Solid Surface Free Energy Research Articles

Suppressing the Leidenfrost effect by air discharge assisted electrowetting-on-dielectrics

The Leidenfrost effect for a droplet on an over-heated substrate always results in a superhydrophobic state, significantly hindering the water evaporation for heat dissipation. Here, we demonstrate a strategy of air discharge assisted electrowetting-on-dielectrics (ADA-EWOD), overcoming this challenge. This strategy increases the solid surface free energy by generating air discharge near the three-phase contact line of the droplet and combines it with the electromechanical force to decrease the contact angle, which makes ADA-EWOD have stronger wetting capabilities than traditional electrically control methods that only rely on electromechanical force. The water contact angle on an over-heated surface (above 350 °C) is decreased from nearly 180° down to less than 10°. This superhydrophilicity at high temperature reduces the droplet lifetime by at least 10 times, well inhabiting the Leidenfrost effect. Furthermore, we use ADA-EWOD in droplet evaporation for heat dissipation, where a heated silicon wafer at 600 °C is cooled down to less than 200 °C within 20 s. We believe that the present work provides a perspective on suppressing the Leidenfrost effect, which may have important potential applications in the field of heat dissipation.

Kinetics and mechanism of ultrasonic enhanced separation of zinc from tannin germanium residue

Tannin germanium residue (TGR) is the main enrichment in the process of hydrometallurgical extraction of germanium (Ge). The removal effect of impurity zinc (Zn) from TGR greatly affects the purity of TGR and the subsequent production process. In this study, a new method of high efficiency separation of impurity Zn from TGR by ultrasonic intensification was proposed. The results of single factor experiment showed that under the optimal purification conditions of ultrasonic frequency of 20 kHz, ultrasonic power of 225 W, acid concentration of 20 g/L, liquid-solid ratio of 7.5:1, reaction temperature of 40 ℃ and reaction time of 30 min, the removal efficiency of impurity Zn from TGR was 88.57%, which is 5.22% higher than the conventional purification method. The kinetic analysis based on the shrinking core model indicated that the removal of impurity Zn in TGR was controlled by the diffusion step, and the apparent activation energy of the reaction was reduced by the introduction of ultrasonic. The microscopic mechanism of ultrasonic action showed that the ultrasonic increased the surface free energy of solids and improved the wettability of the purified reagent to solids. When the ultrasonic power was 225 W, the ultrasonic can not only destroy the impurity inclusions and break the solid reactants into small particles, but also avoid the secondary agglomeration of small particles.

From Raw to Finished Cotton—Characterization by Interface Phenomena

Abstract Interface phenomena that occur at the solid–liquid interface, such as wettability, adsorption, and particle aggregation, depend on the kind and magnitude of the solid surface free energy and electrokinetic properties found in water solutions. These phenomena are crucial for textile dyeing, finishing, and care. They characterize the material surface and change with different material pretreatment and finishing. In this paper, electrokinetic potential, isoelectric point, point of zero charge, a specific amount of surface charge and surface free energy of raw, enzymatically scoured, bleached, and finished cotton fabrics were investigated. Electrokinetic potential was measured by a streaming potential method and a specific quantity of surface charge by the back-titration method. For determination of the solid surface free energy components, the thin-layer wicking and contact-angle methods were used. On the basis of these results, components of solid surface free energy were calculated and discussed.

Physicochemical Modifications on Thin Films of Poly(Ethylene Terephthalate) and Its Nanocomposite with Expanded Graphite Nanostructured by Ultraviolet and Infrared Femtosecond Laser Irradiation

In this work, the formation of laser-induced periodic surface structures (LIPSS) on the surfaces of thin films of poly(ethylene terephthalate) (PET) and PET reinforced with expanded graphite (EG) was studied. Laser irradiation was carried out by ultraviolet (265 nm) and near-infrared (795 nm) femtosecond laser pulses, and LIPSS were formed in both materials. In all cases, LIPSS had a period close to the irradiation wavelength and were formed parallel to the polarization of the laser beam, although, in the case of UV irradiation, differences in the formation range were observed due to the different thermal properties of the neat polymer in comparison to the composite. To monitor the modification of the physicochemical properties of the surfaces after irradiation as a function of the laser wavelength and of the presence of the filler, different techniques were used. Contact angle measurements were carried out using different reference liquids to measure the wettability and the solid surface free energies. The initially hydrophilic surfaces became more hydrophilic after ultraviolet irradiation, while they evolved to become hydrophobic under near-infrared laser irradiation. The values of the surface free energy components showed changes after nanostructuring, mainly in the polar component. Additionally, for UV-irradiated surfaces, adhesion, determined by the colloidal probe technique, increased, while, for NIR irradiation, adhesion decreased. Finally, nanomechanical properties were measured by the PeakForce Quantitative Nanomechanical Mapping method, obtaining maps of elastic modulus, adhesion, and deformation. The results showed an increase in the elastic modulus in the PET/EG, confirming the reinforcing action of the EG in the polymer matrix. Additionally, an increase in the elastic modulus was observed after LIPSS formation.

Self-stratification studies in waterborne epoxy-silicone systems

Waterborne epoxy‑silicone coating formulations are prepared by combining selected ingredients in optimum quantities to produce ~250–275 μm films (dry film thickness, DFT) applied on polyester, sandblasted steel, smooth steel, acrylic, polypropylene and aluminum substrates. The self-stratification of the applied coatings is then evaluated using Fourier Transform Infrared spectroscopy – Attenuated Total Reflectance (FTIR–ATR) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray (EDX) analysis. The difference in the absorption spectra of the top and bottom surfaces of these films as well as a significant difference in topography seen from the SEM images and elemental mapping through an EDX analysis of the cross-section has confirmed the occurrence of stratification on polyester substrate. The observed stratification results are subsequently compared with a theoretical model where the primary mechanism driving the separation is assumed to be the surface free energy difference of the resins and their respective wetting of the substrate. The influence of using two different theories, Wu's Harmonic Mean Method and Owens-Wendt Method for the interfacial surface tensions and the solid surface free energy computations, on the predictions from this model is also tested. The theoretical predictions support the observed results for most cases of the formulated waterborne systems; except for the case of sandblasted steel, the observed results do not match the predictions. The possible reasons for the difference in between the prediction and observation for this specific case has also been elucidated.

Study on surface wettability of nickel coating prepared by electrodeposition combined with chemical etching

Nickel coatings with different surface characteristics were prepared on copper substrate by electrodeposition. Subsequently, the as-coating surface was etched with a mixed solution of FeCl3, HCl and H2O2 at different times, to obtain different surface morphologies and special microstructures. The surface morphologies, element composition, roughness and liquid contact angle were characterized by scanning electron microscope, energy dispersive X-Ray spectroscopy, 3D digital microscope and contact angle measurement. The van Oss-Chaudhury-Good method was used to calculate the solid surface free energy and discuss the variation of surface wettability. The results show that etching time has a significant effect on surface morphology, roughness and wettability. After long-time physical aging, the multiple polygonal-layered architectures and larger-sized layered microstructure can maintain high surface free energy and keep the liquid metal in a good hydrophilic state, which provides a reference for optimizing the surface morphologies of the casting roll. This will improve the heat transfer efficiency and promote heterogeneous nucleation during twin-roll continuous casting.

Physico–mechanical properties of poly(ethylene‐co‐vinyl acetate)/acrylonitrile‐butadiene rubber/multi‐walled carbon nanotubes nanocomposites

AbstractThe influence of multi‐walled carbon nanotubes (MWCNTs) on thermal, mechanical and wetting properties as well as rheological behavior of poly(ethylene‐co‐vinyl acetate) (EVA)/acrylonitrile‐butadiene rubber nanocomposites were studied. The solid surface free energy (), work of adhesion (), interfacial free energy (), spreading coefficient () and Girifalco‐Good's interaction parameter () were calculated for the materials fabricated. The results showed that the contact angle and decreased; however, , , , and increased with the increase in MWCNTs content. The maximum tensile strength and toughness were obtained for the nanocomposite containing 1 wt% nanofillers. It was also found that both of the relaxation ratio (Rr) and relaxation rate index (n) decreased with increasing in the carbon nanotubes (CNTs) concentration. The results designated that the values of Rr and n for the nanocomposite containing 7 wt% MWCNTs were 19% and 60% smaller than those of the unfilled blend, respectively. The Casson plot analysis indicated that the yield stress of the molten materials incremented with the increase in MWCNTs loading according to a quadratic equation. The analysis of crystallization exotherms and melting endotherms revealed that CNTs triggered the crystallization process at slightly higher temperature, however, melting point slightly and the rate of nucleation noticeably reduced with the increase in the nanofillers content.

Effects of chemical composition on the hydrophobicity and antifouling performance of epoxy-based self-stratifying nanocomposite coatings

Inspired by nature, the superhydrophobic surface has great potential for application in the field of antifouling due to its excellent fouling resistance. Nanocomposite coatings are an economical way to fabricate superhydrophobic surfaces by controlling the surface chemical composition and topography. However, the available open literature assessing the chemical composition effects on hydrophobicity and antifouling performance is rare. In this study, epoxy-based nanocomposite coatings with various wettability were fabricated by using the air-spray technique. The element distributions of the coatings at different depths were characterised by EDS and XPS. The results showed that the top coating surfaces of ER-F, ZnF40-1:5, and ZnF40-1:9 were enriched in F and Si elements, evidencing the self-stratifying. And by reducing the solid surface free energy, the hydrophobicity of ER-F, ZnF40-1:5, and ZnF40-1:9 was significantly improved. And the coating ZnF40-1:9 achieved super-hydrophobic. Based on the Young's equation and the “equation of state for the interfacial tensions”, it was found that the effect of solid surface free energy on CAH was greater than that on WCA due to the greater coefficient of the variable. The superhydrophobic coating ZnF40-1:9 shows great antifouling performance against marine diatom P. tricornutum and its antifouling rate is up to 94.1%. By reducing the surface free energy, the hydrophobicity of nanocomposite coatings can be improved significantly, consequently improving the antifouling performance effectively. These findings might provide insight into developing superhydrophobic nanocomposite coatings to inhibit biofouling.

Surface and Interface Investigations of Matrix–Fillers in Heterogeneous Amorphous Semiconductors

Phosphate glass-based composites are prone to be as effective as amorphous semiconductors, enhancing the glass properties by the addition of a small amount of metallic fillers (Cr, Co, Ni, and Zn) and leading to the creation of composite materials where the conductive particles can be distributed in the glass matrix PbO–P2O5 (PbP) at the micrometer level. This paper deals with scanning electron microscopy (SEM) examination and the wetting behavior of the phosphate glass–metal powder composites. We focused on the filler effect on wetting characteristics, such as interfacial free energy. The change in the contact angle of water on the glass surface was also measured. Scanning electron microscopy images of the composites showed a good dispersion in the fillers within the glass matrix. The contact angles of the composites with water and three polar and apolar solvents were calculated. The total solid surface free energy was analyzed. The interaction parameter between the composites and the liquid has been calculated using Owens–Wendt equation.

Direct determination of Lennard-Jones crystal surface free energy by a computational cleavage method

The surface free energy of solids, γ, plays a crucial role in all physical and chemical processes involving material surfaces. For the first time, we obtained γ directly from molecular dynamics simulations using a crystal cleavage method. The approach was successfully realized in a Lennard-Jones system by inserting two movable external walls, each consisting of a single crystal layer, into a bulk crystal to create flat, defect-free surfaces. The cleavage technique designed allowed us to calculate the surface free energy according to its definition and avoid surface premelting. The temperature dependence of γ was determined for the (100) and (110) crystal planes along the whole sublimation line and its metastable extension, up to T = 1.02 · Tm, where Tm is the melting point. Good agreement with indirect values of γ(T) was found. The proposed computational cleavage method can be applied to other solids of interest, providing valuable insight into the understanding of chemical and physical surface processes, and demonstrates the successful import of the cleavage method, traditionally used in technical preparation and study of crystal surfaces, into a modern atomistic simulation.

Conversion of Polymer Surfaces into Nonwetting Substrates for Liquid Metal Applications.

Liquid metal-based applications are limited by the wetting nature of polymers toward surface-oxidized gallium-based liquid metals. This work demonstrates that a 120 s CF4/O2 plasma treatment of polymer surfaces-such as poly(dimethylsiloxane) (PDMS), SU8, S1813, and polyimide-converts these previously wetting surfaces to nonwetting surfaces for gallium-based liquid metals. Static and advancing contact angles of all plasma-treated surfaces are >150°, and receding contact angles are >140°, with contact angle hysteresis in the range of 8.2-10.7°, collectively indicating lyophobic behavior. This lyophobic behavior is attributed to the plasma simultaneously fluorinating the surface while creating sub-micron scale roughness. X-ray photoelectron spectroscopy (XPS) results show a large presence of fluorine at the surface, indicating fluorination of surface methyl groups, while atomic force microscopy (AFM) results show that plasma-treated surfaces have an order of magnitude greater surface roughness than pristine surfaces, indicating a Cassie-Baxter state, which suggests that surface roughness is the primary cause of the nonwetting property, with surface chemistry making a smaller contribution. Solid surface free energy values for all plasma-treated surfaces were found to be generally lower than the pristine surfaces, indicating that this process can be used to make similar classes of polymers nonwetting to gallium-based liquid metals.

Assessment of spring waters from Lourdes (France) by contact angle method

The geochemical investigations on karst spring waters in the area of Lourdes showed differences between the waters for numerous components, including main solutes and trace elements. In this study, a multidisciplinary approach was proposed, based on the surface free energy of solids and the surface tension of liquids using the contact angle method, by raising the hypothesis that different spring waters diverge in surface tension characteristics, which allow to discriminate waters origin in a natural system. Surface energy characteristics of spring waters from Batsurguère Valley were investigated here using the contact angle measured over time and the tangent method in relation to the variations of the droplet volume. Wettability of various test solids was analyzed using static and kinetic contact angle methods which led to the individuation of the spring waters on the base of the different adhesion processes occurring at the interface between water and solid, particularly on the silica glass solid. The analyses demonstrated that the surface tensiometry technique can discriminate one of such spring waters (i.e. the Massabielle spring) from other studied spring waters, regardless of their natural hydrological conditions. Contact angle hysteresis in kinetic modality, based on several repeated and extemporary set of experiment determinations, suggested a link between the water chemistry and the wettability over time of the test solids, confirming that chemical composition has an influence on the surface tension of spring waters. This investigation demonstrates that the surface tensiometry approach may represent a useful tool for monitoring quality of natural waters.

Biomimetic porous polypropylene foams with special wettability properties

Nowadays, the subject of wettability still has received little attention in the scientific literature, especially in the cellular solids, despite its significant role in the pervasiveness of the water treatment. Herein, inspired by the mussel, a green method was developed to construct nano-/micro-structures on the multi-structural polypropylene (like the dense solids, big hollow, small hollow and hierarchical open/closed-cell porous structure) by the surface-adherent polydopamine. Moreover, the relationship between both surface chemical and physical (such as pore structure and size) properties of the porous media, and wettability were studied. Besides, water–glycerol mixture solutions with various viscosities were used to study the spreading and imbibition behavior in the porous polypropylene foams, and the surface free energy of the different cellular solids (before and after modification) was estimated by Owens-Wendt equation. More interestingly, different kinds of the super-wetting materials like super-amphiphilic and hierarchical super-hydrophilic/super-hydrophobic, etc., were successful prepared. Both fundamental theoretical and numerical analysis were also conducted to reveal the mechanism of the penetration or surface repellency toward a drop, and to predict the transport of fluids in porous media. We believe that this work may serve as a source of inspiration in designing other advanced functional materials.

Microscopic mechanism of ice nucleation: The effects of surface rough structure and wettability

It is significant to inhibit the formation of ice on solid surface under some occasions in industry. The origin of ice formation on solid surface is the heterogeneous ice nucleation, which has drawn a lot of research attention in the last decade. In this work, we investigated the effects of surface fraction, pillar height, and solid surface wettability on ice nucleation using molecular dynamics simulation. Compared with that of smooth surface, the ice nucleation rates on rough surface can be enhanced or inhibited depending on the groove size, which can be attributed to the status of water molecules confined between pillars. In addition, it is found that the ice nucleation rate can be significantly weakened by reducing the solid surface free energy. More importantly, when the surface free energy is low enough, a quenching-induced mobility of nanodroplet is observed, including wandering on the solid surface or jumping away. These findings can address the importance of the coupling effect of the gap width and height of pillars, together with the quenching-induced mobility, which we believe will be helpful to understand more molecular mechanisms of heterogeneous ice nucleation.

Laser-Assisted Tailoring of Surface Wettability - Fundamentals and Applications: A Critical Review

The control of ubiquitous wetting behavior that is determined by the surface free energy of the solid and the surface tension of the interacting liquid is an area of intense research, largely due to its applications in various sectors, ranging from healthcare to automotive industry. The failure and/or toxic nature of the conventionally employed chemical methods to engineer the surface free energy of solids paved the way for utilizing alternative techniques such as laser-based physical texturing of the surface to achieve the desired wettability behavior. This review provides insight into the implementation of lasers to engineer the surfaces of various kinds of materials and how the surface engineering manipulates their wetting characteristics based on the state of the art knowledge of the field. Various theories that explain the wetting behavior of droplets on the surfaces of homogeneous, heterogeneous, and complex structures are discussed with a special emphasis on the role of surface roughness. Further, the article focuses on the lasermatter interaction at different time scales and modification of the surfaces with different kinds of lasers (nano, pico, and femtosecond laser) and their applications. A critical analysis of the effects of experimental parameters such as laser fluence, repetition rate, laser wavelength, inter-pattern spacing on the experimentally observed wetting behavior in the recent literature is also presented. Emerging applications of laser structured surfaces in various fields, including biomedical, water harvesting, anti-bacterial, spectroscopic analysis, oil/water separation, etc. are discussed in detail.

Significance of the receding contact angle in the determination of surface free energy

<p>Surface free energy measurements of solids are a very important issue in various fields of science. Many functional, chemical and physical properties of a given material depend on its surface free energy. The basic method of the surface free energy determination are the contact angle measurements. There are several empirical methods useful to calculate the surface free energy of solids. They are based on the measurements of the contact angle of liquids with the defined surface tension. The aim of this paper is to examine the significance of the receding contact angle measurements in the determination of surface free energy of solids.</p>

Gibbs–Thomson effect on aluminum doping during trench-filling epitaxial growth of 4H-SiC

Diffusion equations were solved to examine how the aluminum concentration in 4H-SiC grown on a trenched substrate is influenced by the Gibbs−Thomson (GT) effect. The GT effect increases or decreases the equilibrium gas-phase concentration of growing species in the vicinity of the top or bottom of the trench, respectively, which results in a part of the incident growing species, excluding the central area of the trench, diffusing inward. After fitting activity coefficient of a conditional constituent AlC in solid solution AlSiC and surface free energy of 4H-SiC, we determined the aluminum fluxes toward the top and bottom in such a way that the flux ratio of aluminum to silicon equals the AlC mole fraction at the top () and bottom (), respectively. Calculated was lower than and almost agreed with the measured in filled trenches, showing the GT effect on AlC being weaker than that on SiC.

Evaluating uncertainty of surface free energy measurement by the van Oss-Chaudhury-Good method

Among the great number of methods for measuring the surface free energy of solids, all originate in contact angle measurements, which are carried out using various probe liquids, and employ certain relationships and assumptions. Contact angle measurement can be distorted by a number of factors that occur during the measurement and distort obtained values, and therefore must be accounted for. The present work describes the application of the van Oss-Chaudhury-Good approach in determining the surface free energy of aluminium sheets with the use of three probe liquids: water, diiodomethane and glycerol, with assumed values of the surface free energy components. The study employed direct measurement of the contact angle with the goniometer. Subsequently, measurement uncertainty with the van Oss-Chaudhury-Good method was evaluated, which revealed that the major source of uncertainty is the discrepancy between constant values describing properties of probe liquids.

How solid surface free energy determines coalescence-induced nanodroplet jumping: A molecular dynamics investigation

Molecular dynamics simulation is performed to investigate how solid surface free energy determines the coalescence-induced jumping of nanodroplet on superhydrophobic surfaces. The nanodroplet-jumping is found highly sensitive to the solid surface free energy represented by the fluid-solid bonding strength parameter β. The coalesced-nanodroplet fails to jump off the surface when β is 0.15 (contact angle being 145°) while succeeds to jump off the surface when β is 0.05 (contact angle being 175°). We find that a small proportion (ca. 2%–4%) of the surface free energy released in both cases is eventually converted to kinetic energy in the jumping direction, which is in the same order as the conversion efficiency previously predicted for microdroplets. A lower solid surface free energy decreases viscous and interfacial dissipation and hence increases the kinetic energy converted and eventually leads to sufficient kinetic energy in the jumping direction for nanodroplet to jump up. Our results also address the importance of the liquid-solid interaction in the coalescence-induced jumping of nanodroplets and the determination of the minimum size of jumping nanodroplets.

Interpreting contact angle results under air, water and oil for the same surfaces

Under-water and under-oil superhydropobicity and superhydrophilicity have gained significant attention over the last few years. In this study, contact angles on five flat surfaces (polypropylene, poly(methyl methacrylate), polycarbonate, TEFLON-FEP and glass slide) were measured in water drop-in-air, air bubble-under-water, oil drop-in-air, air bubble-under-oil, oil drop-under-water and water drop-under-oil conditions. Heptane, octane, nonane, decane, dodecane, and hexadecane hydrocarbons were used as oils. Immiscible water/oil pairs were previously mutually saturated to provide thermodynamical equilibrium conditions and their surface and interfacial tensions were determined experimentally. These pairs were used in the two-liquid contact angle measurements. Surface free energies of the solid surfaces in air were determined independently by using the van Oss–Good method, using the contact angle results of pure water, ethylene glycol, formamide, methylene iodide and α-bromonaphalene. In addition, Zisman’s ‘critical surface tension’ values were also determined for comparison. In theory, the summation of contact angle results in a complementary case would give a total of 180° for ideal surfaces. However, it was determined that there are large deviations from this rule in practical cases and these deviations depend on surface free energies of solids. Three complementary cases of (water-in-air with air bubble-under-water); (oil-in-air with air bubble-under-oil); and (oil-under-water with water-under-oil) were investigated in particular to determine the deviations from ideality. A novel approach, named ‘complementary hysteresis’ [γWA(cosθ1 − cosθ2) and γOW(cosθ6 − cosθ5)] was developed where γWA and γOW represent the interfacial tensions of water/air and oil/water, and θ1, θ2, θ5, and θ6 were the contact angles of water/air, air bubble/water, oil/water and water/oil respectively. It was experimentally determined that complementary hysteresis varies almost linearly with the surface free energy of the flat solid samples. This is the first report showing the relation of the surface free energy of a solid which is determined under-air with the contact angles obtained on the same solid in different three-phase systems.