Jean-Michel Piau

Slip flow of polybutadiene through fluorinated dies

Abstract Wall surface energy affects the flow of polymer melts in extrusion dies. The aim of the present study is to demonstrate and characterise the flow regimes with slip that are specific to low-energy walls. The polymer used is a polybutadiene (PB) of high molar mass. The dies considered are two-dimensional with either steel walls or walls covered with a fluorinated (PTFE) deposit. Flow curves are drawn for each of the dies and compared. The appearance of the extrudate corresponding to the various flow regimes is observed. Additional results are obtained by various techniques such as flow visualisation and local velocity measurement with a Doppler velocimeter, and stress determination by means of birefringence. For PTFE dies, two stable slip regimes are observed, for low and high levels of stress, separated by a transition zone. The second slip regime, corresponding to higher levels of stress, is characterised by extremely low friction at the wall. The occurrence of slip produces major modifications in the velocity and stress fields, as well as in the appearance of the extrudate, which shows a crack-free surface.

Spatially resolved stress birefringence and flow visualization in the flow instabilities of a polydimethylsiloxane extruded through a slit die

A polydimethylsiloxane of high molecular weight is extruded through a two-dimensional die with high energy walls, by means of a controlled pressure device. In addition to flow rate measurements and flow visualization, a photo-elastic modulator set-up is used to measure stress birefringence in the flow locally and instantaneously. The same instabilities are observed as in capillary extrusion, i.e. extrudate cracking (shark-skin), upstream instability (melt-fracture) and wall slip with flow rate jumps. Birefringence measurements and flow visualization both show that the upstream instability starts before the macroscopic slip, with an asymmetric flow which develops close to the entrance plane of the die. It is shown precisely how the stresses vary locally when the macroscopic slip starts.

Velocity field for polymer melts extrusion using particle image velocimetry: Stable and unstable flow regimes

Abstract Extrusion defects and flow instabilities are an important limitation in most polymer processing operations. Observation of the fluid kinematics to deduce the dynamical response of the polymer can be very useful to characterise these instabilities and understand the mechanisms involved in their triggering and enhancement. To do so, the flow of two linear polydimethylsiloxanes (PDMS) through an axisymmetric sudden contraction has been studied using pressure drop measurements as well as particles image velocimetry (PIV). This optical method for measuring flow velocities provides quantitative whole-field information even when flow conditions are non-stationary. It was shown that, for the two polymer melts studied, the flow is symmetric under stable conditions whereas non-symmetric and non-stationary flows are obtained under unstable conditions. These instabilities arise in the upstream part of the contraction. The extruded rod is then excited by these instabilities which generates the phenomenon of melt fracture at the die exit. In addition, the flow is found to be purely elongationnal on the axis during stable and unstable flow regimes. Under the experimental conditions investigated here, the resulting stretch rate on the centreline varies as x−3, x being the distance above the contraction plane. Moreover flow through contractions becomes unstable first around the centreline and close to the orifice plane. The instabilities then develop in the radial direction and invade progressively the entire upstream flow which completely looses its properties of symmetry: the flow is expected to have a spiralling motion and its two-dimensional (2D) projection looks like a knitting system. The frequency of the instabilities remains approximately the same while their amplitude increases with flow regimes. On the flow axis, instantaneous fluctuations of the velocity can reach ±30% of a mean value characteristic of a mean flow. These fluctuations are more important on each side of the axis where they can reach ±105% of a mean value.

Bed slope effect on the dam break problem

We consider the dam break problem upon an inclined plane. The flow of a series of Newtonian fluids is generated by the collapse of a dam in a completely transparent channel with variable slope. The stage at given abscissa and the front wave evolution are accurately investigated using ultrasonic and image analysis facilities respectively. While the Navier Stokes and the continuity equations are properly rendered non dfmensional in the viscous regine and solved with the ID shallow water approximation. A successful comparison is shown between the experimental and the theoretical results.

Blood yield stress in systemic sclerosis

Blood is a weak percolating physical gel at low shear rates, in which clusters of aggregates can be reversibly disaggregated or formed again. This phenomenon is of potential importance in the microvascular pathophysiology of ischemic and vasospastic disorders such as systemic sclerosis. The aim of this work was to determine blood yield stress using low-shear-rate rheometry with a homemade roughened Couette device in 10 patients with systemic sclerosis compared with 10 healthy controls. Biochemical plasmatic parameters were assessed independently. Results showed a significantly increased stress (+56%, P < 0.05 at 60% hematocrit) for scleroderma patients. The best biochemical predictor for yield stress was the ratio of albumin to globulins; 69% of its variance was explained by plasmatic factors (albumin, fibrinogen, and globulins) in scleroderma patients and 23.4% in healthy controls. Additional microscopic observations showed different microstructures. These results support the hypothesis of an abnormal red blood cell organization process in scleroderma patients that could be partly responsible for the severity of ischemic complications of the disease.Blood is a weak percolating physical gel at low shear rates, in which clusters of aggregates can be reversibly disaggregated or formed again. This phenomenon is of potential importance in the microvascular pathophysiology of ischemic and vasospastic disorders such as systemic sclerosis. The aim of this work was to determine blood yield stress using low-shear-rate rheometry with a homemade roughened Couette device in 10 patients with systemic sclerosis compared with 10 healthy controls. Biochemical plasmatic parameters were assessed independently. Results showed a significantly increased stress (+56%, P < 0.05 at 60% hematocrit) for scleroderma patients. The best biochemical predictor for yield stress was the ratio of albumin to globulins; 69% of its variance was explained by plasmatic factors (albumin, fibrinogen, and globulins) in scleroderma patients and 23.4% in healthy controls. Additional microscopic observations showed different microstructures. These results support the hypothesis of an abnormal red blood cell organization process in scleroderma patients that could be partly responsible for the severity of ischemic complications of the disease.

The influence of the shear field on the microstructural and chemical evolution of an oil well cement slurry and its rheometric impact

Well cementing is one of the most important operations performed in an oil well. An adequate rheological characterization of the slurry is required to optimize cementing parameters. A study of chemical and microstructural development of a cement slurry and its impact on rheometric behavior was carried out. Scanning electron microscopy, X-ray diffraction analysis and rotative rheometry were used. Hydration of the slurry was followed at rest and under shear. It is shown that chemical reaction kinetics, microstructure development and rheometric behavior depend markedly on the shear conditions during hydration.

The reduction of viscous extrusion stresses and extrudate swell computation using slippery exit surfaces

Wall slip has been pointed out as a way to reduce cracks and swelling on polymer extrudates. To investigate this phenomenon, the capillary extrusion of a purely viscous generalized Newtonian fluid is computed with the finite element code POLYFLOW with a realistic slip law. This fits with the friction curve data for a polydimethylsiloxane (PDMS) in a steel die. It is based on molecular dynamics theory and contains a critical stress below which there is qualitative adhesion at the wall. The existence of a slip boundary condition is shown to modify the morphology of the velocity field which tends toward a plug flow. It thus largely reduces the fully developed stress level and the exit stress concentration. Localized slip at the die exit appears even for fully developed flow under shear rate values which are lower than the critical shear rate corresponding to the occurrence of slip at the wall in a Poiseuille flow. This is explained by exit stress concentration. The effect of slip length is then examined: it is found that exit stress and extrudate swell can be largely reduced by using a very short slip length only. This result shows the interest of optimising slippery surfaces in the design of extrusion processes.

Pressure drop scaling laws and structural stress contributions for complex flows of flexible polymer solutions in thick solvents

Abstract Previous experiments showed that the flow through an orifice die of dilute solutions of flexible coils in thick solvents is characterised by three flow regimes. In each regime, pressure drop Pg scales with flow rate qv according to a specific law. The objective of this work is to show how these scaling laws can be related to molecular stress-producing mechanisms, via a similarity analysis based on pertinent structural models. The essence of chain conformation is described by a vector and a simple constitutive equation is developed by using assumptions typical of dumbbell models. However, further constitutive elements are incorporated in order to represent molecular distortion and resulting stress at conditions far away from equilibrium. Besides the usual elastic term accounting for chain resistance to stretching, the stress tensor contains an additional dissipative term arising from important hydrodynamic interactions among chains when chains are highly distorted. The analysis shows how each regime is related to a distinct molecular behaviour and associated tensile stress growth. In the initial linear viscous Newtonian regime, where Pg is linear in qv, molecules are slightly deformed and the solvent Newtonian stresses govern the flow. In the intermediate quadratic regime, where Pg varies as q2v, molecules unravel considerably and elastic stresses dominate. In the ultimate linear viscous regime, where Pg is again linear in qv, molecules are fully stretched and viscous stresses due to hydrodynamic interaction prevail. The model indicates that transient effects should be taken into account when experimental results in shear-free spatially varying flows are interpreted in terms of the fluids elongational properties. Its predictions also suggest that an additional scaling of Pg as qv should be observed in the orifice flow of more concentrated polymer solutions.

A new tool for the rheometric study of oil well cement slurries and other settling suspensions

An original rheometric geometry has been used in order to solve experimental problems related to the study of suspensions of coarse, heavy particles. This testing device has been used to characterize oil well cement slurries, which are settling suspensions of big industrial interest. The measurements show that the new geometry remedies efficiently for settlement and slip at the walls. Results are compared with those obtained using conventional geometries, and reveal the importance of the errors to which non-adapted instruments can lead.

Physicochemical study of the hydration process of an oil well cement slurry before setting

Abstract This study deals with the basic phenomena that govern structural development during an oil well cement slurry hydration. This development has been followed from the mixing of cement with water to the beginning of setting. Scanning electron microscopy observations have been carried out, which revealed the form and structure of the hydration products as a function of the experimental conditions (e.g., temperature, stirring conditions). Results are correlated with X-ray diffraction analysis tests, to identify the chemical nature of crystalline formations observed.