Weak Boundary Layer Research Articles

Drastic Improvement in Adhesion Property of Polytetrafluoroethylene (PTFE) via Heat-Assisted Plasma Treatment Using a Heater

The heating effect on the adhesion property of plasma-treated polytetrafluoroethylene (PTFE) was examined. For this purpose, a PTFE sheet was plasma-treated at atmospheric pressure while heating using a halogen heater. When plasma-treated at 8.3 W/cm2 without using the heater (Low-P), the surface temperature of Low-P was about 95 °C. In contrast, when plasma-treated at 8.3 W/cm2 while using the heater (Low-P+Heater), the surface temperature of Low-P+Heater was controlled to about 260 °C. Thermal compression of the plasma-treated PTFE with or without heating and isobutylene−isoprene rubber (IIR) was performed, and the adhesion strength of the IIR/PTFE assembly was measured via the T-peel test. The adhesion strengths of Low-P and Low-P+Heater were 0.12 and 2.3 N/mm, respectively. Cohesion failure of IIR occurred during the T-peel test because of its extremely high adhesion property. The surfaces of the plasma-treated PTFE with or without heating were investigated by the measurements of electron spin resonance, X-ray photoelectron spectroscopy, nanoindentation, scanning electron microscopy, and scanning probe microscopy. These results indicated that heating during plasma treatment promotes the etching of the weak boundary layer (WBL) of PTFE, resulting in a sharp increase in the adhesion property of PTFE.

Effect of Temperature/Humidity Treatment on Interfacial Reliability on Screen-Printed Ag / Polyimide for Advanced Embedded Packaging Technologies

Abstract The effect of temperature/humidity treatment conditions on the interfacial adhesion energy between a screen printed Ag film and a polyimide substrate was evaluated by using a 90° peel test. The measured peel strength values decrease from 254.7 N/m to 59.3 N/m after the temperature/humidity treatment at 85°CC/85% relative humidity for 500 h. X-ray photoelectron spectroscopy analysis of the peeled surfaces indicates that peeling occurs cohesively inside of the polyimide, which is closely related to both the decrease in the interfacial adhesion energy and the polyimide degradation due to weak boundary layer formation.

Mediated electrochemical oxidation of vulcanized polybutadiene/organoclay nanocomposite- characterization and kinetic study

Silver (II) was initially generated electrochemically in nitric acid. It was then used to oxidize the surface of vulcanized polybutadiene/organoclay nanocomposite (BRN) and the results were compared to those of polybutadiene rubber (BR). X-ray diffraction (XRD) pattern along with transmission electron microscopy (TEM) pictures proved intercalated organoclay/Cloisite 20A morphology in the BRN matrix. According to elemental analysis, surface oxygen content was increased markedly which is more pronounced for BR and promoted by oxidation time noticeably. The type of polymer and its bond strength can distinctly influence the extend of oxidation by silver(II). Intercalated organoclay obstructed oxidizing effect of silver (II) mainly by protecting polybutadiene chain bonds and partly by growing silver adsorption. Scanning electron microscopy (SEM) micrographs revealed that weak boundary layer (WBL) on the rubbers surface was clearly removed after oxidation. Atomic force microscopy (AFM) disclosed that surface roughness of both BR and BRN samples were drastically enhanced with a more prominent effect on BR. Although, surface roughness distribution was remained normal for BR it was changed to bimodal for the BRN after oxidation. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy showed different surface functional groups such as hydroxyl and carbonyl formed after oxidation with a higher value for BR. The rubber content was found to have a negligible impact on the rate of surface oxidation. Kinetic study revealed that the concentration of silver (II) decreased quickly (<300 s) upon contacting rubbers and the kinetic data were fitted well to the second order rate equation, Morris-Weber and Longmuir isotherm models with a superior oxidation rate constant for BR. Finally it can be concluded that organoclay escalated oxidation resistivity and diminished oxidation rate constant of BRN.

An hybrid initial value method for singularly perturbed delay differential equations with interior layers and weak boundary layer

Abstract In this paper, an hybrid initial value method on Shishkin mesh is suggested to solve singularly perturbed boundary value problem for second order ordinary delay differential equation with discontinuous convection coefficient and source term. In this method, the original problem of solving the second order differential equation is reduced to solving four first order differential equations. Among the four first order differential equations, three of them are singularly perturbed differential equations without delay and other one is a regular differential equation with a delay term. The singularly perturbed differential equations are solved by the second order hybrid finite difference schemes, whereas the delay differential equation is solved by the improved Euler method. An error estimate is derived by using the supremum norm and it is of almost second order convergence. Numerical results are provided to illustrate the theoretical results.

Mechanical properties of single-walled carbon nanotube reinforced polymer composites with varied interphase’s modulus and thickness: A finite element analysis study

The role of the interphase on mechanical performance of glassy polymer/single-walled carbon nanotube composites has been investigated by finite element (FE) method. The matrix and the interphase are modeled using continuum elements and the nanotube is analyzed by Timoshenko beam elements. Stress distribution and mechanical property of the nanocomposites are quantified as a function of the interphase’s modulus and thickness. For composites that include an interphase, the predicted moduli are comparable to the values calculated by the rule of mixtures, but are much lower than those of an interphase-free composite. For composites consisting of only the CNT and the matrix, the predicted modulus values are in good agreement with those computed by the rule of mixtures and with theoretical data reported in the literature, although the predicted values are considerably higher than those of real polymer/CNT composites. Using Griffin’s fracture analysis for dissimilar materials, we have proved that fracture stress of the weak boundary layer in the CNT/polymer interphase is lower than that of the CNT/polymer interface or of the matrix. The weak boundary layer, which is always existing in a CNT/polymer’s three dimensional interphase, is proposed as the main reason for the large discrepancy between Young’s moduli predicted by the model observed in this study and reported in the literature and those measured experimentally from real-world polymer/SWCNT composites.

Fatigue analysis of adhesive joints with laser treated substrates

Recent literature works focused on the analysis of laser irradiation on the strength of adhesive joints under quasi-static loading conditions. It has been demonstrated that laser surface preparation allows to remove impurity and weak boundary layers from the mating substrates and, depending on the energy density, it is also able to modify surface morphology promoting mechanical interlocking. In previous works, the authors assessed the effect of Yb-fiber laser ablation over the quasi-static strength and toughness, of aluminum and stainless steel adhesively bonded joints. The experimental results demonstrated the ability of laser irradiation to improve the mechanical properties of the joints. The aim of this work is to extend the scope of previous investigations to fatigue loading. Double Cantilever Beam (DCB) samples with laser treated aluminum substrates have been bonded with a two component epoxy adhesive. For comparison standard degreasing and grit blasting have been also deployed for samples preparation. The results have been compared in terms of cycles to failure and the fracture surfaces have been analyzed by means of Scanning Electron Microscopy (SEM) in order to investigate the mechanism of failure.

RACORO continental boundary layer cloud investigations: 3. Separation of parameterization biases single‐column model CAM5 simulations of shallow cumulus

AbstractClimatically important low‐level clouds are commonly misrepresented in climate models. The FAst‐physics System TEstbed and Research (FASTER) Project has constructed case studies from the Atmospheric Radiation Measurement Climate Research Facility's Southern Great Plain site during the RACORO aircraft campaign to facilitate research on model representation of boundary‐layer clouds. This paper focuses on using the single‐column Community Atmosphere Model version 5 (SCAM5) simulations of a multi‐day continental shallow cumulus case to identify specific parameterization causes of low‐cloud biases. Consistent model biases among the simulations driven by a set of alternative forcings suggest that uncertainty in the forcing plays only a relatively minor role. In‐depth analysis reveals that the model's shallow cumulus convection scheme tends to significantly under‐produce clouds during the times when shallow cumuli exist in the observations, while the deep convective and stratiform cloud schemes significantly over‐produce low‐level clouds throughout the day. The links between model biases and the underlying assumptions of the shallow cumulus scheme are further diagnosed with the aid of large‐eddy simulations and aircraft measurements, and by suppressing the triggering of the deep convection scheme. It is found that the weak boundary layer turbulence simulated is directly responsible for the weak cumulus activity and the simulated boundary layer stratiform clouds. Increased vertical and temporal resolutions are shown to lead to stronger boundary layer turbulence and reduction of low‐cloud biases.

Zero dissipation limit and stability of boundary layers for the heat conductive Boussinesq equations in a bounded domain

In this paper, we study the zero dissipation limit of the initial boundary-value problem of the multi-dimensional Boussinesq equations with viscosity and heat conductivity. Such equations are used as models for the motion of multi-dimensional incompressible fluids in atmospheric and oceanographic turbulence. In particular, they describe the thermal convection of an incompressible flow, and constitute the relations between the velocity field, the pressure and the local temperature. Under the Navier slip boundary condition in the velocity field and the thermal isolation boundary condition for the temperature, we prove the existence of weak amplitude characteristic boundary layers. Then, by a standard energy method, we prove theL2convergence of the solutions when both the viscosity and the heat conductivity coefficients tend to 0.

Treatment with Ar–O2 low-pressure plasma of vulcanized rubber sole containing noticeable amount of processing oils for improving adhesion to upper in shoe industry

Vulcanized rubber (L3 rubber) containing intentionally noticeable excess of processing oils in its formulation was treated with Argon–Oxygen (Ar–O2) (2:1, vol/vol) low-pressure (LP) plasma for achieving a satisfactory level of adhesion to waterborne polyurethane adhesive. The effectiveness of the Ar–O2 LP plasma treatment of L3 rubber depended on both the configuration of the plasma chamber shelves and the treatment time. Surface modifications were assessed by attenuated total reflectance-IR and X-ray photoelectron spectroscopy, contact angle measurements, and scanning electron microscopy. Ar–O2 LP plasma treatment in direct configuration provided the most effective surface modification of the L3 rubber, and the increase in the treatment time improved the extent of the surface modifications. However, even important surface modifications were produced by Ar–O2 LP plasma treatment, adhesion of treated L3 rubber was not improved due to the creation of weak boundary layer at the polyurethane–rubber interface after joint formation. Heating at 80 °C for 12 h of the as-received L3 rubber prior to Ar–O2 LP plasma treatment enhanced the extent of the surface modifications, and improved adhesion was obtained for Ar–O2 LP plasma treatment times higher than 600 s.

Improvement of adhesion and paint ability of EVA copolymers with different vinyl acetate contents by treatment with UV-ozone

The surface modifications produced by UV-ozone treatment of two ethylene-vinyl acetate (EVA) copolymers containing 12 and 20wt% vinyl acetate (EVA12 and EVA20 respectively) were studied. The treatment with UV-ozone improved the wettability of both EVAs due to the creation of new carbon–oxygen moieties. The extent of these modifications increased with increasing length of the treatment and the modifications produced in EVA20 were produced for shorter lengths of treatment. The UV-ozone treatment also created roughness and heterogeneities on the EVA surfaces. Whereas roughness formation prevailed on the UV-ozone treated EVA12, important ablation was dominant on the treated EVA20. T-peel strength values in joints made with polychloroprene adhesive increased when the EVAs were treated with UV-ozone. Short length of UV-ozone treatment (1min) produced higher T-peel strength in joints made with EVA20 whereas higher T-peel strength values in joints made with EVA12 were obtained after treatment for 5–7.5min in which a cohesive failure into a weak boundary layer on the treated EVA surface was found. Furthermore, the adhesion of UV-ozone treated EVA20 to acrylic paint increased. Finally, the ageing resistance of the treated EVA/polychloroprene adhesive joints was good and the surface modifications on the UV-ozone treated EVAs lasted for 24h after treatment at least.

Experimental study of turbulent oscillatory boundary layers in an oscillating water tunnel

A high-quality experimental study including a large number of tests which correspond to full-scale coastal boundary layer flows is conducted using an oscillating water tunnel for flow generations and a Particle Image Velocimetry system for velocity measurements. Tests are performed for sinusoidal, Stokes and forward-leaning waves over three fixed bottom roughness configurations, i.e. smooth, “sandpaper” and ceramic-marble bottoms. The experimental results suggest that the logarithmic profile can accurately represent the boundary layer flows in the very near-bottom region, so the log-profile fitting analysis can give highly accurate determinations of the theoretical bottom location and the bottom roughness. The first-harmonic velocities of both sinusoidal and nonlinear waves, as well as the second-harmonic velocities of nonlinear waves, exhibit similar patterns of vertical variation. Two dimensionless characteristic boundary layer thicknesses, the elevation of 1% velocity deficit and the elevation of maximum amplitude, are found to have power-law dependencies on the relative roughness for rough bottom tests. A weak boundary layer streaming embedded in nonlinear waves and a small but meaningful third-harmonic velocity embedded in sinusoidal waves are observed. They can be only explained by the effect of a time-varying turbulent eddy viscosity. The measured period-averaged vertical velocities suggest the presence of Prandtl's secondary flows of the second kind in the test channel. Among the three methods to infer bottom shear stress from velocity measurements, the Reynolds stress method underestimates shear stress due to missed turbulent eddies, and the momentum integral method also significantly underestimates bottom shear stress for rough bottom tests due to secondary flows, so only the log-profile fitting method is considered to yield the correct estimate. The obtained bottom shear stresses are analyzed to give the maximum and the first three harmonics, and the results are used to validate some existing theoretical models.

Geostrophic adjustment with gyroscopic waves: stably neutrally stratified fluid without the traditional approximation

AbstractWe examine nonlinear geostrophic adjustment in a rapidly rotating (small Rossby number Ro) stably neutrally stratified (SNS) fluid consisting of a stratified upper layer with $N&gt;f$ ($N$ is the buoyancy frequency, $f$ the Coriolis parameter) and a homogeneous lower layer, the density and other fields being continuous at the interface between the layers. The angular speed of rotation is non-parallel to gravity; the traditional and hydrostatic approximations are not used. The wave spectrum in the model consists of internal and gyroscopic waves. During the adjustment an arbitrary long-wave perturbation is split in a unique way into slow quasi-geostrophic (QG) and fast ageostrophic components with typical time scales $(Ro\, f)^{-1}$ and $f^{-1}$, respectively. The QG flow is governed by two coupled nonlinear equations of conservation of QG potential vorticity (PV) in the layers. The fast component is a sum of internal waves and inertial oscillations (long gyroscopic waves) confined to the homogeneous layer and modulated by an amplitude depending on coordinates and slow time. On times $t\sim (\, f\, Ro)^{-1}$ the slow component is not influenced by the fast one but the inertial oscillations amplitude is coupled to the QG flow and obeys an equation practically coinciding with that in the barotropic case (Reznik, J. Fluid Mech., vol. 743, 2014, pp. 585–605). A non-stationary boundary layer with large vertical gradients of horizontal velocity develops in the stratified layer near the interface to prevent penetration of the inertial oscillations into the stratified fluid; an analogous weaker boundary layer arises near the upper rigid lid. At large times the internal waves gradually decay because of dispersion and the resulting motion consists of the slow QG component and inertial oscillations confined to the barotropic lower layer.

Physical and optical properties of aerosols in a free tropospheric environment: Results from long-term observations over western trans-Himalayas

Simultaneous and collocated estimates of near-surface mass concentration of black carbon (BC), number size distribution (NSD) and total number concentration (NT) of composite aerosols, columnar spectral aerosol optical depth (AOD) and local meteorological parameters were made from Aug 2009 to July 2012 at the high altitude (4520 m amsl), remote location Hanle in western trans-Himalayas. Even though all the aerosol parameters remain quite low, large annual variations are seen in the monthly mean values of BC (25 ng m−3–181 ng m−3), total number concentrations of composite aerosols (628 cm−3–1500 cm−3) and AOD (0.05–0.16). Size segregated correlation analysis reveals that the BC mass contributes significantly to the size range of 200–400 nm of the submicron aerosol size spectra. The diurnal variation of BC mostly has been dampened; yet seems to be prominent during spring, showing the presence of weak boundary layer dynamics. In contrast, the diurnal fluctuations in total number concentration have been mostly controlled by the new particle formation events, leading to bursts of large concentrations of ultrafine particles, which subsequently undergo coagulation. Spectral dependence of AOD also shows large monthly variations, with the Angstrom exponent varying from ∼0.52 to 1.3, with a mean value of ∼0.95 ± 0.21. The vertical distribution of extinction coefficients, obtained from CALIPSO data, indicates the presence of elevated aerosol layers, attributed mainly to the influence of long range transport of aerosols from the west Asian and Indian desert region.

Microscopic analysis of the wood bond line using liquefied wood as adhesive

The bonding of beech (Fagus sylvatica L.) with liquefied wood (LW) causes deterioration of the wood surface, resulting in a high percentage of wood failure at a relatively low bond shear strength. Light microscopy, scanning electron microscopy, FT-IR micro-spectroscopy and elemental carbon, nitrogen and sulphur (CNS) analysis techniques were used to investigate the formation of such bonds. It was assumed that the degradation of lignin, hemicelluloses and parts of the cellulose occurred in the cells of the wood surface where the LW had been applied. At the elevated temperatures occurring during the bonding process, the deteriorated cells were carbonised to some extent. The weak boundary layer of the bond was determined to be a layer of delignified cells located between the zone of partly carbonised cells on the one side and the cells of the undamaged wood of the adherend on the other side. The bonds which formed during the bonding of wood with LW were found to be very untypical compared to bonds formed by synthetic wood adhesives. No adhesive film was formed, the adhesive-adherend interface was not clear and the cells of the adherend subsurface were damaged.

On Thermally Forced Circulations over Heated Terrain

Abstract A combination of analytical and numerical models is used to gain insight into the dynamics of thermally forced circulations over diurnally heated terrain. Solutions are obtained for two-layer flows (representing the boundary layer and the overlying free troposphere) over an isolated mountainlike heat source. A scaling based on the linearized Boussinesq system of equations is developed to quantify the strength of thermally forced updrafts and to identify three flow regimes, each with distinct dynamics and parameter sensitivities. This scaling closely matches corresponding numerical simulations in two of these regimes: the first characterized by a weakly stable boundary layer and significant background winds and the second by a strongly stable boundary layer. In the third regime, characterized by weak winds and weak boundary layer stability, this scaling is outperformed by a fundamentally different scaling based on thermodynamic heat engines. Within this regime, the inability of wind ventilation or static stability to diminish the buoyancy over the heat source leads to intense updrafts that are controlled by nonlinear dynamics. These nonlinearities create a positive feedback loop between the thermal forcing and vorticity that rapidly strengthens the circulation and contracts its central updraft into a narrow core. As the circulation intensifies under daytime heating, the warmest surface-based air is ventilated into the upper boundary layer, where it spreads laterally to occupy a broader area and, ultimately, restrain the circulation strength. The success demonstrated herein of simple theoretical models at predicting key aspects of thermally forced circulations offers hope for improved parameterization of related processes (e.g., convection initiation and aerosol venting) in large-scale models.

Influence of longitudinal wind on natural ventilation with vertical shaft in a road tunnel fire

The influence of longitudinal wind on natural ventilation with vertical shaft in a road tunnel fire was investigated numerically by Large Eddy Simulation. The smoke flow characteristics of a road tunnel fire under the combined function of longitudinal wind and stack effect of shaft were analyzed. Results show that the stack effect, plug-holing and boundary layer separation are the dominating factors on the natural ventilation performance. Plug-holing occurs at small longitudinal velocity and causes the reduction of exhaust effect. At high longitudinal velocity, the driving force of smoke exhausting is weak and obvious boundary layer separation occurs, resulting in a poor exhaust capacity. There is a critical value of longitudinal wind velocity, in which a better smoke exhausting effect can be obtained.

Application of dynamic global-coefficient subgrid-scale models to turbulent natural convection in an enclosed tall cavity

Large-eddy simulations examining natural convection in an enclosed cavity with the simultaneous presence of laminar, transitional, and turbulent flow regimes were conducted. The Rayleigh number based on height of the cavity is 4.6 × 1010. Different dynamic global-coefficient procedures to compute the Vreman [A. W. Vreman, “An eddy-viscosity subgrid-scale model for turbulent shear flow: Algebraic theory and applications,” Phys. Fluids 16, 3670 (2004)] model coefficient were implemented for the subgrid-scale tensors in both the momentum and energy equations. Based on comparison with experimental and existing numerical data, it is shown that the dynamic model derived from the “global equilibrium” hypothesis gives favorable results in the mean flow and turbulence quantities. Nevertheless, because of higher subgrid-scale dissipation, transition to a turbulent flow is postponed when the Vreman model coefficient is either uniform or determined dynamically using the Germano identity approach. This suggests that much finer grid is desired when using these models in order to better capture the weak transitional boundary layer. Further, by exploring the instantaneous flow dynamics, it is demonstrated that characteristics of the coherent structures which resemble streaks in forced convection boundary layers are somewhat dissimilar in the different models.

Boundary layer analysis of the Navier–Stokes equations with generalized Navier boundary conditions

We study the weak boundary layer phenomenon of the Navier–Stokes equations with generalized Navier friction boundary conditions, u⋅n=0, [S(u)n]tan+Au=0, in a bounded domain in R3 when the viscosity, ε>0, is small. Here, S(u) is the symmetric gradient of the velocity, u, and A is a type (1,1) tensor on the boundary. When A=αI we obtain Navier boundary conditions, and when A is the shape operator we obtain the conditions, u⋅n=(curlu)×n=0. By constructing an explicit corrector, we prove the convergence, as ε tends to zero, of the Navier–Stokes solutions to the Euler solution both in the natural energy norm and uniformly in time and space.

Kinetics and mechanism of formation of adhesive joints based on ethylene-vinyl acetate copolymers

The structure of transient zones in PVC-poly(ethylene-co-vinyl acetate), PET-poly(ethylene-co-vinyl acetate), and steel-poly(ethylene-co-vinyl acetate) adhesion systems is studied. It is shown that PET-poly(ethylene-co-vinyl acetate) and PVC-poly(ethylene-co-vinyl acetate) are related in incompatible and partially compatible systems, respectively. In the temperature range 100–180°C, diffusion coefficients and the activation energy of diffusion are determined. The depth of penetration of copolymer macromolecules into the PVC phase is calculated. The kinetics of adhesive-joint formation is studied. For all systems, the increase in joint strength has a common character; for each temperature, the steady state is attained. The results are analyzed in terms of wetting and diffusion theories. Both models satisfactorily describe the kinetics of joint formation in compatible and incompatible systems (including steel-poly(ethylene-co-vinyl acetate). The effective activation energy of the kinetics of adhesive-joint formation is determined and compared with the activation energies of diffusion, the viscous flow of copolymers, the β transition, and the rate of conformation rearrangements in the surface layers of ethylene-vinyl acetate copolymers. It is suggested that the data obtained can be generalized in terms of the Bikerman theory of a “weak boundary layer.”

Failure of a Waterborne Primer Applied to Zinc Coated Steel

The durability of a high‐performance coating system based on a polyester top coat and a water‐based primer system has been studied by exposure of coated panels to salt spray and 100% RH. The humidity exposure is less deleterious than the salt spray with a higher concentration of an adhesion promoter further enhancing durability in each case. XPS analysis establishes that in both cases failure occurs as a result of a weak boundary layer of zinc oxide corrosion product. Further testing of these pre‐exposed panels using a wet adhesion test and a mechanical pull‐off test confirms the superior performance of the formulation with a higher level of adhesion promoter in both tests. The locus of failure is generally in the organic phase, although those specimens at the lower end of the performance spectrum show a significant amount of failure at or close to the coating/substrate interface. For samples pre‐exposed to the salt spray, the presence of chloride ions at the failed interface indicates that such a treatment may have a deleterious effect on subsequent performance. Copyright © 2012 John Wiley &amp; Sons, Ltd.