Dynamic-mechanical properties of interfacially modified glass sphere filled polyethylene

Model composites of spherical glass particles dispersed in a matrix of high density polyethylene were prepared both with and without interfacial modification by an azidofunctional trialkoxysilane. Dynamic mechanical measurements of the composites in the melt state were recorded. The unmodified composites behave as theoretically predicted and the effect of particle—particle interaction at high volume fractions can be measured. The composites with a modified interfacial region have greater shear moduli due to the effect of a region surrounding the particle modified by the silane. The material in this region is largely bound to the glass surface and was examined by Fourier transform infrared spectroscopy after extraction of the bulk matrix. Theoretical calculations are shown to be useful in calculating the mechanical properties and volume fraction of the interfacial region.     

Rheological and electrical properties of EVA copolymer filled with bamboo charcoal

The electrical and rheological properties of an ethylene vinyl acetate (EVA) copolymer filled with bamboo charcoal were investigated. The composites were prepared by melt process in an internal batch mixer. Size distribution analysis showed that d₍₅₀₎ and d₍₉₀₎ values of the bamboo charcoal particles are 12.7 and 40 μm, respectively, with a mean diameter of 22 μm. Scanning electron microscopy proved that the particles of bamboo charcoal present a rectangular shape. The electrical percolation threshold was observed at 0.18 volume fraction (35 wt%) of bamboo. Beyond the percolation threshold, a considerable increase in electrical properties was observed up to a limit value of 10^-2 S/m. The rheological percolation was studied from different rheological models. As a result, the rheological percolation threshold was observed at 0.3 volume fraction (50 wt%) of bamboo charcoal contents. So, the electrical percolation occurs before the rheological percolation. This is principally due to the filler’s characteristics such as the specific surface area, the aspect ratio, and the surface properties. Finally, the bamboo charcoal confers high electrical properties to the EVA composite without inducing strong changes in its viscoelastic properties.     

Dynamic and transient rheological properties of glass filled polymer melts

The dynamic and transient rheological properties of a low density polyethylene melt and a plasticized polyvinylchloride melt filled with glass beads were measured at 200 °C and 180 °C respectively in a modified Weissenberg Rheogoniometer R-17. Its main modification consisted of the use of a piezoelectric transducer instead of the conventional torsion bar, and of the interfacing of a microcomputer Apple II plus to the Rheogoniometer for data acquisition and analysis. The glass beads were pretreated with silane and titanate coupling agents to observe the effect of the chemical modification of the polymer filler interface on these properties. It was observed that both the dynamic viscosity and the storage modulus increased with the weight fraction, but this last parameter did appreciably affect the stress growth and stress relaxation curves of the polymeric matrices at low shear rates. The effect of coupling agents on these properties was varied.     

Rheological characteristics of glass-fibre-filled polypropylene melts

The rheological properties of glass fibre-filled polypropylene melts have been investigated. A high pressure capillary rheometer has been used for the experimental study. The effect of shear rate, temperature, and fibre concentration on the melt viscosity and viscoelastic properties have been studied. An equation has been proposed to correlate the melt viscosity with shear rate, temperature and fibre content. A master curve relation on this basis has been brought out using the shift factora _T . a _T shift factor (=/ _r ) - A _i coefficients of the polynomical of eq. (1) (i = 0, 1, 2, ,n) - B constant in the AFE equation (eq. (2)) (Pa s) - B constant in eq. (3) - D extrudate diameter - d capillary diameter - activation energy at constant shear rate (kcal/mole) - E activation energy at constant shear stress (kcal/mole) - T melt temperature (K) - X fraction glass fibre by weight - shear rate (s^–1) - shear viscosity (Pa s) - normal stress coefficient (Pa s²) - ₁ – ₂ first normal-stress difference (Pa) - shear stress (Pa) - r at reference temperature     

Rheological behaviour of a filled wax system

The temperature dependent rheological behaviour of a pigment filled wax system is investigated in a cone-and-plate viscometer over a range of shear rates from 60 to 10 000 s^–1. A strong influence of water adsorbed by the pigment on rheological properties of the filled system can be found. The increase of the yield stress and the viscosity at low shear rates can be related to a build-up of pigment structures due to growing water content. The flow behaviour can be described by the Casson equation as well as by the Herschel-Bulkley equation.Both formulations are compared and discussed. The Casson model is evaluated more closely by the calculation of characteristic structural parameters of the suspension which are critically discussed.Dedicated to Prof. Dr. Joachim Meissner on the occasion of his retirement from the chair of polymer physics at the Eidgenössische Technische Hochschule (ETH), Zürich.     

Investigation of the rheological properties of short glass fiber-filled polypropylene in extensional flow

The behavior of short glass fiber–polypropylene suspensions in extensional flow was investigated using three different commercial instruments: the SER wind-up drums geometry (Extensional Rheology System) with a strain-controlled rotational rheometer, a Meissner-type rheometer (RME), and the Rheotens. Results from uniaxial tensile testing have been compared with data previously obtained using a planar slit die with a hyperbolic entrance. The effect of three initial fiber orientations was investigated: planar random, fully aligned in the stretching flow direction and perpendicular to it. The elongational viscosity increased with fiber content and was larger for fibers initially oriented in the stretching direction. The behavior at low elongational rates showed differences among the various experimental setups, which are partly explained by preshearing history and nonhomogenous strain rates. However, at moderate and high rates, the results are comparable, and the behavior is strain thinning. Finally, a new constitutive equation for fibers suspended into a fluid obeying the Carreau model is used to predict the elongational viscosity, and the predictions are in good agreement with the experimental data.     

Rheological behavior of filled propylene/ethylene copolymers

A new family of propylene-ethylene copolymers developed by The Dow Chemical Company has narrow molecular weight distributions for industrial polymers and broad chemical composition distributions. This molecular architecture makes possible high filler loadings while maintaining good processability. To provide a fundamental understanding of the lower than expected viscosity, a study of the shear rheological behavior of two series of composites was performed. The composites consist of stearate-coated calcium carbonate particles suspended in a propylene-ethylene copolymer. The matrix in one series was a new copolymer, and in the other series, it was a traditional metallocene copolymer. For both systems, the viscosity increases dramatically with increasing filler loading; however, the viscosity is lower in the case of the composites of the new copolymer. The stearate coating on the calcium carbonate particles decreases the adhesion of the polymer to the filler surface, allowing particle-matrix interfacial slip. A high temperature atomic force microscopy study has indicated the existence of ethylene-enriched zones in the matrix immediately surrounding the particles in the new copolymer. We hypothesize that these high ethylene content chains around the particle surface enhance particle-matrix interfacial slip resulting in the lower composite viscosity.     

Rheological properties of poly(dimethylsiloxane) filled with fumed silica: I. Hysteresis behaviour

The apparent viscosity and primary normal stress difference were measured for dispersions of fumed silica in poly(dimethylsiloxane). Dispersions with less than 4.75% by weight of filler exhibit hysteresis in both the viscosity and normal stress, when the shear rate was increased and then decreased in discrete steps. The shape of the hysteresis loops were sensitive to the details of the deformation history. By using the appropriate deformation history, the material properties determined during the increasing shear rate part of the hysteresis experiment compare favourably with the steady-state rheological properties. The rheological properties of the dispersion were quite sensitive to the age of the fluid with no hysteresis behaviou exhibited by dispersions less than three days old. For dispersions with at least 4.75% by weight of fumed silica, neither the apparent viscosity nor the primary normal stress coefficient exhibited significant hysteresis behaviour. The relationship between the observed rheological behaviour and the dispersion's microstructure is discussed.     

Rheological evidence of modifications of polypropylene by β-irradiation

Electron beam irradiation can be used to induce chemical changes in polymers. The resulting reactions lead to both degradation and crosslinking of polymer chains, depending on reaction conditions. In neat polypropylene, degradation dominates and results in a decrease of molecular weight and worsening of mechanical properties. Enhanced crosslinking can be achieved by utilising a polyfunctional monomer. Triallylisocyanurate (TAIC) serves this purpose and can be used to crosslink polypropylene effectively. The corresponding changes of the rheological properties can be observed using oscillatory and creep experiments when changing the amount of TAIC added as well as the absorbed radiation dose. Depending on these parameters, we attribute the rheological properties to chain degradation or formation of a network and gelation. The phase angle vs the dynamic modulus plot is a useful analytical tool for characterisation of the resultant structures. Some samples showed behaviour that leads us to assume broadening of the molecular weight distribution and long chain branching. Resulting from the parameter dependencies, a topological state diagram is proposed to map parameter values to corresponding polymer structures.     

Transient shear flow behavior of concentrated long glass fiber suspensions in a sliding plate rheometer

In order to eventually predict the behavior of long fiber suspensions in complex flows commonly found in processing operations, it is necessary to understand their rheology and its connection to the evolution of fiber orientation and configuration in well defined flows. In this paper we report the transient behavior at the startup of shear flow of a polymer melt containing long glass fibers with a length (L) >1 mm, using a sliding plate rheometer (SPR). The operation of the SPR was confirmed by comparing the transient shear viscosity (η⁺) for a polymer melt and a melt containing short glass fibers (L < 1 mm) with measurements obtained from a cone-and-plate device, using a modified sample geometry that was designed to avoid wall effects. For the long fiber systems, measurements could only be obtained in the SPR because these systems would not stay within the gap of the rotational rheometer. Transient stress growth behavior of the long fiber systems was obtained as a function of shear rate and fiber concentration for samples prepared with three different initial orientations. Results showed that, unlike short fiber systems (with a random planar initial orientation) that usually exhibit a single overshoot peak followed by a steady state, η⁺ of the long fiber suspensions often passed through multiple transient regions, depending on the fiber concentration and applied shear rate. Additionally, η⁺ of the long fiber suspensions was found to be highly dependent on the initial orientation of the sheared samples. Finally, the initial and final fiber orientations of the long glass fiber samples were measured and used to initiate an explanation of the viscosity behavior. The results obtained in this research will be useful for future assessment of a quantitative correlation between transient rheology and the evolution of fiber orientation.     

An experimental study of dynamic tensile properties of glass fiber at low-temperatures

Tensile impact experiments of EC8.0−24×7 glass fiber bundles at different low temperaturesT(14°C, −40°C and −10°C) and strain rates ɛ were carried out, and complete stress-strain curves were obtained. Within the range of the experiment temperatures and strain rates, it is found that the initial modulusE, the ultimate strength σ_max and the unstable strain ɛ _b of the glass fiber bundles all increase with ɛ at an identicalT. At an identical ɛ, with the decrease ofT, E and σ_max increase; but ɛ _b increases when 10°C>T>−40°C and decreases when −40°C>T>−100°C. The strain-rate- and temperature-dependent bimodal Weibull statistical constitutive theory was adopted for the statistical analysis of the experimental results, and the Weibull parameters of single fiber were obtained. The results show that the bimodal Weibull distribution function is suitable to represent the strength distribution of the glass fiber at low temperature and different strain rates. The differences in the mechanical properties between EC8.0−24×7 and EC5.5−12 ×14 glass fiber bundles were also discussed. Project supported by the National Natural Science Foundation of China (No. 19772058).     

Dynamic flow properties of vinylon fibre and glass fiber reinforced polyethylene melts

An experimental study of the dynamic shear flow properties of polyethylene melts filled with glass fibers and vinylon fibers was carried out and comparison with the steady shear flow properties was made. The effects of loading level and the characteristics of the fibers on the rheological properties of the fiber-filled systems is discussed. The rigidity and complex viscosity of the fiber-filled systems is sensitive not only to the quantity of fibers but also to their length, distribution and properties. The Cox-Merz empirical law for complex viscosity and steady shear viscosity, and Roscoe's empirical relation for estimating the normal-stress coefficients are both able to be applied to pure polymer melts but not to fiber-filled systems.     

Rheological properties of sputum

Summary Sputum is a mixture of saliva with bronchial secretion which, in its turn, is a mixture of tissue fluid transudate through surface epithelium as well as acid glycoproteins from the specialised secretory cells, the goblet cells of surface epithelium, the serous and mucous cells of the submucosal glands. It has recently been shown (Lamb andReid, 1969) that the acid glycoproteins include several types of sialic acid and sulphate. Since sputum has a low dry weight yield its viscosity probably arises from the nature and interaction of its chemical constituents.Sputum is a non-Newtonian fluid. We have recently studied it over a wide shear rate range using aWeissenberg rheogoniometer with oscillation for lower rates and aFerranti-Shirley viscometer for higher levels. This combination has enabled the pattern of sputum viscosity and elasticity to be established. At very low shear rate a feature has been described for the first time for any biological fluid: it has been found in all sputum samples studied and at the same shear rate, regardless of the disease or of the absolute level of viscosity. It would seem, therefore, to arise from some essential feature of the physico-chemical arrangement of sputum. Elasticity over the same shear rates has also been calculated.The following additional features will be discussed: repeatability of results from different aliquots of the same specimen, time-dependent increase in viscosity, variation between certain diseases and the effect of dehydration.

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Zusammenfassung Sputum (Auswurf) ist eine Mischung von Speichel und Bronchialsekret, das seinerseits eine von dem Oberflächenepithel ausgeschiedene Mischung von Gewebeflüssigkeit ist, die auch saure Glykoproteine enthält, die von spezialisierten Sekretionszellen, den Gobletzellen des Oberflächenepithels, den Serösen- und Mukösenzellen der Subkösendrüsen ausgeschieden werden. Kürzlich wurde gezeigt (Lamb undReid, 1969), daß die sauren Glykoproteine verschiedene Arten von Sialinsäure und Sialinsulfat einschließen. Da Sputum ein geringes Trockengewicht hat, beruht seine Viskosität wahrscheinlich auf der Art und der Wechselwirkung seiner chemischen Bestandteile. Sputum ist eine Nicht-Newtonsche Flüssigkeit. Wir haben neulich über einen weiten Bereich von Schergeschwindigkeiten Untersuchungen durchgeführt; für geringe Geschwindigkeiten und oszillierende Bedingungen wurde einWeissenberg-Rheogoniometer verwendet und für höhere Geschwindigkeiten einFerranti-Shirley-Viskosimeter. Diese Kombination ergab ein Bild der Viskosität und Elastizität von Sputum. Für eine biologische Flüssigkeit wurde zum ersten Mal bei geringer Schubgeschwindigkeit ein Charakteristikum beschrieben: dieses wurde in allen untersuchten Sputum-Proben bei der gleichen Schubgeschwindigkeit gefunden und war unabhängig von Krankheiten oder den absoluten Viskositätswerten. Daraus könnte man schließen, daß dieses wesentliche Charakteristikum durch die physikalisch-chemische Anordnung des Sputum bedingt sei. Ebenfalls wurde die Elastizität über gleiche Schergeschwindigkeiten berechnet.Folgende zusätzliche Gesichtspunkte werden diskutiert: Wiederholbarkeit der Ergebnisse mit verschiedenen Teilen der gleichen Probe, Zeitabhängigkeit der Viskosität, Variation aufgrund verschiedener Krankheiten und Einfluß von Wasserentzug.

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Paper presented to the British Society of Rheology Conference on Rheology in Medicine and Pharmacy, London, April 14–15, 1970.     

Rheological scaling and modeling of shear-enhanced crystallization rate of polypropylene

Shear-induced isothermal crystallization of a commercial isotactic polypropylene (iPP) has been investigated by using a rotational rheometer at the steady shear rates ranging from 0.00012 s⁻¹ to 1 s⁻¹, and the temperatures from 135 to 145 °C. Two time scales can be utilized to characterize the crystallization rates: one is the level-upturn onset time of the viscosity; another is that of the normal force. Plotting the onset times against the corresponding onset strain, a common critical value for all the undercooling temperatures can be identified, below which the shear flows have no significant effect on the crystallization rates. Furthermore, we propose a concept of dimensionless onset work; this parameter can make the normalized onset times approximately temperature-invariant in the range of our experiment. Our modeling of the quiescent crystallization is based on the nucleation theory of Ziabicki; the results indicate two-dimensional crystallite growth on pre-existing nuclei. The shear enhanced crystallization is modeled by estimating the excess free energy induced by the flow, and using the rheological model recently proposed by Marrucci, in which the required relaxation times are derived from our rheological measurements. The results imply that the crystallization under the present low shear rates is still two-dimensional crystallite growth on pre-existing nuclei, thus supporting the athermal nucleation theory proposed by Janeschitz-Kriegl. Compared with the experimental data, the modeling is only partially successful. Further improvements encompassing the effects of shear flows on the non-linear increase of the number density of athermal nuclei and on the acceleration of polymer chain disentanglement are needed.     

Rheological properties of poly(dimethylsiloxane) filled with fumed silica: II. Stress relaxation and stress growth

The steady-state viscosity, stress relaxation, and stress growth function were measured for a 3.0% by weight dispersion of fumed silica in poly(dimethylsiloxane); using a two-step shear rate history. For short rest times the stress growth function η⁺ increased monotonically towards steady-state, while at long rest times η⁺ exhibited large overshoot followed by a monotonic decrease to steady-state. For intermediate rest times η⁺ exhibited more complicated transient behaviour. The effect of both shear rates in the two-step shear-rate history on η⁺ have been studied, and the resulting rheological behaviour is discussed in terms of phenomenological microstructural models.     

Rheological properties of road bitumens

Summary The rheological behaviour of eight specimens of bitumens which can be used in asphalt cements has been studied. These are oxidized bitumens of different group composition and degree of oxidation, as well as a specimen of residual bitumen. They were investigated over a wide range of shear rates, frequencies, deformation amplitudes, and temperatures.The bitumen flow may be attended by accumulation of elastic (recoverable) deformations. In the region of low shear stresses the relationship between recoverable deformation and stress corresponds to the Hooke law in shear, and its value depends on the asphaltene content. In the high-stress region the recoverable deformations of the bitumens investigated may reach 0.6 relative unit.Oxidized and residual bitumens of identical group composition show a practically identical viscoelastic behaviour. Depending on the asphaltene content, the rheological behaviour of bitumens may vary from one typical of polymer systems to one typical of disperse systems. It has been established experimentally that for bitumens with a weakly developed structural skeleton the shear stress in stationary flow coincides with the loss modulus in small-amplitude cyclic deformation provided the shear rate and frequency are numerically equal. For structurized bitumens, no correlation of this kind is observed.Bitumens are systems sensitive to the amplitude and frequency of oscillatory shear. The viscosity and dynamic characteristics of bitumens change substantially under the effect of large-amplitude oscillatory shear. The scope of these changes is essentially determined by the asphaltene content. The frequency dependence of the dynamic characteristic enables one to isolate two characteristic regions of viscoelastic behaviour of bitumens: the flow region and that of transition to the storage modulus plateau. The intensity of the change in the dynamic characteristics in the flow region strongly depends on the group composition of the bitumens.The initial values of the relaxation time of bitumens corresponding to their initial undeformed state have been determined from dynamic measurements at low frequencies by extrapolating to the zero frequency. This is one of the most structure-sensitive parameters of the bitumens. The use of this relaxation time makes it possible to plot a generalized relaxation characteristic of the bitumens, which is invariant to the structure and type of the bitumen, the manufacturing process, the nature, the depth of oxidation, the temperature and frequency of coercion.A generalized temperature dependence of the reduction coefficient described by an equation of the type of Williams-Landell-Ferry has been obtained. The softening point of the bitumen was adopted as the reduction temperature. A method for evaluating the viscoelastic behaviour of bitumens on the basis of the known flow curve at a single temperature, the softening point, and the relaxation time at a single frequency is proposed.

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Zusammenfassung Das rheologische Verhalten von acht Bitumenproben, die für Asphalt-Zemente Verwendung finden, wird untersucht. Es handelt sich dabei um oxidierte Bitumina verschiedener Zusammensetzung und mit verschiedenem Oxidationsgrad, sowie um eine Probe Rückstands-Bitumen. Diese werden über einen weiten Bereich von Schergeschwindigkeiten, Frequenzen, Deformationsamplituden und Temperaturen untersucht.Das Fließen der Bitumina kann von elastischen Deformationen überlagert werden. Im Bereich niedriger Schubspannungen entspricht die Beziehung zwischen rückstellbarer Deformation und Spannung dem Hookeschen Gesetz für Scherung, mit einem vom Asphalten-Gehalt abhängigen Wert des Moduls. Im Bereich hoher Spannungen erreicht die rückstellbare Deformation der Bitumina etwa 60%.Oxidierte und Rückstands-Bitumina von übereinstimmender Zusammensetzung zeigen ein praktisch übereinstimmendes viskoelastisches Verhalten. Je nach dem Asphaltengehalt variiert das rheologische Verhalten der Bitumina vom typischen Verhalten eines Polymersystems bis zu dem eines dispersen Systems. Es wird experimentell nachgewiesen, daß für Bitumina mit einem schwach entwickelten Struktur-Skelett die Schubspannung bei der stationären Scherung mit dem Verlustmodul einer periodischen Deformation bei kleiner Amplitude übereinstimmt, wenn man die Schergeschwindigkeit mit der Frequenz gleichsetzt. Dagegen ist für strukturierte Bitumina kein derartiger Zusammenhang feststellbar.Die Bitumina sind empfindlich gegenüber Amplitude und Frequenz einer periodischen Deformation. Sowohl die Viskosität als auch die dynamischen Moduln ändern sich unter dem Einfluß von Scherschwingungen großer Amplitude erheblich, wobei wiederum der Asphaltengehalt von Bedeutung ist. Die Frequenzabhängigkeit der dynamischen Moduln ermöglicht eine Abgrenzung zweier charakteristischer Bereiche des viskoelastischen Verhaltens, nämlich des Fließbereichs und des Übergangsbereichs zum Plateau des Speichermoduls. Die Intensität der Veränderung in den dynamischen Moduln hängt in ausgeprägter Weise von der Gruppenzusammensetzung der Bitumina ab.Die Anfangswerte der Relaxationszeit, welche dem undeformierten Zustand korrespondieren, werden aus dynamischen Messungen bei niedrigen Frequenzen extrapoliert. Es handelt sich dabei um einen besonders strukturempfindlichen Parameter. Die Verwendung dieser Relaxationszeit ermöglicht die Herstellung einer verallgemeinerten Relaxationscharakteristik der Bitumina, welche bezüglich der Struktur, dem Verarbeitungsprozeß, der Natur, dem Oxidationsgrad, der Temperatur sowie der Frequenz der Zwangsbean-spruchung invariant ist.Eine verallgemeinerte Temperaturabhängigkeit des Reduktionskoeffizienten in einer Gleichung vom WLF-Typ wird erhalten, wobei als Reduktionstemperatur die Erweichungstemperatur gewählt wird. Es wird eine Methode zur Bestimmung des viskoelastischen Verhaltens der Bitumina vorgeschlagen, wobei außer der für eine einzige Temperatur bekannten Fließkurve nur noch der Erweichungspunkt sowie die Relaxationszeit bei einer einzigen Frequenz benötigt werden.

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With 15 figures and 2 tables     

Rheological properties of agarose-gelatin gels

Summary Stress relaxation measurements were made on agarose-gelatin aqueous gels. The results were analysed in terms of a six-element model composed of three Maxwell elements in parallel. The temperature and concentration dependences were examined.E ₁, the elastic modulus of the Maxwell element with the longest relaxation time, reflects the elasticity of the main network structure. The observed values ofE ₁ were compared with those calculated by a formula for the composite system. Since the observed values were much smaller than the calculated values, the gelling process of the agarose was considered not to be a simple dispersion of agarose in a gelatin medium, but rather that gelation was hindered by the gelatin. The stress-strain curves for the system were also obtained.

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Zusammenfassung Es wurden Spannungsrelaxationsmessungen an wäßrigen Gelen von Agarose-Gelatine durchgeführt. Die Ergebnisse wurden anhand eines Sechs-Parameter-Modells analysiert, welches durch Parallelschaltung von drei Maxwell-Elementen dargestellt werden kann. Die Temperatur- und die Konzentrationsabhängigkeit wurden untersucht. Der ElastizitätsmodulE ₁ des Maxwell-Elements mit der längsten Relaxationszeit beschreibt die Elastizität der Haupt-Netzwerk-Struktur. Die beobachtetenE ₁-Werte wurden mit den aus einer Formel für ein zusammengesetztes System berechneten Werten verglichen. Da die Beobachtungswerte erheblich kleiner als die berechneten Werte ausfielen, wird geschlossen, daß der Gelierungsprozeß der Agarose nicht als eine einfache Dispergierung der Agarose in dem Medium Gelatine betrachtet werden kann, sondern daß die Gelatine die Gelierung eher behindert. Ebenso wurden Spannungs-Dehnungs-Kurven für das genannte System aufgenommen.

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With 10 figures and 2 tables     

Rheological properties of peanut butter

The rheological properties of two types of commercial peanut butter have been studied. Both products are concentrated suspensions, and differ by the presence of additives. The first type, referred to as “100% peanuts,” is an unstabilized suspension consisting of solid peanut particles in peanut oil which is a Newtonian fluid. The second type, referred to as “smooth,” consists of the same suspension stabilized with a vegetable oil and contains other ingredients such as salt and sugar in very small quantities. A mean volume particle diameter of 6.6 μm has been determined, the particle diameter distribution was found to be narrow, and the solids volume fraction was estimated to be 0.6. Slip encountered in rheometry was greatly reduced by gluing sandpaper to the parallel plates of the rheometer. Both samples behaved like plastic materials and apparent yield stresses of 24 Pa and 370 Pa have been determined for the unstabilized and the stabilized suspensions, respectively. No linear domain was found for both suspensions and the non-linearity was confirmed by deformed Lissajous curves and higher odd harmonics in the output signal of small amplitude oscillatory shear experiments. The stabilized suspension behaved more like a solid, the elastic modulus being larger than the loss modulus and almost independent of the frequency. This solid-like behavior is supposedly caused by strong repulsive (steric) forces induced by the stabilizing agent. Received: 29 September 1999 Accepted: 9 August 2000