Methods to determine surface energies of natural fibres: a review

To tailor the interaction across composite interfaces especially for the development of green composites, i.e. composites made completely from renewable materials, information about the fibre surfaces is required. We review the current state of the art of methods to determine the surface tension of natural fibres and discuss the advantages and disadvantages of techniques used. Although numerous techniques have been employed to characterise surface tension of natural fibres, it seems that commonly used wetting techniques are very much more affected by the non-ideal character of natural fibres. Inverse Gas Chromatography (IGC) is a much better suited technique to determine the surface energetic properties of natural fibres than wetting techniques. The surface tension of natural reinforcements, determined using IGC, was reported for nanosized bacterial cellulose as well as bamboo, cornhusk, flax, hemp and sisal, covering a wide range of cellulose content. The effect of methods to separate/extract fibres from the plants as well as of a few surface modification procedures on the fibre surface properties is also reviewed. The dispersive part of the natural fibre surface tension γ ^d _S varies from 32 to 61 mJ/m². The fibre surface tension increases with increasing cellulose content of natural fibres. We also found that a higher basicity (Donor Number, K _B to Acceptor Number, K _Aratio) was observed for fibres containing more cellulose. This may be reflective of higher crystalline cellulose content in the surfaces of the fibres, as only the ether linkage of the cellulose is labile for hydrogen bonding.     

Adsorption of sodium bis(2-ethylhexyl) sulfosuccinate and wettability in polytetrafluoroethylene-solution-air system

The role of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) adsorption at water-air and polytetrafluoroethylene-water (PTFE) interfaces in wetting of low energy PTFE was established from measurements of the contact angle of aqueous AOT solutions in PTFE-solution drop-air systems and the aqueous AOT solution surface tension measurements. For calculations of the adsorption at these interfaces the relationship between adhesion tension (γ_LV cos θ) and surface tension (γ_LV), and the Gibbs and Young equations were taken into account. On the basis of the measurements and calculations the slope of the γ_LV cos θ-γ_LV curve was found to be constant and equal −1 over the whole range of surfactant concentration in solution. It means that the amount of surfactant adsorbed at the PTFE-water interface, Γ_SL, is essentially equal to its amount adsorbed at water-air interface, Γ_LV. By extrapolating the linear dependence between γ_LV cos θ and γ_LV to cos θ = 1 the determined value of critical surface tension of PTFE surface wetting, γ_C, was obtained (23.6 mN/m), and it was higher than the surface tension of PTFE (20.24 mN/m). Using the value of PTFE surface tension and the measured surface tension of aqueous AOT solution in Young equation, the PTFE-solution interface tension, γ_SL, was also determined. The shape of the γ_SL-log C curve occurred to be similar to the isotherm of AOT adsorption at water-air interface, and a linear dependence existed between the PTFE-solution interfacial tension and polar component of aqueous AOT solution. The dependence was found to be established by the fact that the work of adhesion of AOT solution to the PTFE surface was practically constant amounting 46.31 mJ/m² which was close to the work of water adhesion to PTFE surface.     

The wettability and interfacial characterization between γ-TiAl alloy and ceramic reinforcements

The wettability and interfacial characterization of γ-TiAl alloy on TiC_0.78, TiN_0.87, and VNx substrates were studied using the Sessile Drop method at 1758 K. The equilibrium apparent contact angle of liquid γ-TiAl alloy are 8° on TiC_0.78 substrate, 22° on TiN_0.87, but deficient for VNx substrate because of the gradual appearance of solidification phenomena. The spreading mechanism of γ-TiAl/TiC_0.78 was ascribed to the product control model, which was determined by the new Ti₂AlC formed at the interface. The decreased apparent contact angle of γ-TiAl on TiN_0.87 substrate with the slow rate resulted from the combined effect of Ti adsorption at the interface and the decreased surface tension and viscosity of the liquid. The γ-TiAl/VNx system was a dissolution system. The effect of dissolution on apparent contact angle is reflected in the initial stage and progressed synchronously with the movement of the triple-phase line.     

The microdroplet test: experimental and finite element analysis of the dependance of failure mode on droplet shape

The microdroplet technique is usually designed as a fibre embedded in a drop of resin and subsequently pulled out while the drop is being supported by two knife edges, resulting in either debonding of the droplets from the fibres, or breakage of the fibres before debonding can occur. In this study, the microdroplet technique was performed using a platinum ring with a 40 μm hole instead of the usual two knife edges, giving an axisymmetric geometry, load and stress distribution. Glass/phenolic and glass/polyester composite systems were tested experimentally and subsequent finite element modelling studies were performed to assess the variation of droplet size, and contact angle between the droplet and fibre. It was found that contact angle is of major influence in the proposed failure model. This study characterizes the influence of the contact angle between the droplet and the fibre on the subsequent stress distribution in the microdroplet specimen.     

Atomic force microscopy characterization of the surface wettability of natural fibres

Natural fibres represent a readily available source of ecologically friendly and inexpensive reinforcement in composites with degradable thermoplastics, however chemical treatments of fibres are required to prepare feasible composites. It is desirable to characterize the surface wettability of fibres after chemical treatment as the polarity of cellulose-based fibres influences compatibility with a polymer matrix. Assessment of the surface wettability of natural fibres using conventional methods presents a challenge as the surfaces are morphologically and chemically heterogeneous, rough, and can be strongly wicking. In this work it is shown that under atmospheric conditions the adhesion force between an atomic force microscopy (AFM) tip and the fibre surface can estimate the water contact angle and surface wettability of the fibre. AFM adhesion force measurements are suitable for the more difficult surfaces of natural fibres and in addition allow for correlations between microstructural features and surface wettability characteristics.     

Determination of the Breakage and Wetting Parameters of Calcite and Their Correlations

This paper presents the breakage and wetting parameters of calcite mineral obtained experimentally and establishes a correlation between these characteristic parameters. The breakage parameter obtained from the different feed sizes of grinding is the specific rate of breakage (S_i). The wettability parameter, obtained from surface chemistry‐based processes such as contact angle measurements or flotation methods, is the critical surface tension of wetting of a solid or mineral (γ_c). Calcite mineral, studied for the determination of the above parameters and their correlations, was ground in a laboratory‐size ceramic ball mill with dry, wet and chemically aided grindings and tested extensively to determine the γ_c values by using a contact angle goniometer and a newly designed micro‐column flotation cell. The highest S_i value obtained was 0.35 min^?1 for sodium dodecyl sulfate (SDDS)‐aided grinding, and the lowest S_i value was 0.26 min^?1 for dry grinding of the ?600+425 feed in the mill. The γ_c values for calcite were obtained as 34.0–34.9 mN/m for SDDS‐treated calcite surfaces, 29.9–31.4 mN/m for sodium oleate‐treated surfaces and >72 mN/m for both dry and wet ground products whose surfaces were not treated chemically. Some correlations were established between the S_i and γ_c parameters; as the S_i increases, γ_c decreases, indicating that relatively more hydrophobic surfaces are broken faster for the largest sizes, resulting in higher S_i values with more fines (lower γ of B_{i, j}) in the finer size distribution region (i.e. ?150 μm).     

In vitro investigation of protein adsorption and platelet adhesion on inorganic biomaterial surfaces

The aim of this paper was to study the surface properties, protein adsorption and platelet adhesion behaviors of diamond-like carbon (DLC) and titanium (Ti) films. The surface energy and microstructures of these films were characterized by contact angle measurement and atomic force microscopy (AFM). A modified Coomassie brilliant blue (CBB) protein assay was used to study the amount of adsorbed proteins. Platelet adhesion was assessed by scanning electron microscopy (SEM). The AFM results show that the DLC film is smoother than Ti. Protein adsorption results from CBB protein assay show that the ratio of adsorbed albumin (Alb) to IgG (R_A/I) on DLC is larger than Ti, which coincide with the sequence of the ratio of interfacial tension between solid surface and Alb (γ_S,Alb) to interfacial tension between surface and IgG (γ_S,IgG) (γ_S,Alb/γ_S,IgG). The DLC film has a preferential adsorption for Alb. The results suggest that the ratio of γ_S,Alb/γ_S,IgG may indicate an Alb/IgG affinity ratio of materials. More platelets adhere on Ti film than on DLC, which may correspond to the surface roughness of materials. The conclusion is the blood compatibility of DLC seems to be better than Ti.     

Adsorption behavior of weak hydrophilic substances on low-energy surface in aqueous medium

Through the methods such as measurements of contact angle and surface tension, calculations of surface energy and interfacial interaction free energy, and four weak hydrophilic substances (WHS) were taken as research objects, some interesting conclusions were obtained as follow. In aqueous medium, the WHS give priority to adsorb on low-energy surface that is low polar or particularly non-polar. There is a clear corresponding relationship between the free energy and Lewis base component γ⁻ or the hydrophile index of low-energy surface, and the specific relationship is obtained. Finally, we find hydrophobic attractive force of the Lewis acid-base interaction is mainly responsible for the absorption of WHS on low-energy surface. In short, an initial insight into adsorption behavior of WHS on low-energy surface is demonstrated in this paper.     

平衡接触角对受热液滴在水平壁面上铺展特性的影响

壁面温度是影响壁面润湿性的重要外部条件. 为解决液滴铺展中三相接触线处应力集中问题, 已有研究多采用预置液膜假设, 但无法探究壁面温度对润湿性的影响. 本文针对受热液滴在固体壁面上的铺展过程, 基于润滑理论建立了演化模型, 通过数值模拟, 从平衡接触角角度分析了温度影响壁面润湿性及铺展过程的内部机理. 研究表明: 随温度梯度增大, 液滴所受Marangoni效应增强, 致使液滴向低温区的铺展速率加快; 铺展过程中, 位于高温区的接触线与液滴主体部分间形成一层薄液膜, 重力与热毛细力先后主导该区域的铺展; 当液-固或气-液界面张力对温度的敏感度高于另两个界面时, 低温区方向的平衡接触角不断增大, 使壁面润湿性恶化, 导致液滴铺展减慢; 而当气-固界面张力对温度的敏感度高于其他两个界面时, 低温区方向上的平衡接触角将减小, 由此改善壁面润湿性, 加快液滴铺展; 在温度影响壁面润湿性和液滴铺展过程中, 平衡接触角起关键作用.     

The role of adsorption of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide surfactants in wetting of polytetrafluoroethylene and poly(methyl methacrylate) surfaces

The role of adsorption of dodecylethyldimethylammonium bromide (C₁₂(EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB) at water-air and polytetrafluoroethylene (PTFE)-water and poly(methyl methacrylate) (PMMA)-water interface, in wetting of PTFE and PMMA surface, was established from the measured values of the contact angle (θ) of aqueous C₁₂(EDMAB) and BDDAB solutions in PTFE (PMMA)-solution drop-air system, and from the measured values of the surface tension of aqueous C₁₂(EDMAB) and BDDAB solutions. Adsorption of C₁₂(EDMAB) and BDDAB at water-air interface was determined earlier from the Gibbs equation. Adsorption at solid-water interface was deduced from the Lucassen-Reynders equation based on the relationship between adhesion tension (γ_LV cos θ) and surface tension (γ_LV). The slope of the γ_LV cos θ-γ_LV curve was found to be constant and equal to −1, and about −0.3 for PTFE and PMMA surface, respectively (in the case of both surfactant studied: C₁₂(EDMAB) and BDDAB, and in the whole range of surfactants concentration in solution). It means that the amount of the surfactant adsorbed at the PTFE-water interface, Γ_SL, was essentially equal to its amount adsorbed at water-air interface, Γ_LV. However, Γ_SL at the PMMA-water interface was about three times smaller as compared to that at water-air interface. By extrapolating the linear dependence between γ_LV cos θ-γ_LV and dependence between cos θ-γ_LV and cos θ = 1 we determined the value of the critical surface tension of PTFE and PMMA surface wetting, γ_c. The obtained values of γ_c for PTFE surface were equal 23.4 and 23.8 mN/m, 23.1 and 23.2 mN/m for C₁₂(EDMAB) and BDDAB, respectively and they were higher than the surface tension of PTFE (20.24 mN/m). On the other hand, the obtained values of γ_c for PMMA surface were equal 31.4 and 30.9 mN/m, 31.7 and 31.3 mN/m for C₁₂(EDMAB) and BDDAB, respectively and they were smaller than the surface tension of PMMA (39.21 mN/m). Using the values of PTFE and PMMA surface tension and the measured values of the surface tension of aqueous C₁₂(EDMAB) and BDDAB solutions in the Young equation, the PTFE (PMMA)-solution interfacial tension, γ_SL, was also determined. Next, the work of adhesion (W_A) was deduced, and it occurred that the dependence between the W_A and the surface tension (γ_LV) for both studied solids was linear. However, the values of the W_A for PMMA change as a function of log C (C—surfactant concentration) changed from 91.7 to 68.5 mJ/m² and from 91.8 to 65.1 mJ/m² for C₁₂(EDMAB) and BDDAB, respectively. On the other hand, the work of adhesion of both studied surfactants solutions to the PTFE surface was practically constant (an average value was equal 45.8 and 45.4 mJ/m², respectively). These values were close to the value of the work of water adhesion to PTFE surface (45.5 mJ/m²).     

激光-电子康普顿散射物理特性研究

对激光-电子康普顿散射物理特性即能量特性和微分截面角分布进行了仔细的研究.计算结果显示出光子能量和微分截面角分布的简单结构.康普顿散射X射线光源具有散射光子的能量易调节、方向性好等特点.在入射电子束能量很高时,X射线近乎单向出射.光源色散度较大,但实验上可以获得色散（带宽）小的X射线.对于各种波长的激光,在很宽的电子束能量范围（1 MeV—10 GeV）内,散射X射线光子的总截面和前向发射圆锥内（半圆锥角1/γ,其中γ=E/m₀ 关键词： 康普顿散射 能量特性 微分截面 角分布     

The wettability of poly(tetrafluoroethylene) by aqueous solutions of ternary surfactant mixtures

Contact angle measurements on poly(tetrafluoroethylene) (PTFE) surface were carried out for the systems containing ternary mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycols), Triton X-100 (TX100) and Triton X-165 (TX165). The aqueous solution of ternary surfactant mixtures were prepared by adding the third surfactant to the binary mixture of the surfactants where the synergetic effect in the reduction of the surface tension of water was determined, to compare the influence of the third surfactants on the values of surface tension of this binary mixture and the values of the contact angle on PTFE. The obtained results and calculations indicate that the ternary mixtures of CTAB + TX165 (αCTAB = 0.2, γ_LV = 60 and 50 mN/m) + TX100 (C = 10⁻⁸ to 10⁻² M) have the biggest efficiency of the reduction of contact angle of water on PTFE in comparison to aqueous solutions of the single surfactants and their binary and ternary mixtures. Also in the case of all studied ternary mixtures of surfactants at concentrations of the bulk phase corresponding to unsaturated monolayer at water-air interface the adsorption of surfactants at PTFE-water interface is different than that at water-air interface, but is the same at concentrations near the critical micelle concentration (CMC). Thus the linear dependences between γ_LV cos θ − γ_LV and cos θ − 1/γ_LV, in the range of concentration studied for all systems confirm the same adsorption at two interfaces only at C near the CMC.     

Superhydrophobic cotton fabric coating based on a complex layer of silica nanoparticles and perfluorooctylated quaternary ammonium silane coupling agent

A superhydrophobic complex coating for cotton fabrics based on silica nanoparticles and perfluorooctylated quaternary ammonium silane coupling agent (PFSC) was reported in this article. The complex thin film was prepared through a sol-gel process using cotton fabrics as a substrate. Silica nanoparticles in the coating made the textile surface much rougher, and perfluorooctylated quaternary ammonium silane coupling agent on the top layer of the surface lowered the surface free energy. Textiles coated with this coating showed excellent water repellent property, and water contact angle (CA) increased from 133° on cotton fabrics treated with pure PFSC without silica sol pretreatment up to 145°. The oil repellency was also improved and the contact angle of CH₂I₂ droplet on the fabric surface reached to 131°. In contrast, the contact angle of CH₂I₂ on the fabric surface treated with pure PFSC was only 125°.     

Fabrication of a superhydrophobic ZnO nanorod array film on cotton fabrics via a wet chemical route and hydrophobic modification

A superhydrophobic ZnO nanorod array film on cotton substrate was fabricated via a wet chemical route and subsequent modification with a layer of n-dodecyltrimethoxysilane (DTMS). The as-obtained cotton sample was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), scanning probe microscope (SPM) and X-ray photoelectron spectroscopy (XPS), respectively. The wettability of the cotton fabric sample was also studied by contact angle measurements. The modified cotton fabrics exhibited superhydrophobicity with a contact angle of 161° for 8 μL water droplet and a roll-off angle of 9° for 40 μL water droplet. It was shown that the proper surface roughness and the lower surface energy both played important roles in creating the superhydrophobic surface, in which the Cassie state dominated.     

Multifunctional TiO2‐Based Particles: The Effect of Fluorination Degree and Liquid Surface Tension on Wetting Behavior

A systematic investigation into the influence of the degree of fluorination on the static and dynamic wetting behavior of TiO₂‐based nanobelt (TNB) particles with various liquids is described. The effect of the degree of fluorination and the surface tension of the liquid on the occurrence and stability of liquid marbles, foams or dispersions are studied and the wetting behavior and arrangement of particles at the air–liquid surface are observed. Using contact angle (θ) measurements, the relation between the type of particle‐stabilized material and θ is established. For liquids of relatively high tension like water or formamide which do not wet the fluorinated particles, a powder‐like material (marble) is formed. For polar oils of intermediate tension (35–50 mN m^?1), which partially wet the fluorinated particles, stable air‐in‐oil foams can be prepared in which particles form a close‐packed layer enveloping air bubbles. Liquids of relatively low tension, e.g., ethanol or polydimethylsiloxane, wet the particles forming a uniform dispersion and partial sedimentation. By contrast, the as‐prepared hydrophilic TNB particles are rapidly wetted by all the liquids as expected due to their high surface energy. The stable cross‐stacked TNB particles with fluoroalkylsilane (FAS) modification could be a versatile platform in a wide range of applications, especially for fluidic devices (e.g., biofluids, gas sensing, and lab‐on‐a‐chip devices). In a proof‐of‐concept study, the oil–water separation performance of fabrics with chemically stable TNB/FAS coating and the liquid isolation by a TNB/FAS shell for highly sensitive gas sensing or reagent assays are investigated.     

Surface energies of metals in both liquid and solid states

Although during the last years one has seen a number of systematic studies of the surface energies of metals, the aim and the scientific meaning of this research is to establish a simple and a straightforward theoretical model to calculate accurately the mechanical and the thermodynamic properties of metal surfaces due to their important application in materials processes and in the understanding of a wide range of surface phenomena. Through extensive theoretical calculations of the surface tension of most of the liquid metals, we found that the fraction of broken bonds in liquid metals (f) is constant which is equal to 0.287. Using our estimated f value, the surface tension (γ_m), surface energy (γ_SV), surface excess entropy (−dγ/dT), surface excess enthalpy (H_s), coefficient of thermal expansion (α_m and α_b), sound velocity (c_m) and its temperature coefficient (−dc/dT) have been calculated for more than sixty metals. The results of the calculated quantities agree well with available experimental data.     

Surface tension (γLV), surface energy (γSV) and crystal-melt interfacial energy (γSL) of metals

The results of temperature-dependent surface tension calculations of pure liquids aluminium (933-1200 K) and iron (1811-2500 K), in the framework of the theoretical considerations suggested by Eyring, are presented. It is observed that the surface tension decreases linearly with temperature. The calculated surface tension data are fitted as γ = 985-0.275(T − Tm) and γ = 1560-0.387(T − Tm) for Al and Fe, respectively. Moreover, the surface tension (γ_LV) at melting point, surface energy (γ_SV) and crystal-melt interfacial energy (γ_SL) are calculated for many metals. The agreement between the calculated and the reported measured values is reasonable.     

Water adsorption on hydrogenated Si(1 1 1) surfaces

The adsorption of water on the hydrogen terminated Si(1 1 1) surface is studied by means of first-principles calculations as well as contact angle measurements. Possible initial adsorption configurations for single water molecules and the potential energy surface are calculated. Only small adsorption energies of the order of meV are predicted. Calculations for higher coverage show that the water-water interactions are stronger than the water-surface bonding. The contact angle formed between a water droplet on the surface approximated from the total-energy calculations amounts to 88°, while our measured value is 91°.     

Estimating critical surface tension from droplet spreading area

Critical surface tension (CST) is a measure of solid surface tension and is mainly determined by measuring the contact angle of a droplet on a target solid surface. The concept of CST makes it possible to determine solid surface tension without any unprovable assumptions such as the Fowkes hypothesis. However, it requires somewhat special devices and skills for measuring the contact angle. In this work, we propose a simple method to determine the CST of a solid by measuring the droplet spreading area. This method is developed by combining the conventional CST with a simple analytical droplet model. The difference in estimated CSTs between our method and the conventional one is within 3.0%. Our method enables a quick and simple evaluation of the solid surface tension without special devices for measuring the contact angle.     

In situ detection of flavonoids in weld‐dyed wool and silk textiles by surface‐enhanced Raman scattering

Luteolin and apigenin flavonoid have been detected in silk and wool fibres dyed with weld (Reseda luteola L.) through surface‐enhanced Raman scattering (SERS) measurements carried out ‘on the fibre’. For such purpose, Ag nanoparticles were produced and immobilised in situ via the laser photoreduction of a silver nitrate water solution in contact with the fibre. Control SERS spectra of pure luteolin and apigenin, as well as of mixtures of them, on analogous Ag nanoparticles were also obtained. In this work flavonoids with a similar molecular structure were identified on dyed fibres for the first time without previously hydrolysing the mordant–dye complex. Copyright © 2008 John Wiley & Sons, Ltd.