Fluoroalkylsilane Research Articles

Vapor phase deposition of fluoroalkyl trichlorosilanes on silicon and glass: Influence of deposition conditions and chain length on wettability and adhesion forces

Vapor phase deposition (VPD) of a homologous series of fluoroalkyl silanes (FAS), containing 3 to 10 carbon atoms in a molecule, on silicon and glass substrates was investigated. Two deposition procedures were compared: 1) VPD carried out at atmospheric pressure, where silanization reactions occur in an unrestricted presence of FAS vapors and atmospheric water and 2) VPD carried out under evacuated conditions. Significant differences between topographies of the coatings obtained in the two VPD processes were found, suggesting that silanization conditions highly influenced molecular packing densities of the coatings. This, in turn, was considered to influence the trends in the acid-base (AB) component of surface free energy (SFE), macroscopic work of water-substrate adhesion and locally measured adhesion forces on these surfaces. The AB interactions were found to significantly influence wetting and adhesive properties of coatings obtained from short chain precursors by atmospheric pressure VPD (AP-VPD). Wettability and adhesive force on coatings from longer chain precursors deposited by AP-VPD and on coatings from the whole range of tested FAS precursors deposited in low-pressure VPD (LP-VPD) were found to be predominantly governed by dispersive interactions. Correlations between locally measured adhesive force and macroscopic work of adhesion were found. Considering the net effect of FAS deposition procedure on the adhesive force strength, LP-VPD was found to more effectively reduce adhesive force on Si substrates, and both deposition methods were found to be equally effective on glass substrates. Advantages and disadvantages of both deposition methods were highlighted, with references to their most suitable applications.

Production of hybrid macro/micro/nano surface structures on Ti6Al4V surfaces by picosecond laser surface texturing and their antifouling characteristics

The development of surfaces which reduce biofouling has attracted much interest in practical applications. Three picosecond laser generated surface topographies (Ti1, Ti2, Ti3) on titanium were produced, treated with fluoroalkylsilane (FAS), then characterised using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), Raman Spectroscopy, Fourier Transform Infra-Red (FTIR) spectroscopy, contact angle measurements and white light interference microscopy. The surfaces had a range of different macro/micro/nano topographies. Ti2 had a unique, surface topography with large blunt conical peaks and was predominantly a rutile surface with closely packed, self-assembled FAS; this was the most hydrophobic sample (water contact angle 160°; ΔGiwi was −135.29mJm−2). Bacterial attachment, adhesion and retention to the surfaces demonstrated that all the laser generated surfaces retained less bacteria than the control surface. This also occurred following the adhesion and retention assays when the bacteria were either not rinsed from the surfaces or were retained in static conditions for one hour. This work demonstrated that picosecond laser generated surfaces may be used to produce antiadhesive surfaces that significantly reduced surface fouling. It was determined that a tri-modally dimensioned surface roughness, with a blunt conical macro-topography, combined with a close-packed fluoroalkyl monolayer was required for an optimised superhydrophobic surface. These surfaces were effective even following surface immersion and static conditions for one hour, and thus may have applications in a number of food or medical industries.

Fluoroalkyl-silane-modified 3D graphene foam with improved Joule-heating effects and high hydrophobicity-derived anti-icing properties

A fluoroalkyl-silane (FS)-modified three-dimensional (3D) graphene foam (GF) composite (FS-GF) with a hierarchical porous microstructure was fabricated via a nickel foam template chemical vapor deposition and followed modification. The 3D GF, as an electrically conductive framework and hydrophobic porous substrate, was grafted with FS to further improve the hydrophobicity due to its long non-polar molecular chains and numerous fluoric/silicon functional groups. Porous FS-GF with high electrical conductivity (~650 S/m) is functionalized as a Joule-heating mesh for airflow, which synchronously demonstrates rapid heating rates, high conversion efficiencies, and uniform temperature distributions. Moreover, FS-GF exhibits self-cleaning and anti-icing properties that are superior to metallic and inorganic coatings. The outstanding performances of FS-GF suggest the potential for functionalizing 3D graphene composites using hierarchical structures at the multi-scale and chemical modifications of functional molecular groups.

Improve the Performance of Micro Heat Pipe by Surface Modification

The appearance of high power LED has put forward higher requirements for thermal management. The micro heat pipe (MHP) has high heat transfer ability and plays an important role in high power LED and other high heat flux device cooling. In this paper, we designed and fabricated a micro heat pipe with fluoroalkyl silane (FAS) surface modified glass cover. The contact angle of the working area of glass cover reached 95.49° and made working fluid drops fall back to micro groove of silicon substrate more quickly. Thus the new glass cover can speed up the circulation of working fluid and enhance the heat transfer. The experimental results also proved that hydrophobic glass cover has a better heat transfer capability. Besides, this novel MHP reached the stable working status faster. When the input heat power was 10 W, the balance temperature of MHP with hydrophobic glass cover was 22 oC lower than traditional MHP, while the balance time is 58 seconds less. The work presented in this paper provides a new direction for optimize the MHP, not only the wick structure in substrate, but also the wettability of cover plate.

Efficient Flame-Retardant and Smoke-Suppression Properties of Mg-Al-Layered Double-Hydroxide Nanostructures on Wood Substrate.

Improving the flame retardancy of wood is an imperative yet highly challenging step in the application of wood in densely populated spaces. In this study, Mg-Al-layered double-hydroxide (LDH) coating was successfully fabricated on a wood substrate to confer flame-retardant and smoke-suppression properties. The chemical compositions and bonding states characterized by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed the coating constituents of Mg-Al LDH. The coating evenly covered the sample wood surfaces and provided both mechanical enhancement and flame-retardancy effects. The limiting oxygen index of the Mg-Al LDH-coated wood increased to 39.1% from 18.9% in the untreated wood. CONE calorimetry testing revealed a 58% reduction in total smoke production and a 41% reduction in maximum smoke production ratio in the Mg-Al LDH-coated wood compared to the untreated wood; the peak heat release rate and total heat release were also reduced by 49% and 40%, respectively. The Mg-Al LDH coating is essentially hydrophilic, but simple surface modification by fluoroalkyl silane could make it superhydrophobic, with a water contact angle of 152° and a sliding angle of 8.6°. The results of this study altogether suggest that Mg-Al LDH coating is a feasible and highly effective approach to nanoconstructing wood materials with favorable flame-retardant and smoke-suppression properties.

Superhydrophobic rice husk ash coating on concrete

The silica particles in rice husk ash are useful to create the roughness required by the superhydrophobic coating on concrete. The superhydrophobic coating was prepared using rice husk ash dispersed in the ethanolic solution containing fluoroalkyl silane, 1H,1H,2H,2H-perfluorodecyl triethoxy silane (2vol.%). The ash solution was sprayed on a layer of commercial adhesive coated on the concrete. The water contact angle of 152.3±0.5° on the coated concrete was observed. The cumulative water uptake was reduced as much as 40.38% while the water sorptivity was reduced up to 44.44%. The penetration of water into the coated concrete under the water pressure of 500kPa was successfully reduced after 72h, but not fully prevented. The ash coating did not affect the surface hardness or the compressive strength significantly, but the ash coating resulted in a higher ultrasonic velocity due to the reduction of surface porosity.

Preparative method for obtaining of higher fluoroalkyl silanes

Preparative method for obtaining of higher fluoroalkyl silanes

Multifunctional superamphiphobic fabrics with asymmetric wettability for one-way fluid transport and templated patterning

In this work, multifunctional superamphiphobic fabrics with special wettability were constructed by a facile dip-coating or electrospraying process using easily available materials, viz. silica nanoparticles, heptadecafluorononanoic, and fluoroalkyl silane. The obtained HFA–FAS–SiO2 NPs@surface exhibited a contact angle (CA) of 166.4 ± 3.7° and 155.9 ± 2.1° to water and hexadecane, respectively. In addition, this surface also showed stable repellency toward various corrosive droplets at a wide range of pH values, including HCl (pH = 1), NaCl (pH = 7), and NaOH (pH = 14) solutions. After immersion in the strong acid and base solutions for 24 h, the cotton surface still maintained excellent anti-wetting property. The surface was durable enough to withstand 120 cycles of abrasion and 5 cycles of accelerated standard laundry and still kept a water CA higher than 140° and an oil CA higher than 120°. Another treatment method adopted in this work, electrospraying has been proved to be able to realize asymmetric wetting with one side displaying highly anti-wetting behavior and the other side retaining the inherent hydrophilic and oleophilic nature of the pristine cotton fabric. Based on this special wettability, the obtained fabric could display a one-way directional transport feature. This method can also be extended to create hydrophilically and oleophilically patterned superamphiphobic cotton fabrics using a template. This novel fabric is useful for the development of intelligent cellulose-based substrates for various applications.

Effect of surface morphology on wettability conversion

A hierarchical structural surface of TiO2 film with reversibly light-switchable wettability between superhydrophobicity and superhydrophilicity on metal substrate was fabricated through simply dip-coating method from TiO2 precursor solution containing TiO2 nanoparticles with the average diameter 25 nm (P25), followed by heat-treatment and modification with fluoroalkylsilane (FAS) molecules. The morphology, phase and crystallographic structure, and chemical composite of the as-prepared TiO2 film were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The wettability of TiO2 film was characterized by a drop shape analyzer. The water contact angle of superhydrophobic TiO2 film was up to 165.6°. Under UV irradiation, the water contact angle decreased and the superhydrophobic TiO2 film became superhydrophilic because of hydroxyl groups absorption on the TiO2 surface. Meanwhile, the surface morphology of TiO2 film, which resulted from the TiO2 nanoparticles added in TiO2 precursor solution, had a significant effect on the wettability conversion of TiO2 film and enhanced the switch from hydrophobicity to hydrophilicity. The wettability could be reversibly switched between superhydrophobicity and superhydrophilicity via alternation of UV exposure and dark storage.

Fabrication and Characterization of Ultra Water Repellent Surfaces on Aluminum Alloy Substrate

The fabrication of metallic ultra water repellent surfaces is of great significance to many industrial and scientific areas. Ultra water repellent surfaces on aluminum alloy substrates were fabricated with acidic etching and fluoroalkyl silane coating method. The prepared surfaces exhibit good water repellent behaviors with water contact angles (WCA) larger than 150° and contact angle hysteresis (CAH) about 5°. The resultant surfaces were examined using scanning electron microscope (SEM). The results show that a kind of hierarchical roughness in micro-nanoscale is formed, which plays a key role for the fabrication of ultra water repellent surfaces. Fancy phenomena such as "chair-shaped flow", "flow orientation" and "sinusoidal flow" were observed when water flowed in a rectangular conduit constructed with one ultra water repellent wall and one super hydrophilic wall on the opposite side. The observations reveal some characteristics of water flow in ultra water repellent conduits.

Engineering Surface Energy and Nanostructure of Microporous Films for Expanded Membrane Distillation Applications.

We investigated the factors that determine surface omniphobicity of microporous membranes and evaluated the potential application of these membranes in desalination of low surface tension wastewaters by membrane distillation (MD). Specifically, the effects of surface morphology and surface energy on membrane surface omniphobicity were systematically investigated by evaluating wetting resistance to low surface tension liquids. Single and multilevel re-entrant structures were achieved by using cylindrical glass fibers as a membrane substrate and grafting silica nanoparticles (SiNPs) on the fibers. Surface energy of the membrane was tuned by functionalizing the fiber substrate with fluoroalkylsilane (FAS) having two different lengths of fluoroalkyl chains. Results show that surface omniphobicity of the modified fibrous membrane increased with higher level of re-entrant structure and with lower surface energy. The secondary re-entrant structure achieved by SiNP coating on the cylindrical fibers was found to play a critical role in enhancing the surface omniphobicity. Membranes coated with SiNPs and chemically modified by the FAS with a longer fluoroalkyl chain (or lower surface energy) exhibited excellent surface omniphobicity and showed wetting resistance to low surface tension liquids such as ethanol (22.1 mN m(-1)). We further evaluated performance of the membranes in desalination of saline feed solutions with varying surface tensions by membrane distillation (MD). The engineered membranes exhibited stable MD performance with low surface tension feed waters, demonstrating the potential application omniphobic membranes in desalinating complex, high salinity industrial wastewaters.

Carbon dioxide capture using a superhydrophobic ceramic hollow fibre membrane for gas-liquid contacting process

This work initiates the development of clean technology in carbon dioxide (CO2) capture using ceramic membrane inspired by gas–liquid contacting system. A low cost, high performance superhydrophobic kaolin-alumina hollow fibre membrane was prepared via phase inversion-based extrusion and sintering techniques, followed by a grafting with fluoroalkylsilane (FAS). The membrane was characterized by scanning electron microscopy (SEM), gas permeation test, contact angle, wetting resistance, X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The fabricated membrane was highly porous, thus increasing the gas permeation rate. By surface modification, the membrane contact angle was increased from 0° to 142°. In fact, wettability resistance of the membrane was also improved. The membrane was subsequently applied in membrane contactor for carbon dioxide (CO2) absorption. The CO2 absorption flux as high as 0.18 mol m−2 s−1 was achieved at the liquid flow rate of 100 mL min−1 which was far above the fluxes of some commercial and in-house made polymeric and ceramic membranes. In conclusion, the modified kaolin-alumina hollow fibre membrane with the superhydrophobic surface, high permeance, and absorption flux is suitable for CO2 post-combustion capture, due to its outstanding chemical and thermal stabilities.

Effect of hydrophobic modification on carbon dioxide absorption using porous alumina (Al2O3) hollow fiber membrane contactor

Porous alumina (Al2O3) hollow fiber membranes were successfully prepared using phase inversion spinning and sintering technique. Hydrophilic surface of porous Al2O3 hollow fiber membrane was modified with a fluoroalkylsilane (FAS) solution that was used as hydrophobic agent for treating the surface of Al2O3 hollow fiber membranes. The unmodified and modified porous Al2O3 membranes were characterized using scanning electron microscopy, mercury porosimetry, gas permeation testing equipment, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, and the contact angle analyzer. The hydrophobicity of the porous Al2O3 hollow fiber membrane increased with increasing FAS coating time. The effect of the FAS coating on the CO2 absorption performance was also investigated. As a result, the CO2 absorption flux decreased with increasing FAS coating time. In particular, these experiments revealed that 2h coating was sufficient to obtain an optimal hydrophobic protection against the wetting process of the Al2O3 hollow fiber membrane pores. In addition, the best performance for CO2 absorption flux was achieved with the Al2O3 hollow fiber membrane modified for 2h coating. The maximum CO2 absorption flux of surface modified Al2O3 hollow fiber membrane was 6.022×10−3mol/m2s with a liquid flow rate of 50ml/min at room temperature. Finally, a 50-hours-long stability test demonstrates that the modified porous Al2O3 hollow fiber membranes are stable and promising for membrane contactors.

Durable, robust and free-standing superhydrophobic poly(vinyl alcohol-co-ethylene) nanofiber membrane

A simple and straightforward method for preparation of durable, robust and free-standing superhydrophobic poly(vinyl alcohol-co-ethylene) nanofiber membrane (PVA-co-PE NFM) is reported. PVA-co-PE nanofibers were synthesized by melt-blend extrusion and phase separation method, and their membrane was prepared by nanofiber suspension coating technique. Post-surface chemical modification on prepared NFM was performed by anchoring low surface-energy fluoroalkylsilane (FAS) molecules using dip-coating to get superhydrophobic NFM. The FAS-modified NFM had depicted a high water contact angle of 151°, robust water-repellence and sustained durability in superhydrophobic behavior against corrosive solutions and UV-radiations.

Controlled fabrication and electrowetting properties of silicon nanostructures

Electrowetting on dielectric is of particular interest in various applications including digital microfluidics, optics, displays and in analysis of biological samples. In this paper, Ag-assisted etching of silicon has been used to prepare superhydrophobic surfaces that may add unique properties to such devices. The porosity is controlled by controlling the size of the Ag deposited on the surface. Etching is carried out in HF/H2O2 solution. From the SEM images, the small-sized Ag particles are found to produce a very porous irregular surface, while the larger Ag-sized particles deposited resulted in regular vertical nanostructures. The fabricated surface was chemically modified with fluoro alkyl silane to make it superhydrophobic. Wetting behaviour was studied by measuring the contact angle and contact angle hysteresis. The contact angle measured was greater than 150° and contact angle hysteresis was less than five. Electrowetting was studied by applying DC voltage between the droplet and silicon substrate.

Fast fabrication of superhydrophobic surfaces on Ti–6Al–4V substrates by deposition of lead

In this paper, superhydrophobic Pb surfaces on Ti–6Al–4V substrates were fabricated based on immersion method in just 30–60 s followed by subsequent low surface energy materials modification. Hierarchical micro- and nanoscale structures were generated on titanium alloy substrates when immersed in Pb(CH3COO)2 solution. Then, the as-immersed specimens were chemically modified via fluoroalkylsilane (FAS) so as to reduce the surface energy. The as-prepared superhydrophobic Pb-coated titanium alloy surfaces, including morphologies, chemical compositions, and wettability were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectrophotometry (FTIR) and contact angle measurements. The results showed that the Pb coatings on Ti–6Al–4V substrates displayed good superhydrophobicity with water contact angle of 165.5° ± 2.4° and sliding angle of 4.6° ± 2.1°. Additionally, variations in micro-morphologies, surface roughness, static and dynamic water contact angles of FAS-modified Pb-coated titanium alloy surfaces with immersion time were also investigated, and the prepared superhydrophobic surfaces exhibited long-term stability in air, good self-cleaning and anti-corrosion properties.

Fabrication of broadband antireflective black metal surfaces with ultra-light-trapping structures by picosecond laser texturing and chemical fluorination

A hybrid method consisting of ultrafast laser-assisted texturing and chemical fluorination treatment was applied for efficiently enhancing the surface broadband antireflection to fabricate black titanium alloy surface with ultra-light-trapping micro-nanostructure. Based on the theoretical analysis of surface antireflective principle of micro-nanostructures and fluoride film, the ultra-light-trapping micro-nanostructures have been processed using a picosecond pulsed ultrafast laser on titanium alloy surfaces. Then fluorination treatment has been performed by using fluoroalkyl silane solution. According to X-ray diffraction phase analysis of the surface compositions and measurement of the surface reflectance using spectrophotometer, the broadband antireflective properties of titanium alloy surface with micro-nano structural characteristics were investigated before and after fluorination treatment. The results show that the surface morphology of micro-nanostructures processed by picosecond laser has significant effects on the antireflection of light waves to reduce the surface reflectance, which can be further reduced using chemical fluorination treatment. The high antireflection of over 98 % in a broad spectral range from ultraviolet to infrared on the surface of metal material has been achieved for the surface structures, and the broadband antireflective black metal surfaces with an extremely low reflectance of ultra-light-trapping structures have been obtained in the wavelength range from ultraviolet–visible to near-infrared, middle-wave infrared. The average reflectance of microgroove groups structured surface reaches as low as 2.43 % over a broad wavelength range from 200 to 2600 nm. It indicates that the hybrid method comprising of picosecond laser texturing and chemical fluorination can effectively induce the broadband antireflective black metal surface. This method has a potential application for fabricating antireflective surface used to improve the photothermal or photoelectric conversion efficiency of solar installations and enhancing the surface wave-absorbing capacity of devices.

Nanostructure-induced icephobic sol–gel coating for glass application

We present a study on materials formulation, self-assembled surface structure, and the correlation between room temperature water contact angle and contact angle hysteresis (CAH) and the icephobic property at −20 °C. Coating materials are based on TiO2-modified polysiloxane (PDMS) incorporated in the inorganic–organic hybrid (sol–gel) materials with a self-assembled surface structure induced by fluoroalkylsilane (FAS) additive and nanoparticle dispersion. A home-made single-drop ice tester is used to record the ice formation and ice sliding process on the tilted surfaces with adjustable chamber temperatures below 0 °C. It is found that a CAH less than 10°, surface roughness in the range of 10–80 nm, and a two-scale surface structure with nanospikes are the essential features for automatic ice sliding at −20 °C at a tilting angle of 30°. We achieve such features by a proper formulation of sol–gel matrix containing PDMS and 2 % FAS, with incorporation of 0.5–1 wt% alumina nanoparticle dispersion in water/solvent 1:1 ratio. Preferred particle size is about 100 nm with single peak distribution as measured by a Marvin Zetasizer. Surface morphologies of coatings are measured by atomic force microscopy technique. Coatings are applied by an air atomizing spraying process and thermally cured. Chemical analyses by XPS, FTIR, and SEM–EDX evidenced the designed composition with PDMS and fluoro-group on the coated surface. Accelerated weathering test proved the stability of the coating for at least 1000 h. Home-made single-drop ice tester (a) and the video-captured image of ice droplets on uncoated glass (b) and icephobic-coated glass during ice sliding down (c) at −20 °C.

Self-healing superhydrophobic polyvinylidene fluoride/Fe3O4@polypyrrole fiber with core–sheath structures for superior microwave absorption

Self-healing superhydrophobic polyvinylidene fluoride/Fe3O4@polypyrrole (F-PVDF/Fe3O4@PPyx) fibers with core–sheath structure were successfully fabricated by electrospinning of a PVDF/Fe3O4 mixture and in situ chemical oxidative polymerization of pyrrole, followed by chemical vapor deposition with fluoroalkyl silane. The F-PVDF/Fe3O4@PPy0.075 fiber film produces a superhydrophobic surface with self-healing behavior, which can repetitively and automatically restore superhydrophobicity when the surface is chemically damaged. Moreover, the maximum reflection loss (RL) of the F-PVDF/Fe3O4@PPy0.075 fiber film reaches −21.5 dB at 16.8 GHz and the RL below −10 dB is in the frequency range of 10.6–16.5 GHz with a thickness of 2.5 mm. The microwave absorption performance is attributed to the synergetic effect between dielectric loss and magnetic loss originating from PPy, PVDF and Fe3O4. As a consequence, preparing such F-PVDF/Fe3O4@PPyx fibers in this manner provides a simple and effective route to develop multi-functional microwave absorbing materials for practical applications.

Preparation of alumina membranes comprising a thin separation layer and a support with straight open pores for water desalination

A green tape was formed by phase inversion tape casting using an alumina slurry and a graphite slurry. After firing the graphite layer was burned out, and a ceramic membrane was obtained consisting of a two-layered structure: a skin layer of thickness ~8µm and a finger-like bulk layer of thickness ~730µm. The skin layer contained randomly distributed pores with diameter 0.8µm, and thus could act as a functional layer for separation, while the bulk layer contained finger-like straight open pores with diameter 10–100µm, acting as a support with low transport resistance. The overall porosity of the membrane was ~62%, as determined by the Archimedes method, while the porosity of the skin layer was ~25%, as determined by SEM image analysis. For comparison, another membrane was derived from only an alumina slurry resulting a three-layered structure: a finger-like bulk layer sandwiched by a low porosity layer at each side. The two membranes were modified with fluoroalkylsilane (FAS) to convert the surface from hydrophilic to hydrophobic. Water desalination was determined by exposing the FAS-modified skin layer side to a hot aqueous solution of 4wt% NaCl and sweeping the other side with dry nitrogen. A salt rejection of over 99% was attained for both membranes. A water permeation flux of 21Lm−2h−1 was obtained for the two-layered membrane with the NaCl feed solution of 90°C, which was twice as that of the three-layered membrane. The preparation method for the two-layered structure alumina membrane can be generalized for other membrane systems and porous materials.