Slip Layer Research Articles

Navier number and transition to turbulence

The motivation behind this article is to explain a role of the Navier number (Na – dimensionless slip-length) in prediction of closures for laminar to turbulent transition undergoing via eddies detachment from the slip layer in nano-cannals. Additionally the role of the Navier number Na in universal modeling of phenomenon of enhanced mass flow rate reported in micro- and nano-channels has been explained. The Na number should be treated as a ratio of internal viscous to external viscous momentum transport and therefore this notion cannot be extended onto whole friction resistance phenomena. Our proposal for unique expressing of a critical point in turbulence transition is that on a plane of fD – Re one needs two coordinates. The second critical coordinate has been discovered by Stanton and Pannell and is known to be the Stanton-Pannell number StPa. Finally dependence of the Stanton- Pannell number StPa on Navier number Na and Reynolds number Re is presented.

Lubrication by Polyelectrolyte Brushes.

We develop a scaling model relating the friction forces between two polyelectrolyte brushes sliding over each other to the separation between grafted surfaces, number of monomers and charges per chain, grafting density of chains, and solvent quality. We demonstrate that the lateral force between brushes increases upon compression, but to a lesser extent than the normal force. The shear stress at larger separations is due to solvent slip layer friction. The thickness of this slip layer sharply decreases at distances on the order of undeformed brush thickness. The corresponding effective viscosity of the layer sharply increases from the solvent viscosity to a much higher value, but this increase is smaller than the jump of the normal force resulting in the drop of the friction coefficient. At stronger compression we predict the second sharp increase of the shear stress corresponding to interpenetration of the chains from the opposite brushes. In this regime the velocity-dependent friction coefficient between two partially interpenetrating polyelectrolyte brushes does not depend on the distance between substrates because both normal and shear forces are reciprocally proportional to the plate separation. Although lateral forces between polyelectrolyte brushes are larger than between bare surfaces, the enhancement of normal forces between opposing polyelectrolyte brushes with respect to normal forces between bare charged surfaces is much stronger resulting in lower friction coefficient. The model quantitatively demonstrates how polyelectrolyte brushes provide more effective lubrication than bare charged surfaces or neutral brushes.

Simple, Sensitive, and Quantitative Electrochemical Detection Method for Paper Analytical Devices

We report a new type of paper analytical device that provides quantitative electrochemical output and detects concentrations as low as 767 fM. The model analyte is labeled with silver nanoparticles (AgNPs), which provide 250,000-fold amplification. AgNPs eliminate the need for enzymatic amplification, thereby improving device stability and response time. The use of magnetic beads to preconcentrate the AgNPs at the detection electrode further improves sensitivity. Response time is improved by incorporation of a hollow channel, which increases the flow rate in the device by a factor of 7 and facilitates the use of magnetic beads. A key reaction necessary for label detection is made possible by the presence of a slip layer, a fluidic switch that can be actuated by manually slipping a piece of paper. The design of the device is versatile and should be useful for detection of proteins, nucleic acids, and microbes.

Shear viscosity and wall slip behavior of a viscoplastic hydrogel

The coupled apparent slip and viscoplastic behavior of a hydrogel consisting of 0.2 wt. % aqueous solution of poly(acrylic acid) was analyzed employing steady torsional and circular tube (capillary) flows. Transparent disks and capillaries fabricated out of borosilicate glass were used to allow velocity measurements. The steady torsional flow of the hydrogel was dominated by wall slip which gave rise to plug flow over the apparent shear rate range of 0.1 to 1 s−1, in agreement with the plug flow observed in the capillary under similar shear stresses. The transition from plug flow provided the yield stress of the hydrogel, which was found to be consistent with velocity data collected over the 0.1–200 s−1 apparent shear rate range of steady torsional and capillary flows. The availability of both pressure drop versus flow rate and wall slip velocity data enabled the validation of correction procedures proposed earlier for the determination of the slip-corrected wall shear rate [Yilmazer and Kalyon, “Dilatancy of concentrated suspensions with Newtonian matrices,” Polym. Compos. 12, 226–232 (1991); D. Kalyon, “Apparent slip and viscoplasticity of concentrated suspensions,” J. Rheol. 49, 621–640 (2005)] and the determination of the shear viscosity parameters. The apparent slip layer thicknesses were found to be consistent with the elastohydrodynamic mechanism proposed by Martin et al. [“Wetting transitions at soft, sliding interfaces,” Phys. Rev. E 65, 031605 (2002)] and Meeker et al. [“Slip and flow in pastes of soft particles: Direct observation and rheology,” J. Rheol. 48, 1295–1320 (2004)] for steady torsional flow but not for capillary flow, emphasizing the role the applied pressure plays in shaping the apparent slip behavior.

Study on Wall Slip in Pipeline Flow of Dense Paste

The apparent wall slip flows of incompressible and viscoplastic fluids in plane pipeline were analyzed assuming that the apparent slip layer consists solely wall slip layershear layerslug flow part and they are independent of the flow rate. Taking coal slurry for example,the experiment is conducted on pressure pipe rheological and resistance characteristic test system. It studies how two key factors (the particle concentration and the pipe diameter) influence the dense paste wall slip velocity. The assumed apparent slip mechanism provides methodologies for the determination of the slip velocity values that are consistent with the traditional Mooney method and furthermore allows the determination of the true shear rate of the dense paste at the wall and the yield stress. The analysis of the slip data of various dense pastes of rigid particles reveals that the apparent slip layer thickness is related to the particle concentration and the pipe diameter.

Crust and upper mantle resistivity structure at middle section of Longmenshan, eastern Tibetan plateau

Abstract Longmenshan is located in the eastern edge of the Tibetan Plateau. It is one of the hotspots of geophysical and geological studies in the world for its special tectonic characters and dynamic mechanism, especially after the 2008 Wenchuan earthquake (Ms 8.0). From 2008 to 2010, a 570 km Long-period magnetotelluric (LMT) and broadband magnetotelluric (MT) sounding profile across the Longmenshan fault zone has been carried out to investigate the crust and upper mantle resistivity structure. The coupling relationship of Longmenshan with the Songpan–Ganzi block and with the Sichuan basin of Yangtze block has been studied respectively. The analysis of the deep resistivity structure is of importance for the study of the geodynamic characteristics of the Wenchuan earthquake. The inversion reveals a high–low-high resistivity layer in the Songpan–Ganzi block, a low-high resistivity layer in the Sichuan basin and a complex resistivity structure in the Longmenshan area, respectively. There is a low resistivity layer in the middle and lower crust at the depth about 20–45 km beneath the Songpan–Ganzi block, which has shown that there may be a continuous slip layer in the crust, while there is no such a layer in the crust beneath the Sichuan basin. The thrust imbrication of resistivity structure in the upper crust beneath the Longmenshan indicates that the Songpan–Ganzi block is over thrusting onto the Yangtze block. And the low resistivity layer in the lower crust stretching downward to the bottom of the lithosphere beneath Longmenshan indicates that the Songpan–Ganzi block subducts below the lithosphere of Yangtze block. The complex structure of high resistivity massif in the lithosphere beneath Longmenshan provides some important evidence for the study of the mechanism of the 2008 Wenchuan earthquake.

Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

AbstractAnalytic results are derived for the apparent slip length, the change in drag and the optimum air layer thickness of laminar channel and pipe flow over an idealised superhydrophobic surface, i.e. a gas layer of constant thickness retained on a wall. For a simple Couette flow the gas layer always has a drag reducing effect, and the apparent slip length is positive, assuming that there is a favourable viscosity contrast between liquid and gas. In pressure-driven pipe and channel flow blockage limits the drag reduction caused by the lubricating effects of the gas layer; thus an optimum gas layer thickness can be derived. The values for the change in drag and the apparent slip length are strongly affected by the assumptions made for the flow in the gas phase. The standard assumptions of a constant shear rate in the gas layer or an equal pressure gradient in the gas layer and liquid layer give considerably higher values for the drag reduction and the apparent slip length than an alternative assumption of a vanishing mass flow rate in the gas layer. Similarly, a minimum viscosity contrast of four must be exceeded to achieve drag reduction under the zero mass flow rate assumption whereas the drag can be reduced for a viscosity contrast greater than unity under the conventional assumptions. Thus, traditional formulae from lubrication theory lead to an overestimation of the optimum slip length and drag reduction when applied to superhydrophobic surfaces, where the gas is trapped.

The mechanics of moisture-expansion cracking in fired-clay ceramics

Samian ware (or terra sigillata) is a type of fired-clay ceramic produced at a number of sites in France in the period 50 BC to 200 AD and widely traded in Western Europe. It has a characteristic high-gloss surface, formed by application of a non-calcareous clay slip to the green body before firing. New SEM observations show that the slip layer is frequently crazed, although the cracks are not usually visible to the unaided eye. We discuss the mechanics of the crazing, and show that the cracking is driven by rehydroxylation (RHX) moisture expansion. Observations and analysis aid in understanding the RHX dating of archaeological pottery by showing that craze networks permit efficient transport of moisture through the slip layer.

Die drool theory

Abstract When molten plastic is extruded from a die, it sometimes collects on the open face of the die. Known as die drool, this phenomenon costs plastics manufacturers by requiring die cleaning. This has been attributed to many causes, but none of these has led to an equation for the drool rate. In this work, we provide an exact analytical solution for the drool rate, and we base this solution on a postulate of a cohesive slip layer near the die walls. We thus attribute die drool to cohesive failure within the fluid at an internal surface where the fluid slips on itself. We adimensionalize the drool rate with the production rate, and call this the buildup ratio, BR. We provide an exact analytical solution for BR when the cohesive slip layer either slips at the die wall, or when it does not. We examine two important extrusion geometries: slit (which we then extend to pipe) and tube flow. We identify two new experiments: one to measure BR as a function of pressure drop, and another as a function of the die aspect ratio, and we then use our new theory to design droolometers.

The generation of jet and mixing induced by the interaction of shock wave with R22 cylinder

Based on the large eddy simulation, combined with the 5th order weighted essentially non-oscillatory scheme and the immersed boundary method, the shock wave interacting with an R22 cylinder is numerically simulated. Our numerical results present clearly the deformation of cylinder induced by the Richtmyer-Meshkov instability due to the interaction of shock wave with R22 cylinder, which accords well with previous experimental results of Haas and Sturtevant [Haas J F and Sturtevant B 1987 J. Fluid Mech. 181 41]. In addition, the numerical results reveal the generation process of a jet induced by the refracted shock focusing near the right interface of the inner cylinder, as well as the roll-up of the secondary vortexes along the slip layer of two main vortexes. The mixing mechanism of R22 gas and air is also expatiated. Furthermore, the evolution of R22 cylinder under reshock condition is numerically simulated with two different end wall distances. For the long distance case, the reflected wave interacts with severely distorted R22 volume, making it further compressed on the x-axis. While for the small case, two Mach reflections occur between the reflected shocks during their propagating upstream within the cylinder. The two high pressure areas behind two triple points can accelerate the boundary of the R22 cylinder while they are passing through it and induce two jets.

Effect of boundary slip and surface charge on the pressure-driven flow

Drag reduction in micro/nanofluidic systems is an important issue. The effect of boundary slip and electrical double layer (EDL) induced by surface charge on the pressure-driven flow in a micro/nanochannel has been widely studied. However, change in electrical conductivity as a result of ionic redistribution caused by surface charge, which can affect the EDL-induced electrical force exerted on the flow, is often neglected. In addition, the effect of surface charge on the slip length is not considered. In this work, a model incorporating the effect of surface charge on electrical conductivity and slip length was developed to investigate the effect of boundary slip and EDL induced by surface charge on the volumetric flow rate and skin friction coefficient. The underlying mechanisms for the results regarding the effect of slip and surface charge on the flow were analyzed.

Influence of visco-elastic binder properties on ram extrusion of a hardmetal paste

The influence of the viscous and elastic components of a visco-elastic binder phase on the extrusion behaviour of a tungsten carbide–cobalt hardmetal paste was investigated by studying the paste over a range of temperatures, 30–42 °C, where the binder changed from a semi-solid gel to a viscous liquid. The extrusion behaviour of the paste was studied by ram extrusion and quantified by the Benbow–Bridgwater (BB) method. The elastic and viscous components of the binder were studied separately by means of oscillatory and steady shear techniques in a controlled stress rheometer using rough parallel plates. The paste behaviour fitted the four parameter BB model well: the initial bulk yield stress parameter, σ0, increased linearly with the binder elastic modulus, G′, while both the velocity dependence parameters α and β were linearly related to the binder steady shear viscosity. Analysis of paste-wall slip parameters gave estimated slip layer thicknesses of 2–5 particle diameters.

Cytoplasmic streaming in plant cells: the role of wall slip

We present a computer simulation study, via lattice Boltzmann simulations, of a microscopic model for cytoplasmic streaming in algal cells such as those of Chara corallina. We modelled myosin motors tracking along actin lanes as spheres undergoing directed motion along fixed lines. The sphere dimension takes into account the fact that motors drag vesicles or other organelles, and, unlike previous work, we model the boundary close to which the motors move as walls with a finite slip layer. By using realistic parameter values for actin lane and myosin density, as well as for endoplasmic and vacuole viscosity and the slip layer close to the wall, we find that this simplified view, which does not rely on any coupling between motors, cytoplasm and vacuole other than that provided by viscous Stokes flow, is enough to account for the observed magnitude of streaming velocities in intracellular fluid in living plant cells.

Numerical investigations on the Schardin's problem

When shock waves passes through a triangle wedge (Schardin's problem), some complicated physical phenomena, including shock wave Mach refection and diffraction, wedge wake, vortexlet, etc. may occur. In this paper, the Schardin's problem is investigated numerically with the combination of the third-order WENO scheme, structured adaptive mesh refinement (AMR) method and immersed boundary method. Our numerical results show clearly the interaction between the incident shock wave and the triangle wedge, the Mach reflection on the wedge surface, its diffraction at the wedge tips, and the induction of the main vortex, which accord excellently with Schardin's experimental and previous numerical results. In addition, our numerical results display in detail the induction process of the vortexlet along the slip layer of the main vortex and the interaction of the shock wave with the vortexlet, and the generation of the serial acoustic waves, which have not been reported in the literature.

Shearing mechanisms of MgSiO3at conditions of the Earth's D″ layer

[1] It is shown that the mechanic properties of MgSiO3 at temperature and pressure conditions of the Earth's D″ layer are strongly affected by perovskite/post-perovskite stacking faults and, more generally, perovskite → post-perovskite → perovskite phase transitions. We employ molecular dynamics simulations to explore different shearing routes for {010} layers in the perovskite structure. While the yield strength was found as about 1 GPa for layer displacement along [100], [101] and [001], the underlying mechanisms differ considerably. Shear along [101] induces the formation of a perovskite/post-perovskite stacking fault. Once such a stacking fault arrangement is formed, further shearing is facilitated and the yield strength is reduced to 0.25 GPa. Thus, perovskite/post-perovskite stacking faults and/or perovskite/post-perovskite phase interfaces are suggested as the preferential slip layers of MgSiO3 in the Earth's D″ layer.

On the mass and momentum transport in the Navier–Stokes slip layer

The Navier–Stokes slip boundary conditions are considered as conditions following from the mass and momentum balances within a thin, shell-like moving boundary layer. A problem of consistency between different models, that describes the internal and external friction in a viscous fluid, is stated within the framework of a proper form of the layer momentum balance. Appropriate constitutive equations for friction forces are formulated. The common features of the Navier, Stokes, Reynolds, and Maxwell concepts of a boundary slip layer are revalorized and discussed. Different mobility mechanisms connected with the transpiration phenomena, important for flows in micro- and nanochannels, are classified as a part of equations for the external friction.

Numerical Limit Equilibrium Method of Three-Dimensional Stability for Landslide

The three-dimensional model of slop in numerical calculation is established. And high precision eight-node hexahedron element is used to build the finite element grid of slip layer. The slip layer material is simulated by an ideal elastic-plastic model based on the Mohr-Coulomb Criterion. Three-dimensional stress state and slide direction of the slip layer zone is obtained by numerical computation. So, based on the analysis of the three-dimensional stress state of the slip layer zone, in this article, the author defines the point safety factor as the ratio of the shear strength of the slip zone to the shear stress parallel to the sliding direction, and defines the safety factor of the landslide as the average of the zone point safety factor with its zone as weight number. The sliding mechanism is studied through zone point safety factor, and stability of landslide can be evaluated through the safety factor. Sample case shows that the calculated result is in accordance with the actual case.

An investigation of Hellenistic period pottery production technology from Harabebezikan/Turkey

Upper ancient Mesopotamia is a part of Southeastern Anatolia and irrigated by the Euphrates and Tigris rivers. It has several sites, ruins and monuments belonging to ancient civilizations. Among these, an ancient site namely Harabebezikan, is on the east bank of Euphrates (Fırat) river and located 17 km south of Birecik in Şanlıurfa/Turkey. It is left under water by the reservoir lake of Karkamış (Carchemish) dam today. An archaeological salvage project was carried out in 1999 shortly before the reservoir filling. One of the most important results of the excavations was to discover a ceramic workshop of the Hellenistic period. In this study, some selected Hellenistic period potsherds were studied in order to investigate the technological parameters of pottery production such as raw materials used in manufacturing, firing temperatures and conditions. The selected potsherds are probably production of this workshop. Wavelength dispersive X-ray fluorescence (WDXRF) and X-ray diffraction (XRD) analysis were performed for chemical and phase analysis, respectively. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX) were further performed for microstructural and microchemical characterization. Micro-Raman analysis was performed for assessing the mineralogical components of the slip layers. Finally, thermal behavior of the potsherds was characterized by thermal gravimetric-differential thermal analysis (TG-DTA) techniques. The obtained results showed that calcareous materials including clays were used for pottery production and the potsherds were fired in the temperatures from 600 to 1000 °C. Furthermore, iron rich slip layers in different colors suggest that Hellenistic period potters at Harabebezikan were aware of adjusting the redox conditions of firing to obtain the desired color.

Scanning electron microscopy and micro-Raman spectroscopy of slip layers of Hellenistic ceramic wares from Dorylaion/Turkey

Anatolia (Asia Minor) was inhibitated by several civilizations during the history. Dorylaion at Eskişehir/Turkey, an ancient site, is on the crossroads of many ancient civilizations. Artifacts belonging to different historical periods and cultures have been uncovered during the excavations carried out there since 1989. One of the two important groups of ceramic findings uncovered in these excavation works is the moldmade bowls, familiarly known as the Megarian bowls and the other is the West Slope wares. Both types of wares were probably the fashion cups around the east Mediterranean basin of the Hellenistic period. In this study, an attempt was made to enlighten the technological parameters and production technology of selected Megarian bowls and West Slope wares by characterizing their slip layers. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX) were performed for microstructural and microchemical characterization. Micro-Raman spectroscopy was further applied for assessing the mineralogical components. The slip layers of both wares have similar elemental diversity, apart from the fact that the West Slope wares have more iron content than the Megarian bowls. The iron rich composition of the slip layers in different colors showed that hematite (α-Fe 2O 3), maghemite (γ-Fe 2O 3) and magnetite (Fe 3O 4) are the principal coloring agents for the slip layers of the investigated Hellenistic ceramic wares. However, firing conditions affecting the formation and the abundance of these minerals were probably adjusted in order to obtain the desired color.

Apparent wall slip velocity measurements in free surface flow of concentrated suspensions

In this work we have experimentally measured the apparent wall slip velocity in open channel flow of neutrally buoyant suspension of non-colloidal particles. The free surface velocity profile was measured using the tool of particle imaging velocimetry (PIV) for two different channels made of plane and rough walls. The rough walled channel prevents wall slip, whereas the plane wall showed significant wall slip due to formation of slip layer. By comparing the velocity profiles from these two cases we were able to determine the apparent wall slip velocity. This method allows characterization of wall slip in suspension of large sized particles which cannot be performed in conventional rheometers. Experiments were carried out for concentrated suspensions of various particle volume concentrations and for two different sizes of particles. It was observed that wall slip velocity increases with particle size and concentration but decreases with increase in the viscosity of suspending fluid. The apparent wall slip velocity coefficients are in qualitative agreement with the earlier measurements. The effect of wall slip on free surface corrugation was also studied by analyzing the power spectral density (PSD) of the refracted light from the free surface. Our results indicate that free surface corrugation is a bulk flow response and it does not arise from boundary problem such as development of slip layer.