Wall Slip Research Articles

Calendering of thermoplastics: models and computations

Abstract John Vlachopoulos (JV) started his polymer processing career with the process of calendering. In two landmark papers with Kiparissides, C. and Vlachopoulos, J. (1976). Finite element analysis of calendering. Polym. Eng. Sci. 16: 712–719; Kiparissides, C. and Vlachopoulos, J. (1978). A study of viscous dissipation in the calendering of power-law fluids. Polym. Eng. Sci. 18: 210–214 he introduced the Finite Element Method (FEM) to solve the governing equations of mass, momentum, and energy based on the Lubrication Approximation Theory (LAT). This early work was followed by the introduction of wall slip (with Vlachopoulos, J. and Hrymak, A.N. (1980). Calendering poly(vinyl chloride): theory and experiments. Polym. Eng. Sci. 20: 725–731). The first 2-D simulations for calendering PVC were carried out with Mitsoulis, E., Vlachopoulos, J., and Mirza, F.A. (1985). Calendering analysis without the lubrication approximation. Polym. Eng. Sci. 25: 6–18. In the intervening 35 years, other works have emerged, however our understanding has not been drastically improved since JV’s early works. Results have also been obtained for pseudoplastic and viscoplastic fluids using the general Herschel-Bulkley constitutive model. The emphasis was on finding possible differences with LAT regarding the attachment and detachment points of the calendered sheet (hence the domain length), and the extent and shape of yielded/unyielded regions. The results showed that while the former is well predicted by LAT, the latter is grossly overpredicted. More results have been obtained for 3-D simulations, showing intricate patterns in the melt bank. Also, the transient problem has been solved using the ALE-FEM formulation for moving free-boundary problems. The results are compared with the previous simulations for the steady-state and show a good agreement. The transient simulations capture the movement of the upstream and downstream free surfaces, and also provide the attachment and detachment points, which are unknown a priori. Finding these still remains the prevailing challenge in the modeling of the calendering process.

Effect of opening and wall boundaries on CFD modeling for submarine landslide-ambient water-pipeline interaction

Submarine landslide-ambient water-pipeline interactions are receiving extensive attention in ocean engineering, especially the impact forces caused by landslides on pipelines. Computational fluid dynamics (CFD) approaches are widely used to obtain these forces; however, as necessary definite solution conditions, the boundary conditions of CFD modeling are challenging to reasonably determine. In this study, the relationship between the finite volume method (FVM) in CFD modeling and boundary conditions is presented, and a classic physical flume experiment is reproduced using CFD modeling, verifying the effectiveness of the established modeling. The quantitative effect of the opening and free slip wall boundaries under variable complex working conditions (i.e., Reynolds number, pipeline span height, landslide impact velocity, and landslide rheological model) is explored via a general geometry model. The evolution characteristics of impact forces on pipelines under different boundary conditions are given. The mechanism caused by impact force changes is explained through the distributions of the velocity field, pressure field and streamlines around pipelines. Furthermore, a method for determining boundary conditions is briefly proposed, providing a basis for optimizing the CFD modeling of submarine landslide-ambient water-pipeline interactions.

NEW AND HIGHLY EFFICIENT METHOD TO MEASURE STEADY SHEAR VISCOSITY AND WALL SLIP OF RUBBER COMPOUNDS: CLOSED-BOUNDARY RHEOMETER (RPA)

ABSTRACT The rubber process analyzer was commercialized by Monsanto Rubber Instrument and Equipment company in the early 1990s. In its initial version, the RPA was able to perform only oscillatory-type tests, which varied in frequency and strain. Later, it offered a controlled strain–stress relaxation test. Only recently and not for all models has a controlled steady shear test been made available. Using this type of test, the instrument has successfully measured steady shear viscosity with high repeatability without correction. The results fit well with other rheometers or viscometers when no-slip conditions are ensured. The closed-boundary configuration prevents edge fracture, as commonly experienced with open-boundary rheometers (DMA) on high-viscosity, high-elasticity materials. A comparison of results using grooved dies (no-slip) and polished dies (slip) readily provides wall slip velocity under constant pressure. The results of wall slip versus shear stress follow a power-law function per the Navier slip law [F(V) = −k(Vr)e]. This method separates the shear rate from pressure effects on wall slip. It questions pressure-driven flow instruments (capillary rheometer), which use pressure measurements for shear stress calculations, and prevents an easy and controlled change of the die surface roughness.

Effect of micelle breaking rate and wall slip on unsteady motion past a sphere translating steadily in wormlike micellar solutions

Many prior experimental studies have found the existence of an unsteady or fluctuating flow field around a solid sphere when falling in wormlike micellar solutions. Based on the two-species Vasquez–Cook–McKinley constitutive model for micelles, a recent numerical study shows that the breakage of long micelles downstream of the translating sphere causes this unsteady motion [C. Sasmal, “Unsteady motion past a sphere translating steadily in wormlike micellar solutions: A numerical analysis,” J. Fluid Mech. 912, A52, (2021)]. This numerical study further shows that the micelle breakage rate and wall slip can strongly influence this phenomenon. In particular, we find that the onset of this unsteady motion is delayed to higher values of the Weissenberg number as the micelle breakage rate decreases, or in other words, micelles become hard to break. Additionally, we observe that at some values of the micelle breakage rate, again, a transition in the flow field from unsteady to steady occurs at high Weissenberg numbers. Therefore, there is a window of the Weissenberg number present to observe this unsteady motion past the translating sphere. On the other hand, we show that the presence of wall slip on the sphere surface suppresses this unsteady motion past the translating sphere, and a probable explanation is also provided for the same.

Molecular dynamics study of spontaneous capillary flow and heat transfer in nanochannels

Passive cooling technology has become increasingly used in microelectronic devices, with capillary force being the most commonly used driving method. In this study, the capillary flow and heat transfer mechanisms and characteristics in nanochannels are studied based on molecular dynamics (MD), which have rarely been explored in previous studies. The capillary speed increases as the solid-liquid interaction intensity (εwl) increased, but when the wall is completely wetted, the capillary speed slows slightly an increasing εwl. By introducing wall slip, the Lucas-Washburn (L-W) equation is modified and a better fit is obtained. Furthermore, the effects of εwl, the temperature difference of the solid-liquid and surface nanostructure on capillary flow and heat transfer are analyzed. The increase in εwl and wall temperature enhances the capillary speed and heat transfer, but a continuous increase in the wall temperature will accelerate liquid evaporation and cause climb stagnation. The introduction of nanostructures slows the capillary speed, but can enhance the solid-liquid heat transfer. Finally, the microscopic mechanism of the above findings is analyzed by potential energy and density analyses. The reduction of the liquid potential energy at the solid-liquid interface can accelerate the capillary speed to a certain extent, but when the potential energy is too low, the liquid atoms will gather near the wall surface, blocking the capillary climbing. The expansion of the low potential energy region and the high mass density region enhances the solid-liquid heat transfer.

Linear stability of a falling film over a heated slippery plane.

A detailed parametric study on the linear stability analysis of a three-dimensional thin liquid film flowing down a uniformly heated slippery inclined plane is carried out for disturbances of arbitrary wavenumbers, where the liquid film satisfies Newton's law of cooling at the film surface. A coupled system of boundary value problems is formulated in terms of the amplitudes of perturbation normal velocity and perturbation temperature, respectively. Analytical solution of the boundary value problems demonstrates the existence of three dominant modes, the so-called H mode, S mode, and P mode, where the S mode and P mode emerge due to the thermocapillary effect. It is found that the onset of instabilities for the H mode, S mode, and P mode reduces in the presence of wall slip and leads to a destabilizing influence. Numerical solution based on the Chebyshev spectral collocation method unveils that the finite wavenumber H-mode instability can be stabilized, but the S-mode instability and the finite wavenumber P-mode instability can be destabilized by increasing the value of the Marangoni number. On the other hand, the Biot number shows a dual role in the H-mode and S-mode instabilities. But the P-mode instability can be made stable with the increasing value of the Biot number and the decreasing values of the Marangoni number and the Prandtl number. Furthermore, the H-mode and S-mode instabilities become weaker, but the P-mode instability becomes stronger, with the increasing value of the spanwise wavenumber. In addition, the shear mode emerges in the numerical simulation when the Reynolds number is large, which can be destabilized slightly with the increasing value of the Marangoni number; however, it can be stabilized with the increasing value of the slip length and introducing the spanwise wavenumber to the infinitesimal perturbation.

Effect of wall slip on the viscoelastic particle ordering in a microfluidic channel.

The formation of a line of equally spaced particles at the centerline of a microchannel, referred as "particle ordering," is desired in several microfluidic applications. Recent experiments and simulations highlighted the capability of viscoelastic fluids to form a row of particles characterized by a preferential spacing. When dealing with non-Newtonian fluids in microfluidics, the adherence condition of the liquid at the channel wall may be violated and the liquid can slip over the surface, possibly affecting the orderingefficiency. In this work, we investigate the effect of wall slip on the ordering of particles suspended in a viscoelastic liquid by numerical simulations. The dynamics of a triplet of particles in an infinite cylindrical channel is first addressed by solving the fluid and particle governing equations. The relative velocities computed for the three-particle system are used to predict the dynamics of a train of particles flowing in a long microchannel. The distributions of the interparticle spacing evaluated at different slip coefficients, linear particle concentrations, and distances from the channel inlet show that wall slip slows down the self-assembly mechanism. For strong slipping surfaces, no significant change of the initial microstructure is observed at low particle concentrations, whereas strings of particles in contact form at higher concentrations. The detrimental effect of wall slip on viscoelastic ordering suggests care when designing microdevices, especially in case of hydrophobic surfaces that may enhance the slippingphenomenon.

Diffusioosmosis-driven dispersion of colloids: a Taylor dispersion analysis with experimental validation

Diffusiophoresis refers to the movement of colloidal particles in the presence of a concentration gradient of a solute and enables directed motion of colloidal particles in geometries that are inaccessible, such as dead-end pores, without imposing an external field. Previous experimental reports on dead-end pore geometries show that, even in the absence of mean flow, colloidal particles moving through diffusiophoresis exhibit significant dispersion. Existing models of diffusiophoresis are not able to predict the dispersion and thus the comparison between the experiments and the models is largely qualitative. To address these quantitative differences between the experiments and models, we derive an effective one-dimensional equation, similar to a Taylor dispersion analysis, that accounts for the dispersion created by diffusioosmotic flow from the channel sidewalls. We derive the effective dispersion coefficient and validate our results by comparing them with direct numerical simulations. We also compare our model with experiments and obtain quantitative agreement for a wide range of colloidal particle sizes. Our analysis reveals two important conclusions. First, in the absence of mean flow, dispersion is driven by the flow created by diffusioosmotic wall slip such that spreading can be reduced by decreasing the channel wall diffusioosmotic mobility. Second, the model can explain the spreading of colloids in a dead-end pore for a wide range of particle sizes. We note that, while the analysis presented here focuses on a dead-end pore geometry with no mean flow, our theoretical framework is general and can be adapted to other geometries and other background flows.

Analysis of third‐grade liquid under the influence of wall slip and variable fluid properties in an inclined peristaltic channel

AbstractThe current model investigates the effect of wall slip on third‐grade liquid flow through an inclined peristaltic channel. The variation in viscosity and thermal conductivity are taken into account, along with wall properties. The governing equations are simplified using long wavelength and small Reynolds number approximations. The transformed equations are solved by using the perturbation technique. Physiological quantities such as velocity, streamlines, temperature, and concentration are obtained for different parameters of interest. The findings show that increasing the variable viscosity and slip term value improves the velocity profile. Furthermore, elasticity factors help flow, but damping causes fluid particles to slow down. Similarly, when the slip, variable viscosity, and inclination parameter values rise, the size of the trapped bolus grows, resulting in more bolus forms. Furthermore, the inclusion of variable properties helps understand the complex rheological properties of blood flowing through narrow or micro arteries.

Wall Slip of Bimodal Polyethylene

Wall Slip of Bimodal Polyethylene

Study on the influence of surface roughness on wall slip effect (in rheological measurement) of magnetic fluid

Magnetic fluid is a new type of smart material, and its rheological properties have always been a hot research topic, and how to measure quickly and accurately is also a research difficulty, among which wall slip is one of the important influencing factors. In this work, we studied the effect of wall slip on the flow curve and yield stress of the magnetic fluid. To this end, we used multiple rheometer rotors with different surface roughness. The flow curves of two magnetic fluids with different viscosities were drawn, and the Herschel-Bulkley model was used to fit them to determine the yield stress. In addition, a forced shear oscillation test was carried out on the magnetic fluid. We found when using a rotor with a smooth measuring surface to measure the flow curve of the magnetic fluid, the shear stress of magnetic fluid is underestimated at low shear rates. At the same time, the fitted values of yield stress of various roughness measurement surfaces were similar and greater than those of smooth measurement surfaces. From an engineering point of view, the fitting values of non-smooth measurement surfaces were more reliable. Affected by the wall slip, the storage modulus curve of the magnetic fluid often dropped suddenly during the frequency sweeping oscillation process. By changing the surface roughness of the measuring rotor, the wall slip during the oscillation sweep process was effectively restrained. The research results will have certain guiding significance for the study of the state of the solid–liquid interface junction in applications such as magnetic fluids sealing and lubrication.

Influence of wall slip, thixotropy and lubrication regime on the instrumental sensory evaluation of topical formulations.

Drawing parallels from rheotribology can be used to develop a robust instrumental protocol for non-subjective characterization, product development and design of topical dosage forms with desired sensory attributes. However, instrumental characterization of cosmetic products can be influenced by the measurement protocol, thixotropy, flow anomalies like shear banding or wall slip and nature of the film formed on the skin surface. In this study, we evaluated the influence of above parameters on the instrumental sensory evaluation of 12 topical formulations of different galenic forms. Oscillatory strain sweep measurements (SAOS and LAOS) were performed to investigate the influence of frequency and wall slip on the material parameters. The textural attributes at different consumer touchpoints were evaluated by accounting time-dependent simulation of viscoelastic flow. Further, the influence of film thickness and sample drying on the tactile properties of the topical formulations were studied on a non-biological skin model using a sliding probe tribometer. The study shows that the flow properties of the semi-solid formulations depend on the timescale of the problem. A few formulations exhibited wall slip to varying degrees in the linear viscoelastic regime where the behaviour was found not to be characteristic of a particular topical dosage form. The material functions obtained from the Lissajous plots suggest that the non-linear flow behaviour of different galenic forms is least influenced by the boundary conditions imposed by the measurement geometry. The results were statistically analysed using principal component analysis where the attributes used for discriminating skin creams during pick up and rub out are found to be closely associated with non-linear rheology. The friction coefficient exhibited speed dependence where it formed different parametric group with rheological data depending on the lubrication regime. The study highlights that correlations are possible amongst rheological, tribological and instrumental textural analysis data, which can act an impetus for the development of models to predict attributes that drive perception at different consumer touchpoints. However, the choice of instrumental settings, anomalies associated with rheological measurements and friction dependence on a number of parameters can influence the model prediction.

Friction Dynamics of Foams under Nonlinear Motion

Foams are viscoelastic soft materials with complex mechanical properties. Here, we evaluated the friction dynamics of foams between acrylic plates using a sinusoidal motion friction evaluation system and we found some interesting characteristics under accelerated conditions. On a typical solid surface, a symmetrical friction profile, in which static and kinetic frictions are observed, is obtained under reciprocating nonlinear motion. Meanwhile, significant lubricant effects and velocity-dependent friction profiles without static friction were observed in foams. The friction force in foams increased in proportion to the power of velocity, with a power index of <1. These characteristic and dynamic phenomena in foams were observed in this study. They had been caused by the formation of a thick lubricant film and various dissipative modes including surfactant diffusion, viscous dissipation, and wall slip of bubbles. Moreover, the addition of a thickener increased the friction force and the delay time of friction response and improved the foam durability against normal force and shear. These findings are useful for understanding dynamic phenomena in soft materials.

Modeling of the Peristaltic Lithogeic Bile Flow in the Calculus Duct Under the Influence of Heat Transfer with Slip Boundary Conditions

This paper deals with a mathematical model that represents non-Newtonian bile flow through a calculus duct under the influence of heat transfer with wall slip conditions. The peristaltic flow of bile is characterized by a generalized Carreau's model. An ordinate transformation is initiated to map the cosine geometry of the stone into a rectangular grid. It allows evaluation velocity, pressure distribution, flow rate, and reflux occurrence conditions, adopting the perturbation technique, the analytical solution obtained under various parameters, such as Knudsen number, amplitude ratio, Grashof number, Weissenberg number, and power index. These mathematical expressions are analyzed by plotting the graph in MATLAB R2018b software and observed that the axial velocity and the pressure gradient are strongly affected by heat and wall slip parameters.

Optical investigation of dense suspensions with non-Newtonian matrix under pulsating pumping

Pressure-driven Poiseuille flow of a bidisperse, non-colloidal, dense granular suspension with non-Newtonian matrix under pulsatile pumping is studied experimentally. The flow properties are recorded using a transmitted light optical system. The setup allows to derive velocity profiles with a high spatial and temporal resolution. Statements can be made about the near-wall region, which are difficult to achieve experimentally in dense suspensions. The results are compared in detail with velocity profiles based on theoretical models from the field of suspension rheology and particle migration under the conditions present in the experiment. • Spatially and temporally resolved velocity profiles of a pulsating pipe flow. • Complete comparison of experimental data with rheological and particle migration models. • Determination of wall slip coefficients and comparison with predictions of theoretical models. • Evaluation of velocity profiles during acceleration and deceleration phases (pulsating pumping). • Investigation of different particle distributions with regard to the evolving velocity profiles and wall slip.

A novel ecological, highly-utilizable manufacturing technology for standard bamboo units and its deformation laws

Equal-arc shaped bamboo split (EASB) is an ecological material, and the material utilization rate is as high as 85%. With EASB as the basic unit, various arc-shaped products can be manufactured widely. EASB cleverly uses the indispensable drying process in the bamboo industry to manufacture. The bamboo splits deformation mainly undergoes two processes: uniform heating to complete softening and rapid deformation. After the arc is fixed, a wider arc-shaped bamboo split is obtained, with a width up to 100 mm. The sclerenchyma fibers showed displacement along the tensile stress direction, and the distance between vascular bundles was slightly reduced. The parenchyma’s cell wall-to-lumen area ratio was reduced by 19.4%, and cracks and plastic deformation appeared in the cell wall slippage, but there is no evident densification in the bamboo split. EASB overcomes the challenge of use in standard arcs due to its different shapes, while retaining the natural texture and gradient structure of bamboo. Analyzing the deformation in the EASB manufacturing process is beneficial to understanding the EASB deformation law. Moreover, it provides a basis for the later EASB manufacturing process with different radians and biomass biomimetic materials.

Thermo-dynamical investigation of constitutive equation for rate type fluid: a semi-analytical approach

ABSTRACT A thermo-dynamical model of rate type fluid has been developed from a thermodynamic standpoint that is capable of describing the response of viscoelastic fluids through newly modeled constitutive equations. The key findings of this article are to analyze an unsteady Oldroyd-B fluid flow over an exponentially accelerated plate accompanied by continuous temperature in the existence of wall slip conditions. By using Caputo’s time-fractional derivative, a fractional model is introduced for constitutive equations. The Laplace transform is invoked to have semi-analytical solutions from non-dimensional equations of temperature, velocity, shear stress, and Nusselt number. By using the Laplacian computational inversion algorithm of Stehfest, the results of velocity as well as shear stress have been achieved within thermodynamical aspects. The impacts of several parameters on fluid temperature, velocity, and shear stress are prescribed with the help of graphical diagrams. The temperature and velocity profiles are declined for higher values of Prandtl number. The velocity profile and shear stress are increased by growing the values of Grashof number and retardation time. The velocity profile is declined with larger values of slip parameter and relaxation time.

Parallel-Disk Viscometry of a Viscoplastic Hydrogel: Yield Stress and Other Parameters of Shear Viscosity and Wall Slip.

The rheology, i.e., the flow and deformation properties, of hydrogels is generally a very important consideration for their functionality. However, the accurate characterization of their rheological material functions is handicapped by their ubiquitous viscoplasticity and associated wall slip behavior. Here a parallel-disk viscometer was used to characterize the shear viscosity and wall slip behavior of a crosslinked poly(acrylic acid) (PAA) carbomer hydrogel (specifically Carbopol® at 0.12% by weight in water). It was demonstrated that parallel-disk viscometry, i.e., the steady torsional flow in between two parallel disks, can be used to unambiguously determine the yield stress and other parameters of viscoplastic constitutive equations and wall slip behavior. It was specifically shown that torque versus rotational speed information, obtained from parallel-disk viscometry, was sufficient to determine the yield stress of a viscoplastic hydrogel. Additional gap-dependent data from parallel-disk viscometry could then be used to characterize the other parameters of the shear viscosity and wall slip behavior of the hydrogel. To investigate the accuracy of the parameters of shear viscosity and apparent wall slip that were determined, the data were used to calculate the torque values and the velocity distributions (using the lubrication assumption and parallel plate analogy) under different flow conditions. The calculated torques and velocity distributions of the hydrogel agreed very well with experimental data collected by Medina-Bañuelos et al., 2021, suggesting that the methodologies demonstrated here provide the means necessary to understand in detail the steady flow and deformation behavior of hydrogels. Such a detailed understanding of the viscoplastic nature and wall slip behavior of hydrogels can then be used to design and develop novel hydrogels with a wider range of applications in the medical and other industrial areas, and for finding optimum conditions for their processing and manufacturing.

Impact of Newtonian Heating via Fourier and Fick’s Laws on Thermal Transport of Oldroyd-B Fluid by Using Generalized Mittag-Leffler Kernel

In this manuscript, a new approach to study the fractionalized Oldroyd-B fluid flow based on the fundamental symmetry is described by critically examining the Prabhakar fractional derivative near an infinitely vertical plate, wall slip condition on temperature along with Newtonian heating effects and constant concentration. The phenomenon has been described in forms of partial differential equations along with heat and mass transportation effect taken into account. The Prabhakar fractional operator which was recently introduced is used in this work together with generalized Fick’s and Fourier’s law. The fractional model is transfromed into a non-dimentional form by using some suitable quantities and the symmetry of fluid flow is analyzed. The non-dimensional developed fractional model for momentum, thermal and diffusion equations based on Prabhakar fractional operator has been solved analytically via Laplace transformation method and calculated solutions expressed in terms of Mittag-Leffler special functions. Graphical demonstrations are made to characterize the physical behavior of different parameters and significance of such system parameters over the momentum, concentration and energy profiles. Moreover, to validate our current results, some limiting models such as fractional and classical fluid models for Maxwell and Newtonian are recovered, in the presence of with/without slip boundary wall conditions. Further, it is observed from the graphs the velocity curves for classical fluid models are relatively higher than fractional fluid models. A comparative analysis between fractional and classical models depicts that the Prabhakar fractional model explains the memory effects more adequately.

Experimental study of rheological behavior of foam flow in capillary tubes

• Rheological behavior of the pre-generated foam was studied in capillary tubes • Tube materials were: PTFE and FEP (hydrophobic) and glass tubes (hydrophilic) • Foam behaves as shear-thinning fluid in all capillary tubes • Transition foam quality in tubes depends on the material type and tube diameter • Foam's apparent viscosity is higher in hydrophobic tubes than in hydrophilic tube due to wall-slip velocity • Wall-slip velocity depends on the tube material type and diameter • Foam texture in tubes is highly dependent on foam quality and flow rate Studies of foam flow in highly permeable porous media are still limited due to foam's complex behavior and discrepancies in foam research. Specifically, it is still unclear how foam flows in capillary tubes and what the effects of material and tube diameter are. We have investigated the rheology of pre-generated foam in capillary tubes. Experiments were carried out using two types of capillary tubes: hydrophobic (PTFE – polytetrafluoroethylene; FEP – fluorinated ethylene propylene) and hydrophilic (GT – glass tubes). The foam was previously formed in the sand-pack by co-injecting a surfactant solution and nitrogen gas. We investigated the effect of material and tube size on foam rheology versus gas fraction (for a fixed flow rate) and flow rate (for a fixed gas fraction). A three-parameter Herschel-Bulkley model was used to describe foam rheology in capillary tubes. Pictures of foam flow in GT and FEP tubes were taken to determine the mean bubble area using image analysis. We estimated wall-slip velocity in capillary tubes and compared the results with the bulk-foam rheology using an analytical expression of the Herschel-Bulkley model for volumetric flow through the circular tubes. We observed shear-thinning behavior in all capillary tubes (FEP, PTFE, GT), and the Herschel-Bulkley model successfully fitted its behavior. The foam in PTFE tubes behaved as a yield-stress fluid, while yield stress was not observed in GT and FEP tubes. We also found that transition foam quality depends on the material type and tube diameter. The results show that foam's apparent viscosity is higher in hydrophobic tubes (FEP and PTFE tubes) than in hydrophilic glass tubes. This was explained by the wall-slip velocity being higher in glass tubes than in FEP and PTFE tubes because of the difference in surface roughness. The corrected flow rate without wall slip matches the flow rate calculated using the measured bulk foam viscosity better. Therefore we conclude that considering wall-slip velocity is important when studying foam flow in porous media.