Macroscopic Wetting Research Articles

Influence of the water content in polyamide 6 on atmospheric pressure plasma jet pre-treatment and adhesion for adhesive bonding

AbstractTo quantify the influence of absorbed water in PA6 on the pre-treatment and bonding process, an unfilled and unreinforced PA6 material is investigated in a dried and saturated state. The material is pre-treated by atmospheric pressure plasma jet (APPJ) with varying jet distances. The surfaces are investigated by contact angle measurements, DSC and FTIR to detect molecular and morphological changes in the surface. To evaluate the bonding strength, samples are bonded with a two-component polyurethane adhesive and a two-component acrylate adhesive and tested in a lap shear and a tensile configuration. The results show that water content has a significant influence on the effectivity of the pre-treatment process and the resulting bonding strength and failure mechanism. The adhesion is majorly affected, however these effects do not influence the macroscopic wetting behavior and cannot be measured in contact angles. FTIR spectra and DSC scans do not show significant changes in molecular groups or crystallinity that would explain the observed adhesion improvement in dried samples. High bonding strength is only achieved with adherents at low water content.

Experimental and molecular dynamics simulation of the effect of surfactant concentration on the wettability of bituminous coal

Currently, studies on the mechanism of coal dust wetting have largely ignored the key factor of surfactant concentration, so it is of significant scientific value to systematically investigate the effect of concentration change on the efficacy of surfactant wetting of coal dust. In this paper, with the help of macroscopic wettability experiments and molecular dynamics simulations, the wetting characteristics of SDBS surfactant on coal at different concentrations were analyzed in depth. An increase in the concentration of the SDBS surfactant solution causes the wettability of the coal to show a crest change that increases and then decreases. Coal wettability is optimized when the solution concentration reaches a specific threshold of 7 wt%. The simulation results show that SDBS surfactant mostly forms hydrogen bonds with water molecules in the form of strong hydrogen bonds, and the hydrogen bonding ability with water molecules is stronger than that of coal. There is a positive correlation between the onset distribution of water molecules within coal and the overlap region at the coal/water interface. The dynamic properties of water molecules show the order E<F<D<C<B<A. The number of highly active sites does not continuously increase with the amount of SDBS molecules adsorbed, but begins to decrease after a critical point is reached. The results of the study contribute to a theoretical framework to guide and use different types of surfactants to improve coal dust wettability at various concentration levels.

Building Functional Liquid-Based Interfaces: From Mechanism to Application.

Functional liquid-based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid-based interfaces and provide a perspective on future research directions.

Building Functional Liquid‐Based Interfaces: From Mechanism to Application

AbstractFunctional liquid‐based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid‐based interfaces and provide a perspective on future research directions.

Analysis of the hydration water on the surface of human hair using a combination of infrared absorption vibrational spectroscopy and multivariate curve resolution

Modern society's keen regard for aesthetics made hair products an integral part of a multi‐billion‐dollar cosmetic industry. Hair care products (e.g., shampoos and conditioners) and chemical treatments (e.g., bleaching and permanent waving) result in various effects on the morphological attributes of hair. Generally, water adsorbed on the hair surface is known to significantly dictate the hair's mechanical characteristics (smoothness and friction), and hair's macroscopic wettability has been commonly used to indicate its surface properties. However, an approach to selectively characterize the hydration water in the hair surface is required to accurately understand the intermolecular events between the hair and its vicinal water. In this paper, we successfully obtained the infrared (IR) absorption spectra of the hydration water of human hair. We employed the multivariate curve resolution‐alternating least square (MCR‐ALS) method to separate the hydration and bulk water spectra from the whole spectra. Comparing the IR spectra of the hydration water of chemically untreated and bleached hair samples, we conclude that water molecules form strong hydrogen bonds with the bleached hair surface due to the destruction of the hair's hydrophobic outer layer and the consequent formation of hydrophilic residues.

Real-time three-dimensional micro-imaging of liquid front in spreading sessile droplet under transient spreading states.

Dynamic wetting behaviors of sessile droplets on substrates play crucial roles for various industrial chemical processes. In the case of complete wetting, it has been proposed that a precursor film which is nanometer-order thickness and micrometer-order length further expands outside the macroscopic contact line of the sessile droplet. While the time evolution of the precursor film is believed to strikingly affect the macroscopic wetting behavior (e.g., spreading velocity), it had been hard to visualize the three-dimensional shape of the precursor film at the early stage of wetting. Further, although the spreading velocity rapidly decreases upon time at the early stage of the wetting (transient state) and then converged to be a constant at later time (steady state), conventional fluid mechanics theories generally describe only the wetting behavior at the steady state. Therefore, experimental observation of time evolution of the precursor film shape in the transient state is essential to proceed the theories to the next step. Here, a monochromatic laser interference microscope was developed to visualize three-dimensional shape of the wetting front of sessile droplets in real time. By detecting an interference of the laser reflected at the liquid and substrate surfaces, the precursor film was successfully visualized with a time resolution of 20ms and a thickness resolution of about 3.5nm at worst. For 10 cSt silicone oil on Si substrate, a 60-nm-thick and 70-μm-long precursor film was observed for 2-10min after dropping and its spreading velocity which decreased with time was quantitatively analyzed.

Wetting of typical silica materials by molten Sn3.5Ag-(2, 4)Ti at 800 °C

Wetting of molten SAT(Sn3.5Ag-(2, 4)Ti) alloys on typical silica substrates (vitreous silica, sintered quartz, translucent fused silica and microlite glass) and on the surface of the interfacial precipitation layer was investigated by a modified sessile drop method at 800 °C under high vacuum. After cooling the wetting couples, both TiO2-x and a small amount of Ti5Si3 were precipitated at the interface and Sn5Ti6 - segregation above TiO2-x layer was observed. However, this does not indicate that the precipitated TiO2-x determines the final wettability, rather the isothermal wetting process in which Ti is segregated and adsorbed at the solid/liquid interface, significantly improving the wettability. Among them, vitreous silica and microlite glass in one group; sintered quartz and translucent fused silica in another group, have the similar interfacial microstructures, which linked to the difference in macroscopic wettability and apparent spreading dynamics.

Influence of rock permeability and surface conditioning on carbonate wettability: A link between contact angle and Amott-index

The wettability of crude oil/brine/rock system is of great importance in determining oil recovery during water displacement processes. The wetting behavior of rocks has traditionally been assessed via a range of experimental techniques such as the Amott tests and contact angle measurements. While the Amott wettability is known to be influenced by a range of factors, still the impact of rock permeability, rocks surface/saturation condition and initial water saturation remains unclear. Furthermore, a link between Amott wettability and contact angle is missing which is vital to characterize macroscopic vs. core scale wettability. Therefore, in this study, we investigated the effect of permeability, rocks surface/saturation condition and initial water saturation on Amott tests and contact angle measurements using Indiana limestone. We further attempt to experimentally explore the relationship between the contact angle and Amott-Harvey wettability index (IAH) for different rocks. We demonstrated that, in addition to surface texture (roughness) and surface chemistry (heterogeneity), rock surface wetting behavior depends also on the rock permeability and surface/saturation condition (Dry,Sw=1,Sor,SwirrandSo=1). Moreover, a comparison of the observed values for the Amott and contact angles, showed that only when the rock has moderate to high permeability (up to 15.17 mD) will the Amott index corresponds to the wetting state estimated using contact angle method. The influence of several factors on wettability via IAH and contact angle is evident. While permeability demonstrated a positive correlation with wettability, Sor exhibited a negative correlation. For water-wet rocks, contact angle showed weak positive correlation with IAH, while negative correlations were observed in the case of intermediate- and oil-wet rocks. In addition, exceptions were found in the correlations; unlike the IAH, contact angle is weakly correlated with both Sor and Swi obtained from the imbibition experiments. Lastly, spontaneous imbibition time becomes slower and tends to underestimate IAH as rock permeability decreases. The findings from this study will thus broaden the understanding of the interplay of different petrophysical parameters and the relationship between different wetting indices.

Controllable construction of the underwater superoleophobic surface from superhydrophilicity to superhydrophobicity in oil

Superwetting that is being subdivided to multiple superlyphobic states exists unique superiority in a specific interfacial application. Fine controlling superwetting surface is lack of systematic research. Due to the harsh conditions on the creation of underoil superhydrophilicity and suerhydrophobicity in oil of underwater superoleophobic surface, so far controllable fabrication strategies of the underwater superoleophobic surfaces in oil still remain a challenge. Herein, underwater superoleophobic surfaces from superhydrophilicity to superhydrophobicity in oil are controllable constructed by mixing commercial titanium dioxide nanoparticles, aluminum phosphate, and n-Alkyl acids, where the surface heterogeneous chemistry is finely controlled by changing the chain length of n-alkanoic acids. Under the rational surface energy, underoil superhydrophilicity and underoil superhydrophobicity of the underwater superoleophobic surfaces can be efficiently regulated via simply adjusting the chain length of n-alkanoic acids. The under-liquid dual superlyophobic surface can be achieved when an appropriate parameter denoted chain length of n-Alkyl acids (n) is between 14 and 16. However, when the parameter (n) is to be within 4 and 6, the superhydrophilicity in oil of underwater superoleophobic surface can be realized. This finding can connect the molecular-level conformational transformation and the conversion of macroscopic surface alternative wettability, providing the molecular-level design principle of superwetting interfacial in under-liquid environments.

A new methodology for measuring solid/liquid interfacial energy

HypothesisThe interfacial energy γsl between a solid and a liquid designates the affinity between these two phases, and in turn, the macroscopic wettability of the surface by the fluid. This property is needed for precise control of fluid-transport phenomena that affect the operation/quality of commercial devices/products. Although several indirect or theoretical approaches can quantify the solid/liquid interfacial energy, no direct experimental procedure exists to measure this property for realistic (i.e. rough) surfaces. Makkonen hypothesized that the frictional resistance force per unit contact-line length is equal to the interfacial energy on smooth surfaces, which, however, are rarely found in practice. Consequently, the hypothesis that Makkonen’s assumption may also hold for rough surfaces (which are far more common in practice) arises naturally. If so, a reliable and simple experimental methodology of obtaining γsl for rough surfaces can be put forth. This is accomplished by performing dynamic contact-angle experiments on rough surfaces that quantify the relationship between the frictional resistance force per unit contact-line length acting on an advancing liquid (Fp,a) and the surface roughness in wetting configurations. ExperimentWe perform static and advancing contact-line experiments with aqueous and organic liquids on different hydrophilic surfaces (Al, Cu, Si) with varying Wenzel roughnesses in the range 1-2. These parameters are combined with the liquid’s known surface tension to determine Fp,a. FindingsFp,a rises linearly with the surface roughness. Analysis based on existing theories of wetting and contact-angle hysteresis reveals that the slope of Fp,a vs.Wenzel roughness is equal to the solid/liquid interfacial energy, which is thus determined experimentally with the present measurements. Interfacial energies obtained with this experimental approach are within 12% of theoretically predicted values for several solid/liquid pairs, thereby validating this methodology.

Fabrication of hierarchically textured aluminum-based superhydrophobic surfaces for anti-frosting application

Rational design of solid interfaces with an ability to prevent frost formation is crucial in many industrial systems such as refrigeration, air-conditioning, cryogenics, etc. In this work, we fabricated two hierarchically textured aluminum-based superhydrophobic surfaces by simple steps such as chemical etching and dip/spray coating. We used perfluorooctyltriethoxysilane (PFOTES) and commercial Glaco solution as a hydrophobic coating material. The fabricated substrates have shown excellent superhydrophobicity with a low contact angle hysteresis of below 5°. The frost formation on both the coatings was different despite having similar macroscopic wettability. We analyzed the surface topography and its effects on frost formation comprehensively. PFOTES coated substrate has shown a sweeping mode of drop drainage, whereas the Glaco coated substrate showed the droplet jumping mode. Rapid ejection of condensed microdroplets from the Glaco coated substrate is delayed the frost formation due to the significant reduction in the drop contact time. This study elucidates the dependence of nanostructured coatings over the microstructures during frost formation. The insights from this study are helpful for the fabrication of anti-frosting surfaces for industrial applications.

Hydrophilicity and Hydrophobicity Control of Plasma‐Treated Surfaces via Fractal Parameters (Adv. Mater. Interfaces 19/2021)

Wettability Control of Plasma-Treated Surfaces via Fractal Parameters Functional properties of nanocomposites are linked to their morphological characteristics. In article number 2100724, Cecilia Piferi, Claudia Riccardi, and co-workers demonstrate that polyethylene-terephthalate samples treated with oxygen plasmas display scale-invariant morphology that evolves as a function of treatment time and leads to super-hydrophilic properties. Fractal dimension is identified as a bridge between surface morphology at nano- and micro-scale and macroscopic wettability.

Relationship Between Zeta Potential and Wettability in Porous Media: Insights From a Simple Bundle of Capillary Tubes Model

Hypothesis & MotivationExperimental data suggest a relationship between the macroscopic zeta potential measured on intact rock samples and the sample wettability. However, there is no pore-scale model to quantify this relationship. MethodsWe consider the simplest representation of a rock pore space: a bundle of capillary tubes of varying size. Equations describing mass and charge transfer through a single capillary are derived and the macroscopic zeta potential and wettability determined by integrating over capillaries. Model predictions are tested against measured data yielding a good match. FindingsMixed- and oil-wet models return a macro-scale zeta potential that is a combination of the micro-scale zeta potential of mineral-brine and oil-brine interfaces and the relationship between macro-scale zeta potential and water saturation exhibits hysteresis. The model predicts a similar relationship between zeta potential and wettability to that observed in experimental data but does not provide a perfect match. Fitting the model to experimental data allows the oil-brine zeta potential to be estimated at conditions where it cannot be measured directly. Results suggest that positive values of the oil-brine zeta potential may be more common than previously thought with implications for surface complexation models and the design of controlled salinity waterflooding of oil reservoirs.

Dewetting of non-polar thin lubricating films underneath polar liquid drops on slippery surfaces

HypothesisThe stability of thin lubricating fluid-coated slippery surfaces depends on the surface energy of the underlying solid surface. High energy solid surfaces coated with thin lubricating oil lead to the dewetting of the oil films upon depositing aqueous drops on them. Hence such surfaces are very suitable to investigate dewetting of thick films (thickness > 500 nm), which otherwise is not possible using a conventional dewetting system. ExperimentsLubricating films of different thicknesses are coated on hydrophilic solid surfaces, and glycerol drops are deposited on them. Fluorescence imaging of lubricating films and macroscopic wetting behavior of glycerol drops are analyzed to understand the dewetting phenomenon. FindingsUnderneath lubricating films undergo initial thinning and subsequently dewet. The dewetting dynamics during hole nucleation and growth and the final pattern of the dewetted oil droplets depend strongly on the thickness of the lubricating films. Ultrathin films dewet spontaneously via homogeneous nucleation, whereas thicker films dewet via heterogeneous nucleation. During dewetting, the apparent contact angle and radius of glycerol drops follow universal scaling behavior.

Molecular Desorption by a Moving Contact Line.

The interaction of the contact line with topographical or chemical defects at the nanometer scale sets the macroscopic wetting properties of a liquid on a solid substrate. Based on specific atomic force microscopy (AFM) experiments, we demonstrate that molecules physically sorbed on a surface are removed by a dynamic contact line. The mechanism of molecules desorption is directly determined by the capillary force exerted at the contact line on the molecules. We also emphasize the potential of AFM to clearly decorrelate the effects of topographical and chemical defects and monitor, with a subsecond time resolution, the dynamics of molecules adsorption on a surface.

Plumbing the depths of the graphene wetting controversy

Plumbing the depths of the graphene wetting controversy

Directional Metastable Wetting Evolution of Droplets on Artificial Patterned Microcavity Surfaces

AbstractControllable wetting transition on artificial microtextured surfaces has significant applications in many industrial fields. In this work, the droplet spreading and the directional wetting transition are investigated on the substrate surface with the patterned microcavities. The results show that the macroscopic inward wetting transition from the periphery to the center of the droplet strongly depends on the mesoscopic sequential transition from Cassie to Wenzel state in the microcavities on the substrate surfaces. The semiquantitative relationship between sagging depth of meniscus in the microcavities and the droplet spreading velocity is set up by utilizing scaling‐law analysis in terms of mechanical equilibrium of the meniscus. This finding is expected to help to clarify the issues on the mechanism and main affecting factors of the wetting transition on the patterned surface.

Potential of Mean Force between Bare or Grafted Silica/Polystyrene Surfaces from Self-Consistent Field Theory.

We investigate single and opposing silica plates, either bare of grafted, in contact with vacuum or melt phases, using self-consistent field theory. Solid–polymer and solid–solid nonbonded interactions are described by means of a Hamaker potential, in conjunction with a ramp potential. The cohesive nonbonded interactions are described by the Sanchez-Lacombe or the Helfand free energy densities. We first build our thermodynamic reference by examining single surfaces, either bare or grafted, under various wetting conditions in terms of the corresponding contact angles, the macroscopic wetting functions (i.e., the work of cohesion, adhesion, spreading and immersion), the interfacial free energies and brush thickness. Subsequently, we derive the potential of mean force (PMF) of two approaching bare plates with melt between them, each time varying the wetting conditions. We then determine the PMF between two grafted silica plates separated by a molten polystyrene film. Allowing the grafting density and the molecular weight of grafted chains to vary between the two plates, we test how asymmetries existing in a real system could affect steric stabilization induced by the grafted chains. Additionally, we derive the PMF between two grafted surfaces in vacuum and determine how the equilibrium distance between the two grafted plates is influenced by their grafting density and the molecular weight of grafted chains. Finally, we provide design rules for the steric stabilization of opposing grafted surfaces (or fine nanoparticles) by taking account of the grafting density, the chain length of the grafted and matrix chains, and the asymmetry among the opposing surfaces.

Macroscopic and Microscopic Wettability of Graphene.

Interactions between water and graphene can be probed on a macroscopic level through wettability by measuring the water contact angle and on a microscopic level through water desorption kinetic studies using surface science methods. The contact angle studies of graphene pinpointed the critical role of sample preparation and measurement conditions in assessing the wettability of graphene. So far, studies of water desorption from graphene under the conditions of ultrahigh vacuum provided superior control over the environment but disregarded the importance of sample preparation. Here, we systematically examined the effect of the morphology of the growth substrate and of the transfer process on the macroscopic and microscopic wettability of graphene. Remarkably, the macroscopic wetting transparency of graphene does not always translate into microscopic wetting transparency, particularly in the case of an atomically defined Cu(111) substrate. Additionally, subtle differences in the type of substrates significantly alter the interactions between graphene and the first monolayer of adsorbed water but have a negligible effect on the apparent macroscopic wettability. This work looks into the correlations between the wetting properties of graphene, both on the macroscopic and microscopic scales, and highlights the importance of sample preparation in understanding the surface chemistry of graphene.

Epigenetic Formation of Clay Surface Properties: Experimental Modeling

Quaternary diluvium soil samples were tested for particles’ cohesion, contact angle, porosity and their geoengineering properties were compared. The effect of particle size, applied external pressure, hy dration and the presence of organomineral surfactants on the soil surface properties of a clay preparation was evaluated. These characteristics determine the strength of clay particles’ cohesion to each other and are important for the development of techniques for geoengineering management. The macroscopic wetting contact angle was estimated using the captive bubble method by the author’s technique. An external pressure in excess of 40 kPa applied to a clay preparation on a glass substrate results in more than one layer of clay particles due to cohesion. It was observed that the thickness of the clay layer retained on the preparations due to cohesion decreases during hydration. This decrease is more noticeable for samples with particles up to 45 μm compared to samples containing larger particles up to 250 μm. The positive effect of organomineral surfactants on the cohesion of clay particles has been established. The addition of these surfactants initially leads to an increase in wetting heterogeneity.