garnet films. It is shown here that this enhancement is not due to elemental substitution or compositional variations. Nor is it due to photon trapping or resonance effects. Comprehensive experimental and theoretical analysis of the Faraday rotation reveals a dramatic seven-fold amplification in the magneto-optic gyrotropy within only 2 nm of the air-surface interface, corresponding to just a couple of atomic monolayers as a result of symmetry-breaking at the air-film interface. This finding opens up an avenue to the application of monolayer magnetic garnets for the control of light.

Observation of enhanced magnetic anisotropy in PLD YIG thin film on GGG (1 1 1) substrate

Abstract Magnetic properties and FMR of pulsed laser deposited Y3Fe5O12 (YIG) thin film on Gd3Ga5O12 (1 1 1) substrate have been investigated in the temperature range 4.2 ≤ T ≤ 300 K. The effective saturation magnetization (4πMeff), obtained from Ferromagnetic Resonance at room temperature, is found to be 2600 G, which is higher than SQUID measured 4πMS value of 1770 G. This implies that the value of 4πMeff can only be accounted by considering a negative anisotropy field (HU) of around 830 Oe at room temperature. Such an anisotropy is attributed to a compressive stress (−1.70 × 1010 dyne/cm2) induced in the film closer to the film-air interface. The presence of this stress in YIG film was confirmed using multiple {hkl} stress measurements with grazing incidence X-ray diffraction performed at different depth of penetration on the YIG film.

Cone-shaped optical beam induced by nanohole array

We observe an interesting optical effect of beam shaping by exploiting a laser beam incident at an arbitrary angle to a nanohole array fabricated in a dielectric aluminum oxide film. The light field with wave-vector component projected in a plane across the nanohole array exhibits omni-directional scattering process, while the light field of the wave-vector component along the elongated nanohole simply propagates toward the film–air interface where the light wave is partially retro-reflected. As a result, the optical fields with different wave vectors construct two symmetric optical beam cones about the film. We simulate the interaction process of the incident laser with nanoholes, showing excellent accordance with our experimental results. The beam shaping mechanism of nanohole array is also validated by our experimental study about scattering polarization.