Wall Slip Effects Research Articles

Relative Effects of Asymmetry and Wall Slip on the Stability of Plane Channel Flow

The effect of wall velocity slip on the stability of a pressure-driven two-dimensional asymmetric channel flow is examined by considering Navier slip condition on the channel walls. The two-parameter families of mean velocity profiles are considered to approximate the underlying asymmetric basic flow. Competing effects of skewness and maximum velocity on the stability of the flow are explored for a range of model parameters. The Orr–Sommerfeld system of the asymmetric flow is solved using a Chebyshev spectral collocation method for both symmetric and non-symmetric type slip boundary conditions. Numerical results indicate that moderate asymmetry in the basic flow has a significant role on the stability of the Poiseuille-kind parallel/nearly parallel flows. Wall slip shows a passive control on the instability of the asymmetric flow by increasing or decreasing the critical Reynolds number and the set of unstable wave numbers. The stabilizing/destabilizing effect of slip velocity on the flow instability is weak or strong depending on the presence of velocity slip at the upper or lower wall. Velocity slip has a profound grip on the flow behaviour by changing the shear rate inside the perturbed flow.

Viscous behavior and wall slip of barite-weighted water-based drilling fluids containing a high particle fraction

Accurate rheological measurements of barite-weighted ultrahigh density water-based drilling fluids with a rotational viscometer may be hindered by the wall slip effect. This paper describes a correction method that is based on the principle of flow curves gap dependence. A two-stage Tikhonov regularization numerical strategy was developed to obtain the true viscous behavior and wall slip of barite-weighted drilling fluids containing a high particle fraction. The proposed approach does not depend on the narrow gap assumption or the prescribed form of rheological models. The results show that barite-weighted ultrahigh density drilling fluids start to slip after reaching a small value of critical shear stress, and the wall slip velocity increases linearly as a function of shear. There are significant discrepancies between the slip-corrected rheological models and the apparent models. Curve fitting results show that the slip-corrected flow curves are linear as would be expected for Bingham plastic fluids, while the uncorrected apparent flow curves are nonlinear as one would expect for yield power law fluids. Failing to correct for the wall slip effect would result in seriously underestimating the bulk viscosity of barite-weighted ultrahigh density water-based drilling fluids.

Yield stress materials in soft condensed matter

We present a comprehensive review of the physical behavior of yield stress materials in soft condensed matter, which encompass a broad range of materials from colloidal assemblies and gels to emulsions and non-Brownian suspensions. All these disordered materials display a nonlinear flow behavior in response to external mechanical forces, due to the existence of a finite force threshold for flow to occur: the yield stress. We discuss both the physical origin and rheological consequences associated with this nonlinear behavior, and give an overview of experimental techniques available to measure the yield stress. We discuss recent progress concerning a microscopic theoretical description of the flow dynamics of yield stress materials, emphasizing in particular the role played by relaxation time scales, the interplay between shear flow and aging behavior, the existence of inhomogeneous shear flows and shear bands, wall slip, and non-local effects in confined geometries.

Bulk fluidity and apparent wall slip of aqueous kaolin suspensions studied using the cone-cone (KK) sensor: Effect of the sensor surface quality

Influence of sensor surface quality on apparent wall slip (AWS) effects in aqueous kaolin suspensions was studied using AWS rotational viscometry with four unconventional cone-cone (KK) sensors of different materials and surface finishing. The fluidity φ[σ] and slip χ[σ] material functions were obtained for three concentrations of kaolin (30, 35, and 40wt.%) applying several gap thicknesses (between 0.6 and 1.6mm). Positive slip coefficients were found in the sensors made from titanium or stainless steel with smooth or sand-blasted surfaces. A negative AWS effect was detected for the sensor made of aluminum alloy with eloxal coated surfaces. This negative AWS effect is interpreted as a consequence of possible formation of a stagnant layer at the walls.

Describing flow behavior of polymer gel in fracture using rheology

In many successful conformance controls to reduce excessive water production, large volumes of gels were extruded through fractures during placement. The pressure gradient for gel extrusion depends strongly on fracture width and gel composition. Extrusion experiments directly measure gel properties in fractures, but they are both expensive and time-consuming. The rheology behavior of the gels showed a strong parallel to the results obtained from previous gel extrusion experiments. The previous researches showed two discrepancies and one similarity between the rheology estimation and extrusion experiment. The two discrepancies are: the pressure gradient required to extrude a gel through a fracture substantially greater than anticipated based on rheology measurements; different relationships of pressure gradient versus fracture width. The similarity is the relationships between pressure gradient versus flow rate and shear stress versus shear rate are fairly close. This paper examined these discrepancies and the similarity, in hopes of understand and ultimately substituting the rheology measurements for extrusion experiments.Extensive experiments established that wall slip and first normal stress difference were not responsible for the pressure gradient discrepancy. Steady shear and oscillatory shear data were collected with a rheometer using both smooth and rough parallel-plate geometries and employing various gap heights. Wall-slip effects were present with smooth plates but negligible with rough plates. To explain the discrepancy, we noted that the aperture for gel flow (for mobile gel wormholing through concentrated immobile gel within the fracture) was much narrower than the width of the fracture.Considering the shear-thinning properties of the gels, two models were developed combining power-law model and shell momentum balances. The first model explained the discrepancy of the relationships between pressure gradient versus fracture width. The second model correlated pressure gradient, shear stress, flow rate, and shear rate to bridge the gap between gel rheology in fractures versus in a rheometer and explained the similarity between the rheology measurement and extrusion experiment.

Evaluation of wall slip effects on the flow characteristics of petroleum coke–water slurry flow along pipelines

AbstractFlow characteristics of petroleum coke–water slurry (PCWS) were investigated in a slurry transportation facility. True rheological properties, drag reduction, and flow resistances of PCWS under the influence of wall slip were studied using the Tikhonov regularization method. Results show that the shear viscosity is insensitive to changes in the shear rate when the solid loading of PCWS is 59.8 wt%. However, the PCWS of 61.1 wt% exhibits typical dilatant fluid properties. As the shear rate increases, the PCWS of 63.4 wt% initially exhibits shear thinning, and then, the viscosity increases sharply when the shear rate exceeds 250 s−1. In addition, the slurry also displays shear thinning at the shear rates exceeding 600 s−1. Modified friction factor‐generalized Reynolds number correlation is established to describe the resistance properties of a laminar flow of PCWS. When increasing the velocity, the drag reduction produced by the wall slip behavior reduces for the 59.8 wt% PCWS but increases for the 61.1 wt% PCWS. The drag reduction decreases at the outset followed by an increase for the 63.4 wt% PCWS. A decrease in the inner diameter of pipeline could enhance the effect of drag reduction of PCWS flowing in a pipeline. © 2017 Curtin University and John Wiley & Sons, Ltd.

Pressure dependent wall slip of wood flour filled polymer melts

The rheological properties of a 50 wt.% wood flour filled polypropylene have been measured at 195°C using an instrumented slit die attached to a single screw extruder. Wall slip and non-Newtonian effects have been corrected through the Mooney and Rabinowitsch procedures, thus equations for viscosity and wall slip have been obtained. Interestingly, the pressure drop profiles calculated using these equations are not perfectly consistent with the experimental pressure measurements, in that the theoretical curves overestimate the experimental data. An explanation has been sought by assuming that slip velocity increases in the last portion of the slit die, i.e. close to the exit, where pressure drops down considerably. A pressure dependent slip model has been then proposed, its specific equations numerically solved and the results fitted to the experimental pressure data. The fitting is better than the one coming from the model that comprises slip depending on shear stress only, thus, for the fluid that has been studied, the assumption of pressure dependent slip appears to be justified.

Dynamic wall slip behavior of yield stress fluids under large amplitude oscillatory shear

Wall slip occurs under large amplitude oscillation shear (LAOS) for yield stress fluids. In this work, we investigated how the boundary conditions affect the nonlinear behavior under LAOS and proposed a simple methodology based on the geometric average of Lissajous curves to study the dynamic wall slip behavior under oscillatory shear. The results show that the stress-mean strain curve is a good candidate to define material's functions since it is almost not influenced by the wall slip effect. Meanwhile, the stress-mean strain rate curves from smooth plates and rough plates can be used to determine the wall slip velocity. It is found that the intercycle maximum slip strain rate follows the generalized Navier's law, while the intracycle slip behavior can be well described by a Maxwell-like dynamic slip model, which helps to determine the slip relaxation time. It is also found that the slip Deborah number is independent of the angular frequency and is a monotonically decreasing function of the reduced stress. Moreover, the slip Deborah number depends on the reduced stress through a power law, and there is an evident transition of the power law exponent at the yield stress.

Influence of combined electromagnetohydrodynamics on microchannel flow with electrokinetic effect and interfacial slip

We investigate analytically the combined consequences of electromagnetohydrodynamic forces and interfacial slip on streaming potential mediated pressuredriven flow in a microchannel. Going beyond traditional Debye-Huckel limit, we first derive a closed-form analytical solution for velocity field by considering nonlinear electrical potential distribution, wall slip effects, externally imposed transverse magnetic field, and laterally applied electric field in the plane of flow. The effects of electrical double-layer (EDL) formation and the consequent interfacial phenomena are critically examined under such situations. An expression for induced streaming potential in the microchannel is deduced considering EDL formation and the consequences of finite conductance of the immobilized Stern layer. This simplified analytical expression is later on critically assessed against three-dimensional simulation paradigm of streaming potential mediated flows, which is a first effort of this kind. We demonstrate that flow rate increases progressively with increasing surface potential and eventually approaches to a limiting value. Combination of electromagnetohydrodynamic effect with liquid slip is shown to amplify the flow rate, even at lower values of surface potential. Our study brings out the possibility of achieving an optimum flow rate by judicious application of combined electromagnetohydrodynamics. The present analysis has significant consequence in the design of advanced microfluidic devices with improved efficiency and functionality.

Theoretical Modeling of Biological Fluid Deformation during Dynamic Acoustic Tweezing

Acoustic tweezing rheometry is a novel non-contact method for real-time measurement of rheological properties of biological fluids, without artifacts such as contamination, wall slip effect and other phenomena associated with contact of a sample with device walls. This technique can operate in two regimes - quasi-static elastometry and dynamic measurement of sample viscoelasticity. In this presentation, we report the development of theoretical models for dynamic acoustic tweezing tests in which the sample fluid is modeled using the viscoelastic fluid or viscoelastic solid models (Jeffrey's or Kelvin-Voigt equations, respectively). By asymptotic analysis of the characteristic equation for sample shape oscillation, we obtained analytical relationships between rheological properties (viscosity, relaxation time, elastic modulus) and the frequency and damping rate of shape oscillations, which are directly measured in experiments. To test our models against experiments, we have conducted dynamic acoustic tweezing tests with human whole blood and blood plasma samples. By applying our models to the data of these experiments, we have obtained physically consistent results on blood rheology and showed that the viscosity of blood samples increases by a factor of 4-6 within six minutes of coagulation onset. The presented theoretical models are invaluable for non-contact assessment of biological fluid rheology and understanding the mechanical behavior of biological fluids subjected to the acoustic tweezing field.

Wall slip and non-integer order derivative effects on the heat transfer flow of Maxwell fluid over an oscillating vertical plate with new definition of fractional Caputo-Fabrizio derivatives

This article is focused on natural convection of unsteady flow of generalized Maxwell fluid over an oscillating vertical flat plate with constant temperature at the boundary. The Maxwell fluid with classical derivatives, describing one dimensional flow has been generalized to non-integer order derivatives known as fractional derivative with term of buoyancy. A modern definition of fractional derivative, recently introduced by Caputo and Fabrizio has been used to formulate the considered problem. Semi analytical solutions of the dimensionless problem have been obtained by using the Laplace transform. The solutions for temperature, velocity and shear stress are obtained with numerical inversion techniques of Laplace transform namely, Stehfest’s and Tzou’s algorithms. At the end, graphical illustrations for temperature, velocity, Nusselt number and shear stress are plotted. We have studied especially the influence of fractional parameter on temperature, velocity and shear stress respectively. We have observed that temperature can be enhanced for increasing the fractional parameter α while velocity and shear stress can be increased by decreasing the value of fractional parameter α.

Comparisons of the rheological behaviors of polypropylene and polymethyl methacrylate in a capillary die

ABSTRACTIn this study, the rheological characteristics of polypropylene (PP) melt at 210, 220, and 230 °C and polymethyl methacrylate (PMMA) melt at 230, 240, and 250 °C in a micro die were investigated. The experiments were performed over a shear rate range of 3 × 102 to 5 × 103 s−1 using an advanced twin‐bore capillary rheometer. Dies with diameters of 1.0, 0.5, and 0.25 mm were used. The results indicated that the geometric dependences of the PP and PMMA viscosities were not identical at different shear rates and temperatures and that the micro size effect had a profound influence on the PP viscosity. The analysis demonstrated that the variations in the shear viscosity of the PP and PMMA melts in the micro die were partially attributed to the contribution of the pressure applied to the polymer melts. Additionally, the effect of wall slip on the PP and PMMA viscosities in the tested dies was investigated based on the modified Mooney method. The results implied that wall slip easily occurred in the PP melt flowing through the 0.25 mm die at 210 °C due to the distinct size effect. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44617.

The effect of wall slip on the dynamics of a spherical particle in Newtonian and viscoelastic fluids subjected to shear and Poiseuille flows

We address the effect of wall slip on the dynamics of a spherical particle suspended in an inertialess Newtonian or viscoelastic shear-thinning fluid under shear or Poiseuille flow. The study is performed through 3D direct finite element simulations by employing an Arbitrary Lagrangian-Eulerian method for the particle motion.In both shear and Poiseuille flows, wall slip reduces the difference between the particle translational velocity along the flow direction and the velocity of the unperturbed fluid, and slows down the particle rotational velocity. Remarkably, in a viscoelastic fluid, the presence of wall slip reverses the migration direction as compared to the no-slip case. Hence, for sufficiently high slip coefficients, all the particles migrate toward the channel midplane in shear flow and toward the channel centerline in Poiseuille flow, regardless of their initial position through the channel.

A multiscale transport model for Lennard-Jones binary mixtures based on interfacial friction.

We propose a one-dimensional isothermal hydrodynamic transport model for non-reacting binary mixtures in slit shaped nanochannels. The coupled species momentum equations contain viscous dissipation and interspecies friction term of Maxwell-Stefan form. Species partial viscosity variations in the confinement are modeled using the van der Waals one fluid approximation and the local average density method. Species specific macroscopic friction coefficient based Robin boundary conditions are provided to capture the species wall slip effects. The value of this friction coefficient is computed using a species specific generalized Langevin formulation. Gravity driven flow of methane-hydrogen and methane-argon mixtures confined between graphene slit shaped nanochannels are considered as examples. The proposed model yields good quantitative agreement with the velocity profiles obtained from the non-equilibrium molecular dynamics simulations. The mixtures considered are observed to behave as single species pseudo fluid, with the interfacial friction displaying linear dependence on molar composition of the mixture. The results also indicate that the different species have different slip lengths, which remain unchanged with the channel width.

The Effect of Slip on the Discharge From Partially Filled Circular and Fully Filled Lens and Figure 8 Shaped Pipes

In this work, we examine the effect of wall slip for a gravity-driven flow of a Newtonian fluid in a partially filled circular pipe. An analytical solution is available for the no-slip case, while a numerical method is used for the case of flow with wall slip. We note that the partially filled circular pipe flow contains a free surface. The solution to the Navier–Stokes equations in such a case is a symmetry of a pipe flow (with no free surface) with the free surface as the symmetry plane. Therefore, we note that the analytical solution for the partially filled case is also the exact solution for fully filled lens and figure 8 shaped pipes, depending on the fill level. We find that the presence of wall slip increases the optimal fill height for maximum volumetric flow rate, brings the “velocity dip” closer to the free surface, and increases the overall flow rate for any fill. The applications of the work are twofold; the analytical solution may be used to verify numerical schemes for flows with a free surface in partially filled circular pipes, or for pipe flows in lens and figure 8 shaped pipes. Second, the work suggests that flows in pipes, particularly shallow filled pipes, can be greatly enhanced in the presence of wall slip, and optimal fill levels must account for the slip phenomenon when present.

Development lengths in Newtonian Poiseuille flows with wall slip

The effect of wall slip on the development of planar and axisymmetric Newtonian Poiseuille flows is studied by means of finite element simulations. The Navier slip law is employed. i.e., it is assumed that the slip velocity varies linearly with the wall shear stress. In addition to the standard definition of the development length, L, as the length required for the maximum velocity to attain 99% of its fully-developed value, the wall development length Lw is also relevant in the presence of slip. This is defined as the length required for the slip velocity to decrease to 1.01% of its fully-developed value. The numerical simulations for the planar and the axisymmetric geometries showed that both L and Lw increase with slip passing through a maximum and vanish at a critical value of the slip parameter corresponding to the full slip case. Moreover, the velocity overshoots and the axial pressure minimum near the entrance become less pronounced as slip becomes stronger. The calculations of L for the planar flow are in good agreement with available results in the literature. An interesting result is that in contrast to the axisymmetric flow in which Lw<L, the planar flow develops more slowly at the wall than at the midplane, i.e. Lw>L in the planar case.

Two-layer electro-osmotic flow and heat transfer in a hydrophobic micro-channel with fluid–solid interfacial slip and zeta potential difference

Of concern in this paper is an investigation of two-layer fluid flow and heat transfer in a hydrophobic micro-channel by considering the combined influence of pressure gradient and electro-osmotic forces. The wall slip effects due to hydrophobic interactions at the fluid–solid interface has been taken into account. The governing equations consisting with the linearized Poisson–Boltzmann equation, the Cauchy momentum equation and thermal energy equation are solved analytically within the framework of the Debye–Hückel approximation. The effects of different dimensionless parameters and the interface zeta potential difference on the axial velocity as well as the temperature distribution are presented graphically. The slip velocity on the fluid flow and the Joule heating effects on the temperature are furthermore investigated. The study reveals that the temperature enhancement is strongly depends on the Joule heating parameter as well as the Brinkman number. The zeta potential difference plays an important role in controlling fluid velocity in the micro-channel. For validation of our results, a comparison has been made with the results of those available in the scientific literature and shows excellent agreement.

Rheological Characterization of Ethylcellulose-Based Melts for Pharmaceutical Applications.

Rheological characterization of ethylcellulose (EC)-based melts intended for the production, via micro-injection moulding (μIM), of oral capsular devices for prolonged release was carried out. Neat EC, plasticized EC and plasticized EC containing solid particles of a release modifier (filler volume content in the melt around 30%) were examined by capillary and rotational rheometry tests. Two release modifiers, differing in both chemical nature and particle geometry, were investigated. When studied by capillary rheometry, neat EC appeared at process temperatures as a highly viscous melt with a shear-thinning characteristic that progressively diminished as the apparent shear rate increased. Thus, EC as such could not successfully be processed via μIM. Plasticization, which induces changes in the material microstructure, enhanced the shear-thinning characteristic of the melt and reduced considerably its elastic properties. Marked wall slip effects were noticed in the capillary flow of the plasticized EC-based melts, with or without release modifier particles. The presence of these particles brought about an increase in viscosity, clearly highlighted by the dynamic experiments at the rotational rheometer. However, it did not impair the material processability. The thermal and rheological study undertaken would turn out a valid guideline for the development of polymeric materials based on pharma-grade polymers with potential for new pharmaceutical applications of μIM.

Effects of wall slip on the rheological measurement and extrusion die design of a filled rubber compound

We investigate the rheological properties of a filler-reinforced rubber compound using a rotorless shear rheometer specially designed for rubber and a rotational rheometer with parallel-plate geometry. Our aim is to evaluate the effects of wall slip on both the oscillatory and steady shear modes of the rotational rheometer. The rheological measurements show that the slip generally does not affect the oscillatory shear but does exist in the steady shear flow and tends to make the measured shear viscosity lower than the true value. Besides, we extend the investigation into engineering applications. The extrusion die for a given extrudate profile is obtained using a finite-element model in which the wall slip is considered as a boundary condition. To validate the die design, an extrusion experiment is carried out and the results confirm that, for filled elastomers, it is necessary to consider the effect of wall slip in the extrusion die design.

Normal stress measurement in foams and emulsions in the presence of slip

Steady shear measurements on three yield-stress fluids, a foam and two types of emulsions, were carried out using a smooth torsional parallel-plate geometry, with different gap spacings to assess the effect of wall slip on the normal stress differences. The classical Mooney analysis gave slip-corrected flow curves that agreed with data from a roughened cone-and-plate for all three materials, despite the fact that the compatibility requirement between the flow curve and slip velocity functionalities was not satisfied, indicating that the method is robust for this geometry. All three materials exhibited finite normal stresses following yielding, with power-law exponents of approximately 0.4, as well as shear stresses that can be represented by the Herschel–Bulkley model. The systems showed complete slip for shear stresses below the yield stress, with the stress transmitted to the unyielded material through a thin liquid layer causing an elastic deformation that was measurable for the unyielded foam, which can be represented by the Mooney–Rivlin elastic constitutive equation. The unyielded foam exhibited large sample-to-sample variations in the apparent modulus for identical loading conditions.