Weak Boundary Layer Research Articles

Surface Migration of Fatty Acid to Improve Sliding Properties of Hypromellose-Based Coatings

Hypromellose (HM) is a cellulose-derived polymer of pharmaceutical grade that forms easily from thin films and coatings. As few studies concern HM-formulated systems, this study focuses on the formulation of HM films by incorporating a fatty acid additive, making it possible to control surface properties such as wetting and slip behavior for pharmaceutical or medical applications. The results show that the addition of a very small amount (from 0.1 to 1% w/w) of fatty acid additive reduces HM film affinity for water and water vapor transmission rate, while film appearance and gloss are rather preserved. Surface properties were probed using wettability measurements, Tapping Mode AFM, ATR-FTIR spectrometry, and friction measurements. Tapping Mode AFM images show that the surface roughness reduces by up to 65%. Wettability results show that the surface energy decreases from 43 to 31 mJ.m−2, whereas surface FTIR spectrometry measurements demonstrate that fatty acid molecules migrate on the surface of the formulated films, the driving force being the microphase separation between the polar HM macromolecules and the hydrophobic additive, leading to the formation of a weak boundary layer with poor cohesion. As a consequence, the surface coefficient of friction significantly reduces from 0.38 to 0.08, and fatty acid molecules thus act as a lubricant, improving the sliding properties of HM-based coatings.

Influence of shale reservoir properties on shale oil mobility and its mechanism

Given the tremendous potential for continental shale oil in China, many oilfields in the central and eastern parts of the country are involved in the exploration and development of shale oil resources. Besides engineering factors, shale oil mobility is the key to determining its commercial viability. This study explores the Hetaoyuan Formation in the Biyang Depression as an example to determine the influence of reservoir properties on the movable oil volume and its mechanisms. Multiple techniques were used, including displacement nuclear magnetic resonance (NMR), low-temperature nitrogen adsorption (LTNA), X-ray diffraction (XRD) bulk mineral analysis, and scanning electron microscopy (SEM), and the results suggest that large average pore diameter, high throat to pore ratio, single pore morphology, and small specific surface area can weaken the boundary layer effect and reduce the amount of adsorbed oil. Our observations reveal that compared to the dissolution pores and intergranular pores in brittle minerals, the intercrystalline pores in terrigenous clastic clay minerals are more affected by compaction. Furthermore, authigenic clay minerals notably block the intergranular pores in the interbedded sandstones. Clay minerals are identified as the main contributor to the specific surface area, with high clay mineral content enhancing the pore heterogeneity of the reservoir. Thus, positive shale oil mobility occurs in shale with a weak boundary layer effect, which is attributed to the high brittle mineral content, large average pore diameter, small specific surface area, single pore morphology, and reservoir homogeneity.

Asymptotic analysis for the homogeneous model of wind‐driven oceanic circulation in the small viscosity limit

In this paper, we study the asymptotic expansion for the homogeneous model of wind‐driven oceanic circulation with the nonslip boundary condition when both of the beta‐plane parameter and the Reynolds number go to infinity. By asymptotic analysis under certain constraints on wind tensor, we derive a formal expansion including the geophysical boundary layer in the large beta‐plane parameter and high Reynolds number limit, from which one sees that the external force, wind tensor, has an important effect on the behavior of the boundary layers. Moreover, when the external force and initial data satisfy certain constraints, to exclude the appearance of strong boundary layers, we construct approximate solutions with steady boundary layers to the homogeneous model of wind‐driven oceanic circulation in the large beta‐plane parameter and high Reynolds number limit. By using an energy method, we obtain the ‐stability of small perturbation around this approximate solutions, which verifies the validity of the asymptotic expansion of weak boundary layers in the large beta‐plane parameter and high Reynolds number limit.

The Influence of Low-Pressure Plasma Treatments on the Lap Shear Strength of Laser-Joined AISI 304 Hybrids with Polypropylene and Polyamide 6.6

This paper investigates if the polar groups induced by a plasma treatment can increase the lap shear strength of laser-joined metal and plastic hybrids. Optimal laser joining parameters for cold-rolled AISI304–polyamide 6.6 and sandblasted AISI304–polypropylene hybrids were developed at 2.85 MPa and 4.22 MPa, respectively. The surface free energy was doubled for all used plasma gases to a value of ca. 80 mN m−1 at 180 s. The plasma-treated samples were joined and tested. The arithmetic means of the plasma-treated hybrids’ lap shear strength with polyamide 6.6 varied slightly, but all measured values were within the range of the untreated samples. Residue on the sheared metal samples indicated covalent bonds between AISI304 and polyamide 6.6. The lap shear strengths of the plasma-treated polypropylene hybrids were significantly reduced between −30.8% and −53.3%, depending on the used plasma gas. This was attributed to the over-aging and development of low-molecular-weight oxidized materials, which led to a weak boundary layer. No residue of polypropylene was found on treated or untreated lap shear samples. No correlation between the surface free energy and lap shear strength could be found.

Weldability improvement of immiscible polycarbonate/GFRP by femtosecond laser surface treatment

Poor miscibility is an important reason for the non-weldable performance of most dissimilar polymers and their composites, and traditional surface treatments cannot effectively improve the weldability of dissimilar polymers. Therefore, the non-weldability of dissimilar polymers is one of the important factors that limit the application of polymers and their composites. We proposed a surface modification scheme of laser ablation or etching to improve the weldability of immiscible polymers effectively. Experimental studies on femtosecond ultraviolet (UV) laser ablation/etching, mid-infrared laser transmission welding (LTW), and related characterization and mechanical tests were carried out. The experimental results showed that laser ablation induced the deposition of abundant polar oxygen-containing groups on the surface of polycarbonate (PC), which improved the miscibility of PC and polyamide 66 with a glass fibre volume ratio of 30% (PA66GF30) and reduced the weak boundary layer (WBL), thereby improving their chemical bonding performance. Using the compatibilization mechanism induced by laser ablation, the bonding strength of the PC/PA66GF30 joint obtained by LTW was 13.7 MPa, which was much higher than that of the untreated joint (1.4 MPa). However, the grooves generated by laser etching could induce macro-anchoring effect of the hybrid joint. Based on the compatibilization mechanism and macro-anchoring effect, with the selection of appropriate welding power and etching parameters, the bonding strength of the PC/PA66GF30 joint was further increased to 29.7 MPa. This work has demonstrated the feasibility of laser surface treatment (especially etching) to significantly improve the welding strength of immiscible polymers and their composites.

Theoretical Background of Self-Healing in Asphalt Concrete

The article presents theoretical premises that can be used to describe the mechanism of self-healing of the state of the structure of asphalt concrete containing an encapsulated reducing agent. The article presents the main provisions of the Flory-Huggins theory, according to which the restorative effect can be realized due to the ability of organic binder molecules to spontaneously entangle with each other. The intensity of this process depends on the proximity of the molecules to each other and the rate of spontaneous motion, which increases with increasing temperature. And in accordance with the theory of adhesion, the restoration of the state of the structure of asphalt concrete can be due to the mechanisms of formation of adhesive bonds with the help of an encapsulated modifier. To describe the features of the formation of adhesive bonds during restoration, the approaches described in the mechanical, molecular, chemical, diffuse, relaxation theory of adhesion or the theory of weak boundary layers can be used.

Effect of mainstream oxygen concentration on supersonic film cooling and friction reduction performance using hydrocarbon fuel

One of the effective methods to simultaneously achieve the thermal protection and friction reduction requirement of a scramjet engine is the usage of fuel film cooling. As the combustion process inside the combustor proceeds, the oxygen concentration in the burnt gas gradually decreases and will influence the thermal and friction characteristics of the hydrocarbon film. Therefore, the effects of oxygen concentration in the mainstream on the hydrocarbon film are evaluated by numerical simulations based on RANS framework. The results indicate that the cooling performance of the hydrocarbon film is not sensitive to the oxygen concentration, although the fuel film burns more intense as the oxygen concentration increases. While the friction reduction performance is sensitive to the oxygen concentration. Specifically, about 10% friction reduction can be obtained for 10% oxygen in the mainstream compared to that of inert cases, and the wall friction can be further reduced by 30% for 20% oxygen. Further analysis reveals that the mechanism of the above two drag reductions is different. For weak boundary layer combustion, the decrease of the molecular viscosity contributes to the reduction of friction. While for significant boundary layer combustion, a lower near-wall velocity gradient is the main reason.

Effect of Flame Treatment on Bonding Performance of GF/EP Pultrusion Sheets Used for VARI Process

This paper presents an easy and low-cost flame treatment method to improve the bonding performance of GF/EP (Glass Fiber-Reinforced Epoxy) pultrusion plates, which are using widely for large size wind blades. In order to explore the effect of flame treatment on the bonding performance of the precast GF/EP pultruded sheet vs. the infusion plate, the GF/EP pultruded sheets were treated with different flame treatment cycles and were embedded in the fiber fabrics during the vacuum-assisted resin infusion process (VARI). The bonding shear strengths were measured by tensile shear tests. It is found that after 1, 3, 5, and 7 flame treatments, the tensile shear strength between the GF/EP pultrusion plate and infusion plate increased by 8.0%, 13.3%, 22.44%, and -2.1%, respectively. This indicates that the maximum tensile shear strength can be obtained after five times of flame treatment. In addition, DCB and ENF tests were also adopted to characterize the fracture toughness of the bonding interface with the optimal flame treatment. It is found that the optimal treatment gives increments of 21.84% and 78.36% for G I C and G II C, respectively. Finally, the surficial topography of the flame-treated GF/EP pultruded sheets were characterized by optical microscopy, SEM, contact angle test, FTIR, and XPS. The results show that flame treatment plays an impact on the interfacial performance through the combination of physical meshing locking and chemical bonding mechanism. Proper flame treatment would remove the weak boundary layer and mold release agent on the surface of the GF/EP pultruded sheet, etch the bonding surface and improve the oxygen-containing polar groups, such as C-O and O-C=O, to improve the surface roughness and surface tension coefficient of pultruded sheet to enhance the bonding performance. Excessive flame treatment destroys the integrity of epoxy matrix on bonding surface which results into the exposure of the glass fiber, and the carbonization of release agent and resin on the surface loosen the surficial structure, which reduces the bonding properties.

Singular limits for the Navier-Stokes-Poisson equations of the viscous plasma with the strong density boundary layer

The quasi-neutral limit of the Navier-Stokes-Poisson system modeling a viscous plasma with vanishing viscosity coefficients in the half-space $$\mathbb{R}_+^3$$ is rigorously proved under a Navier-slip boundary condition for velocity and the Dirichlet boundary condition for electric potential. This is achieved by establishing the nonlinear stability of the approximation solutions involving the strong boundary layer in density and electric potential, which comes from the breakdown of the quasi-neutrality near the boundary, and dealing with the difficulty of the interaction of this strong boundary layer with the weak boundary layer of the velocity field.

Direct adhesion between Cu foil and polytetrafluoroethylene without increasing surface roughness for high-frequency printed wiring boards.

Polytetrafluoroethylene (PTFE) serves as a suitable dielectric substrate for high-frequency printed wiring boards (PWBs) owing to its excellent properties at high frequency. However, to the best of our knowledge, no study has investigated the strong adhesion between PTFE and Cu foil with low surface roughness. Therefore, in this study, pure-PTFE comprising a weak boundary layer (WBL) on the surface and glass-cloth-containing PTFE (GC-PTFE), which did not contain a WBL, were subjected to heat-assisted plasma (HAP) treatment. Thereafter, we investigated the surface chemical bonding state, surface morphology, and adhesion properties of the as-prepared PTFE toward Cu foil with low surface roughness. As observed, oxygen-containing functional groups were generated on the HAP-treated PTFE, and the WBL in the as-received pure-PTFE was eliminated via HAP treatment. Moreover, the surface roughness of the HAP-treated PTFE did not increase compared to that of as-received PTFE. After performing thermal compression under atmospheric conditions, the adhesion strength of both HAP-treated pure-PTFE and GC-PTFE was ∼0.9 N mm-1. In addition, the adhesion strength of Cu/pure-PTFE and Cu/GC-PTFE increased after thermal compression under a reduced pressure, and the adhesion strength of 1 N mm-1 was obtained. Although the Cu foil was not roughened, Cu/PTFE realized strong adhesive strength. The developed method is advantageous because maintaining a low interface roughness is crucial for applying PTFE to manufacture high-frequency PWBs.

The role of helicity and fire–atmosphere turbulent energy transport in potential wildfire behaviour

Background Understanding near-surface fire–atmosphere interactions at turbulence scale is fundamental for predicting fire spread behaviour. Aims This study aims to investigate the fire–atmosphere interaction and the accompanying energy transport processes within the convective boundary layer. Methods Three groups of large eddy simulations representing common ranges of convective boundary layer conditions and fire intensities were used to examine how ambient buoyancy-induced atmospheric turbulence impacts fire region energy transport. Key results In a relatively weak convective boundary layer, the fire-induced buoyancy force could impose substantial changes to the near-surface atmospheric turbulence and cause an anticorrelation of the helicity between the ambient atmosphere and the fire-induced flow. Fire-induced impact became much smaller in a stronger convective environment, with ambient atmospheric flow maintaining coherent structures across the fire heating region. A high-efficiency heat transport zone above the fire line was found in all fire cases. The work also found counter-gradient transport zones of both momentum and heat in fire cases in the weak convective boundary layer group. Conclusions We conclude that fire region energy transport can be affected by convective boundary layer conditions. Implications Ambient atmospheric turbulence can impact fire behaviour through the energy transport process. The counter-gradient transport might also indicate the existence of strong buoyancy-induced mixing processes.

Evaluation of the interactions of typical wood extracts on the bonding performance of soybean-based adhesives

The substitution of environmentally friendly, renewable soybean-based adhesives (SBAs) for formaldehyde-based synthetic resins to produce formaldehyde-free wood composites is of great significance for resource recycling and environmental protection. However, SBA-bonded wood composites often exhibit great variations in bonding properties, due to the diverse interactions of the extractive components in wood. In this study, the extracts of four typical commercial fast-growing timbers were isolated, and their effects on the structures and properties of the SBAs were systematically investigated using FTIR, NMR, TGA, contact angle, buffer capacity, sol–gel test, and plywood evaluation. The analytical data indicated that some of the wood extracts could cause important effects on the bonding properties of the SBAs by impacting the PAE cross-linking capacity and unfolding degree of the globular soybean proteins. Fast-growing hardwood extracts from poplar and eucalyptus timbers containing active groups resulted in increased cross-linking density and improved water resistance of the SBAs. Meanwhile, fast-growing softwood extracts from Chinese fir and Scotch pine containing inert hydrophobic components had slight impacts on PAE cross-linking, but resulted in the agglomeration of unfolded soybean proteins. This formed a weak boundary layer on the wood surface, causing a significant decrease in the water resistance of the SBAs by >35%. These findings presented available analytic approaches of wood extract–SBA interactions and useful guidance for the proper applications of SBAs in the commercial production of wood composites.

Adhesion in Bitumen/Aggregate System: Adhesion Mechanism and Test Methods

A literature review of the five main theories describing the interaction mechanisms in the bitumen/aggregate system was conducted: theory of weak boundary layers, mechanical theory, electrostatic theory, chemical bonding theory, and thermodynamic theory (adsorption theory). The adhesion assessment methods in the bitumen/aggregate system are described, which can be divided into three main groups: determination of adhesion forces for bitumen with different materials, determination of bitumen resistance to the exfoliating action of water with different materials, and determination of adhesion as a fundamental value (contact angle measurements, interfacial fracture energy, adsorption capacity and others). It is proposed to evaluate the quality of adhesive interaction in the bitumen/aggregate system in two stages. The authors recommend using the adhesion determination methods for these two stages from the second group of methods the determination of bitumen resistance to the exfoliating action of water with different materials. In the first stage, the adhesion in the bitumen/aggregate system is determined by an accelerated technique in which the used bitumen binder and mineral material are considered as test materials. After the first stage, there are positive results in the second tests on compacted mixtures (indirect tensile strength test, Modified Lottman indirect tension test, immersion-compression test, and Hamburg wheel tracking test).

Enhancing the Wettability of Polyetheretherketone (Peek) Membrane with Ozone for Improving Fuel Cell Performance

Ozone was reacted with the aromatic membrane polyetheretherketone (PEEK) to form oxidized functional groups on the surface to enhance the attraction and transport of protons in fuel cells. Ozonation of unsaturated C-C sp<sup>2</sup> bonds in PEEK formed a primary ozonide which dissociated to primarily produce O=C-O/O=C-OH moieties, and the root mean squared roughness factor (R<sub>q</sub>) decreased from 7.4 nm, for the untreated sample, down to 3.1 nm. The oxidation of the surface and decrease in surface roughness made the surface increase in hydrophilicity as observed by the decrease in the water contact angle (CA) from 80.3° for untreated PEEK down to 21.7°. Washing the treated surface with solvent decreased the O at % on the surface indicating the formation of a weak boundary layer because of bond breakage during the decomposition of the ozonide.

The effect of weak boundary layers on adhesion properties of laser pretreated aluminum alloy EN-AW 6082 surfaces

This study investigates the influence of the laser ablation regimes and the material removal mechanisms on the morphologies, topographies, and chemistries of laser formed surface structures on the aluminum alloy EN-AW 6082. The characteristics of these surface structures are correlated to the adhesion and durability of joints bonded adhesively with PA6. X-ray photoelectron spectroscopy showed that the chosen laser ablation regimes have almost no influence on surface chemistries while microscopic analysis indicated that the surface structures are affected significantly. The adhesion properties and durability of the adhesively bonded joints studied by single lap shear and aging tests are strongly affected by the ablation regimes. Optimal initial shear strengths and aging resistances were obtained for a single step laser treatment with parameters corresponding to the melt displacement mode, while multi-step treatments provided no further benefits. A failure analysis revealed that the micro- and nanostructures developed in melt ejection regime of laser ablation can act as weak boundary layers (WBLs) on metal substrates and affect adversely the adhesion and durability of the joints.

Measurement and Analysis of the Microphysical Properties of Arctic Precipitation Showing Frequent Occurrence of Riming

AbstractDetailed ground‐based observations of snow are scarce in remote regions, such as the Arctic. Here, Multi‐Angle Snowflake Camera measurements of over 55,000 solid hydrometeors—obtained during a two‐year period from August 2016 to August 2018 at Oliktok Point, Alaska—are analyzed and compared to similar measurements from an earlier experiment at Alta, Utah. In general, distributions of hydrometeor fall speed, fall orientation, aspect ratio, flatness, and complexity (i.e., riming degree) were observed to be very similar between the two locations, except that Arctic hydrometeors tended to be smaller. In total, the slope parameter defining a negative exponential of the size distribution was approximately 50% steeper in the Arctic as at Alta. Sixty‐six percent of particles were observed to be rimed or moderately rimed with some suggestion that riming is favored by weak boundary layer stability. On average, the fall speed of rimed particles was not notably different from aggregates. However, graupel density and fall speed increase as cloud temperatures approach the melting point.

Vacuum UV (VUV) Photo-Oxidation of Polyethersulfone (PES)

International need for water quality is placing a high demand on separation technology to develop advanced oxidative processes for polyethersulfone (PES) membranes to help improve water purification. Therefore, VUV photo-oxidation with a low pressure Ar plasma was studied to improve the hydrophilicity of PES by flowing oxygen over the surface during treatment. X-ray photoelectron spectroscopy (XPS) detected a decrease in the C at% (4.4 ± 1.7 at%), increase in O at% (3.7 ± 1.0 at%), and a constant S at% (5.4 ± 0.2 at%). Curve fitting of the XPS spectra showed a decrease in sp2 C-C aromatic group bonding, and an increase in C-O, C-S, O=C-OH, sulphonate (-SO3) and sulphate (-SO4) functional groups with treatment time. The water contact angle decreased from 71.9° for untreated PES down to a saturation level of 41.9° with treatment. Since scanning electron microscopy (SEM) showed no major changes in surface roughness, the increase in hydrophilicity was mainly due to oxidation of the surface. Washing the VUV photo-oxidized PES samples with water or ethanol increased the water contact angle saturation level up to 66° indicating the formation of a weak boundary layer.

Adhesive-Free Adhesion between Plasma-Treated Glass-Cloth-Containing Polytetrafluoroethylene (GC-PTFE) and Stainless Steel: Comparison between GC-PTFE and Pure PTFE.

In this study, the effect of plasma treatment on glass-cloth-containing polytetrafluoroethylene (GC–PTFE) was investigated. Previous plasma studies investigated pure PTFE (which does not contain glass cloth) but not GC–PTFE. The effect of Ar + H2O plasma treatment on GC–PTFE was investigated. The Ar + H2O plasma-treated GC–PTFE sheets were thermally compressed to stainless steel (SUS304) foils without using adhesive, and the GC–PTFE/SUS304 adhesion strengths were measured using a 90° peel test. The adhesion strength increased with the increase in the plasma treatment time (0.8 and 1.0 N/mm at 20 s and 300 s, respectively). Thus, strong adhesion between GC–PTFE/SUS304 was achieved without adhesive. This improvement in the adhesion properties of GC–PTFE can be attributed to the generation of oxygen-containing functional groups and the decrease in the surface roughness of the samples. Thereafter, the adhesion properties of GC–PTFE and pure PTFE were compared. Because, unlike pure PTFE, GC–PTFE has no weak boundary layer, GC–PTFE exhibited better adhesion properties than pure PTFE under short plasma treatment times.

Uniformly Convergent Scheme for Singularly Perturbed Space Delay Parabolic Differential Equation with Discontinuous Convection Coefficient and Source Term

A singularly perturbed delay parabolic problem of convection‐diffusion type with a discontinuous convection coefficient and source term is examined. In the problem, strong interior layers and weak boundary layers are exhibited due to a large delay in the spatial variable and discontinuity of convection coefficient and source. The problem is discretized by a nonstandard finite difference scheme in the spatial variable and for the time derivative, we used the Crank–Nicolson scheme. To enhance the order of convergence of the spatial variable, the Richardson extrapolation technique is applied. The error analysis of the proposed scheme was carried out and proved that the scheme is uniformly convergent of second order in both spatial and temporal variables. Numerical experiments are performed to verify the theoretical estimates.

Boundary layer solutions to singularly perturbed quasilinear systems

<p style='text-indent:20px;'>We consider weak boundary layer solutions to the singularly perturbed ODE systems of the type <inline-formula><tex-math id="M1">\begin{document}$ \varepsilon^2\left(A(x, u(x), \varepsilon)u'(x)\right)' = f(x, u(x), \varepsilon) $\end{document}</tex-math></inline-formula>. The new features are that we do not consider one scalar equation, but systems, that the systems are allowed to be quasilinear, and that the systems are spatially non-smooth. Although the results about existence, asymptotic behavior, local uniqueness and stability of boundary layer solutions are similar to those known for semilinear, scalar and smooth problems, there are at least three essential differences. First, the asymptotic convergence rates valid for smooth problems are not true anymore, in general, in the non-smooth case. Second, a specific local uniqueness condition from the scalar case is not sufficient anymore in the vectorial case. And third, the monotonicity condition, which is sufficient for stability of boundary layers in the scalar case, must be adjusted to the vectorial case.</p>