Stick-Slip Modeling

In this paper, a legless capsule robot (capsubot) comprised of a sealed external body, an internal body, and a rotational actuator is proposed. The aim of this robot is to move in a two-dimensional viscous environment. After the robot is designed and modeled, a four-stage angular velocity profile is proposed to move the robot in one direction. In addition, a camera is used to obtain the position of the robot in the environment while the orientation and the velocity of the robot is estimated with Kalman filter by fusing the IMU, gyro, and the magnetometer sensors. Furthermore, to control the robot a state feedback control is implemented. Finally, experimental results are provided to demonstrate the performance of the robots and the proposed algorithms.

Similar to other rheological properties, slip is an inherent characteristic of a pigmented coating suspension that depends on pigment crowdiness, the state of dispersion, and the flowability of the continuous phase. Slip manifests how a coating behaves in a boundary flow. This is important because coating processes involve boundary flows where traditional rheological measurements of bulk provide only limited information. The principles of wall slippage in polymer melt flows and coating suspensions are discussed. For concentrated suspensions, like coating colors, the local concentration of suspended particles can be lower near a wall than in the bulk. Two methods are presented for calculating the apparent slip velocity at the walls in both capillary and cylindrical viscometric flows. We demonstrate how to calculate the apparent slip velocity in a capillary flow, but there are still questions regarding the accuracy of apparent slip velocity calculations in a cylindrical flow. Slip velocity as a function of shear stress decreases with increasing solids and thickening of the continuous phase. Lubricants as coating additives were found to increase the slip velocity, particularly when using a calcium stearate. This beneficial effect was more pronounced at high (63%) rather than at low (59%) solids. It is speculated that high slip velocities, characteristic of low-solids coatings, may be associated with good coater runnability.

The 1st International Symposium on Rheology and Fracture of Solids ACKNOWLEDGEMENT In the name of ic-rmm Conference Boards I would like acknowledge and say many thanks to our following sponsors for their support in press-campaign and contributions the news and information about The 1 st International Conference on Rheology and Modeling of Materials between their members and in their media:

Souza de Cursi and Sampaio's new book, Uncertainty Quantification and Stochastic Modeling with MATLAB, presents a collection of methods for measuring, quantitatively, the potential error in simulations and models. Uncertainty quantification is a relatively new field within numerical analysis with applications across engineering, social sciences, and natural sciences as computer-based simulations and complex statistical analyses become core analytical methods. The creators of deployed models, such as high-frequency traders and recommendation system developers, will also find this book applicable.

Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply its endorsement by the United States Government or the Jet Propulsion Laboratory, California Institute of Technology.

Flow experiments through capillaries with 0.2% xanthan in aqueous solution and 0.1 N NaC1 brine were carried out to study the influence of the molecular conformation on the flow development at relatively low shear rates, from 20 s-I to 400 s-1. Capillaries with a wide range of length-to-diameter ratios, L/D = 4.5 to 1015 were used. The apparent viscosity as a function of L/D at a constant shear rate shows a continuous decrement of the viscosity as L/D increases, until an asymptotic value is reached. The decrement in the apparent viscosity is partially explained in terms of slip. It was found that slip is a function of L/D as well as shear stress, i.e., slip develops during flow, thus inducing spatial anisotropy in the fluid until a stable state is reached. However, the substantial difference in apparent viscosity between short capillaries and capillaries longer than 300 D may be attributed to dominant elongational flow due to the contraction in the small capillaries and slip in long capillaries. The flow in a sufficiently long capillary can be divided in four regions rather than three, as is usually assumed. In the first region, which corresponds to the entry, elongational and shear flow coexist and elongational flow dominant. In the second region, end effects and slip development are coupled. In the third region the flow is fully developed and end effects are negligible. However, the fluid shows physical characteristics different from those of the fluid at rest, as a consequence of prior slip development. The fourth zone is the exit region in which the velocity rearranges due to the change of boundary conditions. The length of each region depends on the conformation of the macromolecules and shear rate. In addition, it was found that the stiffness of xanthan increases with the increase of the ionic strength. Finally, a performance of Bagley's analysis in the whole range of L/D studied showed that the use of the Bagley correction is not a reliable way to correct for end effects when the flow is not fully developed and/or in the presence of slip.

In this paper a parametric analysis of a sample of responses of a dry-friction oscillator is performed in order to construct a statistical model. The system consists of a simple oscillator moving on a base with a rough surface. Due to this roughness, the mass is subject to a dry-frictional force modeled as a Coulomb friction. It is considered that the base has an imposed stochastic bang-bang motion which excites the system in a stochastic way and, induces stochastic stick-slip oscillations. The base velocity is modeled by a Poisson process for which a probabilistic model is fully specified. The system response is composed by a random sequence alternating stick and slip-modes. With realizations of the system, a statistical model is constructed for this sequence. Statistics and histograms of the random variables which characterize the stick-slip process are estimated. The objective of the paper is to analyze how these estimated statistics and histograms vary with the system parameters...

In this paper the dynamics of a dry-friction oscillator excited by a stochastic base motion is studied. The non-smooth behavior of the dry-frictional force associated with the non-smooth stochastic base motion induces in the system stochastic stick-slip oscillations. The focus of the paper is to analyze the stick-slip dynamics of the system from a probabilistic view point. Defined a time interval for analysis, the variables of interest are the number of time intervals in which stick or slip occur, the instants at which they begin, and their durations. These variables are modeled as stochastic objects and Monte Carlo simulations are employed to compute their statistics.

This paper investigates the dynamics a simple dry-friction oscillator which is composed of a block, modeled as a particle, connected to a fixed support by a spring. The block moves over a continuous belt that is driven by rollers. The frictional force between the block and the belt is modeled as a Coulomb friction. Due to this friction model, the resulting motion of the block can be characterized into two qualitatively different modes, the stickand slip-modes, with a non-smooth transition between them. The focus of the paper is to quantify the percent of time in which the block stays in the stick-mode for a non-constant belt velocity and for different values of the friction coefficient from a deterministic and from a stochastic view point.

This paper investigates the dynamics of a simple dry-friction oscillator which is composed of a block, modeled as a particle, connected to a fixed support by a spring. The block moves over a continuous belt that is driven by rollers. The frictional force between the block and the belt is modeled as a Coulomb friction. Due to this friction model, the resulting motion of the block can be characterized into two qualitatively different modes, the stick-and slip-modes, with a non-smooth transition between them. The focus of the paper is to quantify the percent of time in which the block stays in the stick-mode for different models of periodic belt velocity and for different values of the friction coefficient. Continuous, discontinuous, and random models of belt velocity were considered. The objective is to compare their influence in the duration of the stick-mode. The time of stick represents the fraction of time that the oscillator sticks, it can be viewed as the probability of a biased-coin problem: stick is head and slip is tail.

This paper analyzes the behavior of a stick-slip dry-friction oscillator considering the existence of uncertainties in a friction parameter. The system studied is composed by a block connected to a fixed support by a spring. The block moves over a continuous belt that is driven by rollers. The frictional force between the block and the belt is modeled as a Coulomb friction. Solutions to the phase diagram of the case of belt with constant velocity are analytically developed. By numerical simulations, this simpler case is compared with different imposed non-smooth belt velocity configurations, from a deterministic and from a stochastic viewpoint. The focus of the paper is to determine the duration of the stick-and slip-mode parts of the trajectory through the phase diagram. In the stochastic analysis, the belt speed is modeled as a random process. Monte Carlo simulations are employed to compute the statistics of the response of the stochastic dry-friction oscillator. The variable of interest is the duration of the stick-mode of the trajectory.

Applications of multiphase flows in microchannels as chemical and biological reactors and cooling systems for microelectronic devices typically present liquid slugs alternated with bubbles of elongated shape, the Taylor bubbles. These occupy almost entirely the cross-section of the channel and present a hemispherical front and a liquid layer, the lubrication film, which separates the gas from the tube wall. The Taylor bubble perturbs the surrounding fluids activating many transport mechanisms in the proximity of the gas-liquid interface; therefore, the bubble motion significantly influences the heat and mass transfer rates. Although many works deeply investigate the bubble hydrodynamics in Newtonian fluids, the knowledge about the relation between bubble hydrodynamics and rheological properties is insufficient, and studies where the continuous phase exhibits a shear-thinning behavior are missing. Our numerical analysis tries to fill this gap by investigating the motion of a Taylor b...

Flow experiments through capillaries with 0.2% xanthan in aqueous solution and 0.1 N NaC1 brine were carried out to study the influence of the molecular conformation on the flow development at relatively low shear rates, from 20 s-I to 400 s-1. Capillaries with a wide range of length-to-diameter ratios, L/D = 4.5 to 1015 were used. The apparent viscosity as a function of L/D at a constant shear rate shows a continuous decrement of the viscosity as L/D increases, until an asymptotic value is reached. The decrement in the apparent viscosity is partially explained in terms of slip. It was found that slip is a function of L/D as well as shear stress, i.e., slip develops during flow, thus inducing spatial anisotropy in the fluid until a stable state is reached. However, the substantial difference in apparent viscosity between short capillaries and capillaries longer than 300 D may be attributed to dominant elongational flow due to the contraction in the small capillaries and slip in long capillaries. The flow in a sufficiently long capillary can be divided in four regions rather than three, as is usually assumed. In the first region, which corresponds to the entry, elongational and shear flow coexist and elongational flow dominant. In the second region, end effects and slip development are coupled. In the third region the flow is fully developed and end effects are negligible. However, the fluid shows physical characteristics different from those of the fluid at rest, as a consequence of prior slip development. The fourth zone is the exit region in which the velocity rearranges due to the change of boundary conditions. The length of each region depends on the conformation of the macromolecules and shear rate. In addition, it was found that the stiffness of xanthan increases with the increase of the ionic strength. Finally, a performance of Bagley's analysis in the whole range of L/D studied showed that the use of the Bagley correction is not a reliable way to correct for end effects when the flow is not fully developed and/or in the presence of slip.

Die-land flow on extruding a nonaqueous aluminum nitride plastic body was analyzed using simultaneous orifice and capillary rheometry. Extrudate slip on the wall of the die-land was observed and concluded to involve frictional slip of extrudate particles against the die-wall and the viscous flow of an annular liquid. Results supported a model of a hydroplaning mechanism for the radial consolidation of the particle and binder network with complementary syneresis of liquid. The thickness of the apparent annular slip film was calculated to be submicrometer and to increase with an increase in the extrudate velocity and a decrease in the bulk yield stress and diameter of the die-land. The flow stress on approaching the critical volume concentration of particles terminating plastic flow increased abruptly for shear of the body in the die-entry but more gradually for slip in the die-land. Shear resistance in the die-land was significantly decreased on adding a lubricant and was very dependent on the contact stress between the particle network and the die-wall, controlled by the degree of pore saturation for an unsaturated body and the volume of particles and binder for the saturated extrudate.

A self-excited system with unilateral friction contact modelled by a two degrees-of-freedom mechanical system, where the normal force varies during displacement of a block has been studied. The constructed real laboratory rig approximates the investigated system, and it includes feedback reinforcement of the friction force acting on the vibrated block. Some methods of data acquisition and data handling procedures are proposed for experimentally observed results and data collection. A novel static friction force model for both positive and negative velocities of the base is proposed.

The analysis of the time-resolved fluorescence anisotropy data in light of Stokes-Einstein-Debye (SED) hydrodynamic description reveals significant decoupling of rotational motion of solute with the medium viscosity for a hydroxyl functionalized ionic liquid. This behavior and NMR experiments indicate that the hydroxyl functionalized ionic liquid (IL) is more heterogeneous than the other structurally similar IL. Keeping in mind the recent theoretical investigations (J.

Injected fluid losses in large subsurface channels can be suppressed with the use of low concentrations of viscoelastic surfactants which selectively retard flow in larger apertures. The effect is demonstrated by pumping viscoelastic surfactants through smooth capillaries. The choice of capillary diameters relate to fractures in oil or geothermal reservoirs as well as induced hydraulic fracturing operations in tight gas reservoirs. Selective retardation is favoured at a lower range pressure drops usually associated with oil recovery. The effective apparent viscosity contrasts between different capillary diameters are not as high as those previously observed in permeable flow because the measured effects in the smooth capillaries are mainly shear driven. We expect an elongation contribution to the apparent viscosity in real non-smooth fractures.

Understanding non-Darcian flow of shear-thinning fluids through rough-walled rock fractures is of vital importance in a number of industrial applications such as hydrogeology or petroleum engineering. Different laws are available to express the deviations from linear Darcy law due to inertial pressure losses. In particular, Darcy's law is often extended through addition of quadratic and cubic terms weighted by two inertial coefficients depending on the strength of the inertia regime. The relations between the effective shear viscosity of the fluid and the apparent viscosity in porous media when inertial deviations are negligible were extensively studied in the past. However, only recent numerical works have investigated the superposition of both inertial and shear-thinning effects, finding that the same inertial coefficients obtained for non-Darcian Newtonian flow applied in the case of shear-thinning fluids. The objective of this work is to experimentally validate these results, extending their applicability to the case of rough-walled rock fractures. To do so, flow experiments with aqueous polymer solutions have been conducted using replicas of natural fractures, and the effects of polymer concentration, which determine the shear rheology of the injected fluid, have been evaluated. Our findings show that the experimental pressure loss-flow rate data for inertial flow of shear-thinning fluids can be successfully predicted from the empirical parameters obtained during non-Darcian Newtonian flow and Darcian shear-thinning flow in a given porous medium.

We present a nano-rheometer based on the dynamic drainage flow between a sphere and a plane from bulk regime to highly confined regime. The instrument gives absolute measurements of the viscosity of simple liquids in both regimes. For complex fluids, the measurements involve the viscosity and the elastic modulus. The device operates on distances ranging over four orders of magnitude from 1 nm to 10 μm, bridging rheological properties from the macroscopic to the molecular scale. This allows to measure an hydrodynamic or visco-elastic boundary condition and to explore the causes of the boundary condition at the microscopic level.

The charge transfer (CT) band maximum of N-alkyl pyridinium iodide (NAPI) has been studied as a function of the composition of binary mixed dipolar aprotic solvents. The deviation from linearity of the energy maximum (Ea2) and the mole fraction (of a component solvent) plot is explained as due to a preferential solvation by the more polar cosolvent in the binary mixture. The extent of preferential solvation has been observed to vary with the composition, the maximum being towards the less-polar end. The role of hydrogen bond donating ability of a solvent in preferential solvation is discussed.

Fluorescence anisotropy measurements reveal a non-monotonic density dependence for average rotation time (τ(R)) of a polar solute, coumarin153 (C153) in polar supercritical fluoroform (CHF(3)). The conventional Stokes-Einstein-Debye model, relating τ(R) to the solvent viscosity, fails to explain the observed density dependence, because the experimental viscosity increases monotonously with density for a fluid, in general. Here, the density-dependent τ(R) is calculated by incorporating the wave vector-dependent viscosity of the solvent and the solute-solvent interaction. A molecular hydrodynamic description is used for the wave vector-dependent viscosity which is verified by molecular dynamics (MD) simulation. A justification for the applicability of the present prescription is provided by reproducing the experimental viscosity of supercritical (SC) CHF(3). Solute-solvent interaction has been included via the fluctuating torque acting on the rotating solute. Incorporation of wave vec...

The strengthening of the hydrogen bonding (H-bond) network as well as transition from the tetrahedral-like water network to the zigzag chain structure of alcohol upon increasing the alcohol concentration in ethanol-water and tertiary butanol (TBA)-water mixtures have been studied by using both steady state and time resolved spectroscopy. Absorption and emission characteristics of coumarin 153 (C153), a widely used non-reactive solvation probe, have been monitored to investigate the structural transition in these binary mixtures. The effects of the hydrogen bond (H-bond) network with alcohol concentration are revealed by a minimum in the peak frequency of the absorption spectrum of C153 which occur at alcohol mole fraction ~0⋅10 for water-ethanol and at ~0⋅04 for water-TBA mixtures. These are the mole fractions around which several thermodynamic properties of these mixtures show anomalous change due to the enhancement of H-bonding network. While the strengthening of H-bond network is revealed by the absorption spectra, the emission characteristics show the typical non-ideal alcohol mole fraction dependence at all concentrations. The time resolved anisotropy decay of C153 has been found to be bi-exponential at all alcohol mole fractions. The sharp change in slopes of average rotational correlation time with alcohol mole fraction indicates the structural transition in the environment around the rotating solute. The changes in slopes occur at mole fraction ~0⋅10 for TBA-water and at ~0⋅2 for ethanol-water mixtures, which are believed to reflect alcohol mole fraction induced structural changes in these alcohol-water binary mixtures.

A model to study the dynamics of a stick-slip phenomenon in expansion of tubes was developed. During the permanent deformation process, the system experiences large friction forces at mandrel/tubular interface. This result in an increase of drawing force required for expansion as well as variation in tube thickness along its length. Three different sets of equation; one each for slip phase, stick phase and the transition from stick to slip were derived using equilibrium equations, incompressibility conditions and Karnopp's model. A zero velocity interval was used to define slip, stick and transition phases. The model captured the effect of stick-slip phenomenon on varying thickness reduction along the tube length. The fluctuation in the displacement-time plot clearly indicates the time when the mandrel stuck to the tube. Subsequent slip phase resulted in higher thickness reduction in tube compared to the preceding section. This uneven thickness lowers the structural integrity of the tube during its service life. The results showed that the velocities of mandrel and friction coefficient are critical parameters in minimizing thickness variation after expansion as well as lowering the force required to expand the tube.

This article describes our ongoing efforts to comprehend the role of specific interactions on the dynamical processes such as rotational diffusion and photoisomerization in a typical room temperature ionic liquid. Rotational diffusion studies carried out with a pair of structurally similar non-dipolar solutes indicate that organic solutes do experience strong specific interactions even with the highly associative ionic liquids such that their rotation is hindered. Similar measurements carried out with a nonpolar and a dipolar solute in an ionic liquid and a conventional solvent reveal that even in ionic liquids, apart from the viscosity of the medium, the important parameters, which govern the solute rotation are the solvent-size and free volume in case of non-polar solutes, whereas for charged and dipolar solutes, it is the solute-solvent interaction strength. Photoisomerization studies dealing with a pair of carbocyanine derivatives have shed light on the influence of solvent viscosity and specific interactions on the rates of photoisomerization. Our results point to the fact that the positively charged as well as the negatively charged cyanine derivatives do not experience specific interactions with the ionic liquid such that the isomerization rates are affected. However, when the isomerization rates are compared with a conventional isoviscous solvent, it has been noticed that the rates of isomerization are solely governed by viscosity of the medium in case of the positively charged cyanine derivative. In contrast, photoisomerization rates of the negatively charged cyanine derivative are significantly faster in a conventional isoviscous solvent compared to the ionic liquid due to the specific interactions between the solute and the former, which lower the barrier height for isomerization.

Complex liquids flow is known to be drastically affected by the roughness condition at the interfaces. We combined stresses measurements and observations of the flow during the motion of different rough surfaces in dry liquid foams. We visually show that three distinct friction regimes exists: slippage, stick-slip motion, and anchored soap films. Our stress measurements are validated for slippage and anchored regimes based on existing models, and we propose a leverage rule to describe the stresses during the stick-slip regime. We find that the occurrence of the stick-slip or anchored regimes is controlled by the roughness factor, defined as the ratio between the size of the surface asperities and the radius of curvature of the Plateau borders.

It is now accepted that ionic liquids have heterogeneous spatial distributions of the anions and cations. A number of researchers have shown that solvation and photo-induced electron-transfer reaction dynamics can also be quite heterogeneous.[1,2] We will present an overview of ...

It is now accepted that ionic liquids have heterogeneous spatial distributions of the anions and cations. A number of researchers have shown that solvation and photo-induced electron-transfer reaction dynamics can also be quite heterogeneous.[1,2] We will present an overview of ...

During the drilling process, the drilling system devices can be exposed to several types of phenomena incited by lateral, axial, and torsional vibrations. The latter can lead to severe damages if they are not efficiently controlled and quickly mitigated. This research work is focused on the torsional vibrations, which are stimulated by the nonlinear dynamical interaction between the geological rocks and the drill bit. Wherein, a model with three degrees of freedom was designed to demonstrate the severity of the stick-slip phenomenon as consequence of torsional vibrations. The main objective of this study was to design a robust controller based on hybridizing a conventional PID controller with sliding mode approach in order to mitigate rapidly the torsional vibrations. Moreover, a comparative study between PI, PID and sliding mode controllers allowed us to emphasize the effectiveness of the new hybrid controller and improve the drilling system performances. Furthermore, the chatterin...

Optical coherence tomography velocimetry combined with a rheometer and optical modulation techniques provides increased sensitivity to the low shear rate motion of complex fluid systems. Optical modulation coupled with a new interferometer design yields improved signal to noise ratios and is demonstrated with optically opaque colloidal suspensions. Thus the measurable range of shear velocities with complex fluids can be as low as ∼40 μm s(-1), more than an order of magnitude improvement on the previous lower limit of ∼700 μm s(-1). Furthermore the apparatus demonstrates improved sensitivity to the measurement of velocity. The instrument was used to study two hard sphere colloidal systems, sterically stabilized PVP spheres of 1 μm radius and sterically stabilized polystyrene spheres of 600 nm radius, which display shear banding behavior due to shear induced concentration gradients. OCT velocimetry also allows the velocity fluctuations of the system to be quantified as a function of t...

The rotational dynamics of two structurally similar nonpolar molecules, 2,5-dimethyl-1,4dioxo-3,6-diphenylpyrrolo͓3,4-c͔pyrrole ͑DMDPP͒ and 1,4-dioxo-3,6-diphenylpyrrolo͓3,4-c͔ pyrrole ͑DPP͒ has been studied in glycerol in the temperature range of 300-380 K using both time-resolved and steady-state fluorescence depolarization techniques. While the reorientation times of both the probes are varying linearly as a function of viscosity over temperature, the rotational dynamics of DMDPP is described by the Stokes-Einstein-Debye hydrodynamic theory with slip boundary condition, whereas the reorientation times of DPP are in between slip and stick limits and are about a factor of 1.5 longer than that of DMDPP. This is due to the hydrogen bonding between the two NH groups of the probe molecule and the oxygen atoms of the hydroxyl groups in glycerol. It has also been observed that the rotational dynamics of a nonpolar and noninteracting molecule like DMDPP is essentially the same, both in glycerol and in n-alcohols.

Steady state and time-resolved fluorescence behavior of coumarin153(C153) in bis(1-methyl-1H-imidazol-3ium-3-yl)dihydroborate cation containing room temperature ionic liquid and its mixture with dimethylformamide (DMF) has been investigated. Density functional calculations on the present ionic liquid have been carried out to have ground state structural information of this system. C-H N and C-H O hydrogen bonding interactions between cationic and anionic moiety of the present ionic liquid has been observed. Steady state absorption and emission spectral profiles of C153 are found not to be influenced by the polar cosolvent. Time-resolved fluorescence anisotropy experiments show that the rotational motion of the probe becomes faster in presence of DMF. During time dependent dynamic Stokes shift measurements in ionic liquid-DMF mixtures, the average solvation time is found to decrease with the addition of DMF to the ionic liquid. The decrease in both average solvation and rotational time of probe molecule upon gradual addition of polar organic co-solvent is attributed to the lowering of bulk viscosity of the medium.

Electromechanical systems present an interesting behavior characterized by the mutual influence between the electromagnetic and mechanical parts of the system. The dynamics of an electromechanical system is given by an initial value problem (IVP) comprising a set of coupled differential equations involving mechanics and electromagnetic variables. This work analyzes the dynamics of an electromechanical system with dry-friction, composed of a cart whose motion is excited by a DC motor. The coupling between the motor and the cart is made by a mechanism that transforms the motor rotational motion α into horizontal cart motion in a rail, Figs. 1(a) and 1(b). A voltage is imposed in the motor. The dynamical variables are the current c and the angle α. It is considered the existence of Coulomb dry-friction between the cart and the rail. The resulting motion of the motor can be characterized by two alternate modes, the stickand slip-modes, with a non-smooth transition between them [1, 2]. T...

This paper attempts to present a new analysis for dynamical behavior of two-layer beams with frictional interface which held together in a pressurized environment, including stick-slip nonlinear phenomenon. To achieve a proper outlook of two-layer beam structures behavior, it is essential to realize the mechanisms of motion precisely. Coupled equation of transversal and longitudinal vibration of two-layers in the presence of dry friction is derived and nondimensionalized. Furthermore, free and forced vibration of the mentioned system is investigated and the system dynamics is monitored via Poincare maps and Lyapunov exponent analysis. A comparative study with ANSYS is developed to show the accuracy of the proposed approach.

Rubber compounds are known to exhibit slip at the wall in particular flow conditions. The slip velocity is usually determined by using the classical Mooney method. The rheological behavior of a styrene butadiene rubber (SBR) compound was studied with three different rheometers. Biconical rotational, capillary and slit die rheometers were used to define the true viscous behavior of the compound and the slip velocity. It was shown that it was impossible to apply the Mooney method to our experimental data. New characterizations were thus developed for both capillary and slit die experiments. They were based on the dependency of the slip velocity on the local flow gap. Contrarily to the Mooney method, they provided physically acceptable results and led to a powerlaw relationship between wall slip, wall shear stress and local geometry of the flow.

Rubber compounds are known to exhibit slip at the wall in particular flow conditions. The slip velocity is usually determined by using the classical Mooney method. The rheological behavior of a styrene butadiene rubber (SBR) compound was studied with three different ...

Polymer molecules may be entropically excluded from a region of fluid close to the wall in a narrow capillary or flow channel in a porous medium. This effect results in a layer of polymer-depleted solution close to the wall which causes both apparent viscous slip effects and velocity enhancement of the polymer molecules relative to the solvent molecules. In this paper, theoretical calculations are presented on the effects of the depleted layer on the rheological and transport properties of the polymer solution in capillaries. This work extends earlier simple models to include more complex analytical forms of the layer profile and flow rate effects.

A bicomponent coextrusion process is modelled using a 3-D finite element formulation. The layer uniformity problem in ¢oextrusion is addressed by examining the effects of the polymer melt/polymer melt/die wall contact line boundary condition. It has been observed that the less viscous polymer layer will tend to displace the more viscous polymer layer near the die wall, The behaviour of the contact line is considered to be either a "stick" or "slip" boundary condition, In the "stick" boundary condition, the contact line does not move from its original position after the two polymer layers meet, A slip boundary condition allows the contact line to move along the die wall. The calculated interfaces which result from different contact line assumptions are determined. Results show that if a "stick" boundary condition is appropriate for a given fluid/fluid/solid contact line, then a very thin entrained layer of the more viscous polymer melt will be trapped between the less viscous polymer melt and the die wall. Slip boundary conditions would allow complete displacement of the contact line along the die wall, Both slip and stick boundary conditions produce similar interface profiles far away from the die wall for small viscosity ratios. In certain cases, the displacement of the mdre viscous material by the less viscous material will cease and a static interface structure is produced regardless of die length. Experimental work with polycarbonate melts is compared with the numerical simulations.

Electromechanical systems present an interesting behavior characterized by the mutual influence between the electromagnetic and mechanical parts of the system. The dynamics of an electromechanical system is given by an initial value problem (IVP) comprising a set of coupled differential equations involving mechanics and electromagnetic variables. This work analyzes the dynamics of an electromechanical system with dry-friction, composed of a cart whose motion is excited by a DC motor. The coupling between the motor and the cart is made by a mechanism that transforms the motor rotational motion α into horizontal cart motion in a rail, Figs. 1(a) and 1(b). A voltage is imposed in the motor. The dynamical variables are the current c and the angle α. It is considered the existence of Coulomb dry-friction between the cart and the rail. The resulting motion of the motor can be characterized by two alternate modes, the stickand slip-modes, with a non-smooth transition between them [1, 2]. T...

A model to study the dynamics of a stick-slip phenomenon in expansion of tubes was developed. During the permanent deformation process, the system experiences large friction forces at mandrel/tubular interface. This result in an increase of drawing force required for expansion as well as variation in tube thickness along its length. Three different sets of equation; one each for slip phase, stick phase and the transition from stick to slip were derived using equilibrium equations, incompressibility conditions and Karnopp's model. A zero velocity interval was used to define slip, stick and transition phases. The model captured the effect of stick-slip phenomenon on varying thickness reduction along the tube length. The fluctuation in the displacement-time plot clearly indicates the time when the mandrel stuck to the tube. Subsequent slip phase resulted in higher thickness reduction in tube compared to the preceding section. This uneven thickness lowers the structural integrity o...

The stick-slip transition behaviour of two high density polyethylene (HDPE) melts are studied experimentally by using a capillary rheometer with twin bores at different temperatures. The shear stress-shear rate curves are investigated by the capillary rheometer with two diameters. The results show that the flow curves break at a certain critical shear stress. The broken point of the flow curve implies the occurrence of the stick-slip transition. The critical shear stresses obtained by the two capillaries equal approximately, but extrapolation slip length increases with the diameter of the capillary. It is found that the critical shear stress increases proportionally with absolute temperature, which means increasing temperature can depress or delay the occurrence of slippage to a certain degree. Additionally it is found the slip section's slope of the shear stress-shear rate curve is lower than the sticky section's slope.

The rich diversity of man-made complex fluids and naturally occurring biofluids is opening up new opportunities for investigating their flow behavior and characterizing their rheological properties. Steady shear viscosity is undoubtedly the most widely characterized material property of these fluids. Although widely adopted, macroscale rheometers are limited by sample volumes, access to high shear rates, hydrodynamic instabilities and interfacial artifacts. Currently, microfluidic devices are capable of handling low sample volumes, providing precision control of flow and channel geometry, enabling a high degree of multiplexing and automation, and integrating flow visualization and optical techniques. These intrinsic advantages of microfluidics have made it especially suitable for the steady shear rheology of complex fluids. In this paper we review the use of microfluidics for conducting shear viscometry of complex fluids and biofluids with a focus on viscosity curves as a function of shear rate. We discuss the physical principles underlying different microfluidic viscometers, their unique features and limits of operation. This compilation of technological options will potentially serve in promoting the benefits of microfluidic viscometry along with evincing further interest and research in this area. We intend that this review will aid researchers handling and studying complex fluids in selecting and adopting microfluidic viscometers based on their needs. We conclude with challenges and future directions in microfluidic rheometry of complex fluids and biofluids.

Measurements have been undertaken on pressurised saturated hydrocarbon/water mixtures from the North Sea. Hydrate formation and dissociation was detected, the hydrate dispersion was characterized, and the yield stress of the dispersion was measured. Laboratory rheometers fitted with suitable pressure and temperature control facilities can be used to characterize many of the important parameters associated with the presence of hydrates in production systems.

The aim of this study was the characterization of polymer flows within an extrusion die using particle image velocimetry (PIV) in very constraining conditions (high temperature, pressure and velocity). Measurements were realized on semi-industrial equipments in order to have test conditions close to the industrial ones. Simple flows as well as disrupted ones were studied in order to determine the capabilities and the limits of the method. The analysis of the velocity profiles pointed out significant wall slip, which was confirmed by rheological measurements based on Mooney's method. Numerical
simulations were used to connect the two sets of measurements and to simulate complex velocity profiles for comparison to the experimental ones. A good agreement was found between simulations and experiments providing wall slip is taken into account in the simulation.

A model to study the dynamics of a stick-slip phenomenon in expansion of tubes was developed. During the permanent deformation process, the system experiences large friction forces at mandrel/tubular interface. This result in an increase of drawing force required for expansion as well as variation in tube thickness along its length. Three different sets of equation; one each for slip phase, stick phase and the transition from stick to slip were derived using equilibrium equations, incompressibility conditions and Karnopp's model. A zero velocity interval was used to define slip, stick and transition phases. The model captured the effect of stick-slip phenomenon on varying thickness reduction along the tube length. The fluctuation in the displacement-time plot clearly indicates the time when the mandrel stuck to the tube. Subsequent slip phase resulted in higher thickness reduction in tube compared to the preceding section. This uneven thickness lowers the structural integrity of...

Some of the challenges being faced by the petroleum development industry worldwide are low-cost and more efficient well completion, water-shutoff, other types of zonal isolation, etc. Successfully addressing these issues, solid expandable tubulars (SETs) are finding increasing use for enhanced oil recovery from difficult or damaged wells. All SET applications revolve around cold expansion of the tubular in a casing or in an open hole, by hydraulically pushing a cone through the tubular, or by mechanically pulling it. Issues such as maximum expansion ratio, post-expansion pipe integrity, etc are directly linked with material properties of the tubular before and after expansion. An SET expansion test rig has been designed, fabricated, and commissioned at the Engineering Research Lab, Sultan Qaboos University (SQU) in collaboration with Petroleum Development Oman (PDO). Various types of material testing and characterization equipment augment this facility. The current paper presents so...