Showing papers in "Journal of Rheology in 1997"

Viscoelastic properties of colloidal gels

Abstract: The microstructure and scaling of mechanical properties of dense colloidal gels were investigated as a function of volume fraction and strength of interparticle attraction. Gels were reversibly formed by cooling suspensions of octadecyl silica particles in decalin or tetradecane. Shear history independence of mechanical properties was ensured by preshearing the suspensions in the gelled state. Gelation resulted in suspensions with apparent fractal dimensions of 1.4. Shear densification resulted in an apparent fractal dimension of 2.5 for structures containing many particles. When the gelled suspension was presheared, just after the shear rate was set to zero, elastic moduli were small. Over time, the moduli recovered to a time independent value, G∞′, at a rate α. When measured over a wide range of volume fraction, φ, and strength of interparticle attraction, G∞′, α, and the strain limiting the extent of linear response to oscillatory deformations, γM, fell onto master curves when plotted as a function of ...

Migration of particles undergoing pressure-driven flow in a circular conduit

Abstract: This study focuses on the demixing of neutrally buoyant suspensions of spheres during slow, pressure driven flows in circular conduits. Distributions of the solid fraction of particles, φ, and the suspension velocity, ν, are measured at different lengths from a static in-line mixer. Experiments were conducted over a range of volume average solids fractions, φbulk (0.10⩽φ⩽0.50), and at two different ratios of the particle radius, a, to the radius of the circular conduit, R (a/R=0.0256 and a/R=0.0625). At φbulk⩾0.20, the particles rapidly migrate to the low-shear-rate region in the center of the conduit. This migration results in a blunting of the ν profile, relative to the parabolic profile observed in homogeneous Newtonian fluids. For the flow geometry with the smaller ratio of a/R, the φ profile builds to a sharp maximum or cusp in the center. Particle structures are observed in the experiments with the higher a/R. The entrance lengths for the development of the φ and ν fields, Lφ and Lν, respectively, are strong functions of a/R and φbulk. Lφ and Lν rapidly decrease as φ and a/R increase. Over the range of our data, the ν profiles are observed to develop more rapidly than the φ profiles. The experimental results are compared with fully developed flow predictions from the shear-induced migration (SIM) model and the suspension balance (SB) model. At the smaller a/R, the SIM model more accurately predicts the experimental results. At larger a/R, some qualitative features of the experimental results are better predicted by the SB model, however, neither model provides good quantitative predictions, especially at low φbulk.

Rheology of reconstituted type I collagen gel in confined compression

Abstract: Collagen gels are used extensively for studying cell–matrix mechanical interactions and for making tissue equivalents, where these interactions lead to bulk deformation of the sparse network of long, highly entangled collagen fibrils and syneresis of the interstitial aqueous solution. We have used the confined compression test in conjunction with a biphasic theory to characterize collagen gel mechanics. A finite element method model based on our biphasic theory was used to analyze the experimental results. The results are qualitatively consistent with a viscoelastic collagen network, an inviscid interstitial solution, and significant frictional drag. Using DASOPT, a differential-algebraic equation solver coupled with an optimizing algorithm, the aggregate modulus for the collagen gel was estimated as 6.32 Pa, its viscosity as 6.6×104 Pa s, and its interphase drag coefficient as 6.4×109 Pa s m−2 in long-time (5 h) creep. Analysis of short-time (2 min) constant strain rate tests gave a much higher modulus (...

Some conditions for rupture of polymer liquids in extension

Abstract: Rupture is one of the least investigated and least understood features of the rheological behaviour of polymeric liquids. Some key experimental results on the rupture of polymer melts in uniaxial extension are discussed. Three features of experimental results for narrow molecular weight distribution (MWD) polymers may be highlighted. Firstly, steady elongational flow becomes impossible and rupture of the liquid filament occurs when and if the stored elastic (Hencky) strain reaches 0.5 units. This can be interpreted in terms of a critical Weissenberg number. Secondly, at higher rates of strain (when the elastic strain becomes more than 0.5) the relationship between limiting stress and elastic strain (at the breaking point) is linear. In this case the limiting elastic strain can become at least as high as 2. Thirdly, the strength of a fluid polymer is not a constant but may be characterised by the lifetime, or durability, t* of the filament. This leads to an empirical criterion for rupture: M=(σ3t*/η)=0.30±...

Solution rheology of a hydrophobically modified alkali-soluble associative polymer

Abstract: Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high concentration dependen...

Yield stress in magnetorheological and electrorheological fluids: A comparison between microscopic and macroscopic structural models

Abstract: The yield stress of a magnetorheological suspension is calculated from two different approaches. The first one is based on a mesoscopic description of the structure taking only into account the shape anisotropy of the strained aggregates. The second one is based on a microscopic approach where the interparticle forces, due to the application of the field, are calculated numerically by taking into account the magnetostatics between the particles inside the aggregates. We show that the macroscopic description well applies to suspensions of nonmagnetic particles in a ferrofluid and that a layered structure, consisting of parallel slabs of magnetizable materials should have a yield stress much higher than a structure made of cylindrical aggregates. On the other hand the microscopic approach is appropriated for the description of suspensions of particles of high permeability. In this case, the yield stress is mainly determined by the rupture between pairs of particles and, consequently, it strongly increases with the angle between the line of centers of the pair undergoing the rupture and the field.

Viscoelastic material functions of noncolloidal suspensions with spherical particles

Abstract: We have characterized various steady and time-dependent material functions of suspensions of a non-Newtonian binder, poly(dimethyl siloxane), incorporated with 10%–60% by volume of hollow and spherical glass beads The material functions included storage and the loss moduli, shear stress and first normal stress difference growth and relaxation, relaxation modulus upon step strain and creep and recovery behavior Both constant shear stress and shear rate experiments were carried out using multiple rheometers over a broad temperature range (−35 to 40 °C) while following sample fracture and wall slip effects With increasing volume fraction, φ, of the noncolloidal particles, the strain range, over which linear viscoelastic behavior is observed, became narrower and the relaxation time of the suspension increased Increasing solid content gave rise to the development of the yield stress and the dependence of large amplitude oscillatory shear properties on time and deformation history The yield stress values i

Shear flow of wormlike micelles in pipe and cylindrical Couette geometries as studied by nuclear magnetic resonance microscopy

Abstract: The nonlinear viscosity of the wormlike surfactant system cetyl pyridinium chloride/sodium salicylate (60 mM/100 mM in water) has been investigated in both pipe and cylindrical Couette geometries, using nuclear magnetic resonance to image both velocity and diffusion In pipe flow we observe transitions from Newtonian to non-Newtonian viscosity, to spurt, to unstable flow, and then to a regime where fluctuations are rapid on the timescale of a few milliseconds In the Couette cell we observe apparent slip at the inner wall as well as a high shear rate band located away from the wall in the body of the fluid The banding phenomenon, which has its counterpart in the pipe flow, is consistent with double valuedness in the stress versus rate of strain relationship for this fluid

Transient rheological response and morphology evolution of immiscible polymer blends

Abstract: The response of semi-concentrated model blends (10% disperse phase), consisting of slightly viscoelastic polymers, on a stepwise increase in shear rate is investigated. During the initial stage of the response droplets deform into fibrils. The shear and normal stress transients during the deformation process are modeled by combining the approach of Doi and Ohta with the affine deformation theory for single droplet behaviour. In the proposed equations the scaling relations of Doi and Ohta for transient stresses are preserved. They do not contain any fitting parameter. First, the model predictions are compared with experimental results on model blends. Good agreement is found under conditions for which affine deformation is expected. Second the applicability of the scaling relations of the Doi–Ohta theory is verified experimentally. Although the scaling laws should only apply for 50:50 mixtures of Newtonian liquids with equal viscosity, the experiments show that they hold as well for semiconcentrated system...

Influence of elastic properties on drop deformation in elongational flow

Abstract: We report experimental results on the deformation of a single drop suspended in a medium under uniaxial elongational flow along the central axis of a converging conical channel made of Plexiglas. Both the drop and the continuous phases consist of constant viscosity elastic fluids, so-called Boger fluids. This study reveals several interesting features about the role played by both the drop and matrix elasticities on the drop deformability. In a given matrix fluid, the drop deformation decreases as its elasticity increases. For a given drop fluid, the matrix elasticity has the opposite effect: the drop deformation increases with increasing matrix elasticity. An empirical relation between the drop and matrix deformations is established as a function of the drop and matrix characteristic elastic times.

The rheology of charge stabilized silica suspensions

Abstract: The flow properties of aqueous suspensions of silica particles with diameters ranging from 117 to 780 nm were measured as a function of volume fraction, ionic strength, and continuous phase composition. Dense, aqueous suspensions of all sizes displayed similar behavior with a yield stress in the limit of low shear rate, shear thinning as the stress was raised and, above a critical volume fraction, shear thickening. Master flow curves which are weakly dependent on particle size, volume fraction, surface potential, magnitude of the decay length for the electrostatic forces, or particle size distribution are produced when stress is scaled on the suspension’s elastic modulus, G, and shear rate on G/ηc. Here ηc is the continuous phase viscosity. For suspensions which ordered at rest and of sufficiently high volume faction, shear thickening occurred as a discontinuous decrease in shear rate as the stress increased. This behavior was not observed in suspensions altered so that no order occurred at rest. When the...

Nuclear magnetic resonance visualization of anomalous flow in cone-and-plate rheometry

Abstract: We demonstrate the use of nuclear magnetic resonance (NMR) microscopy to image the velocity distribution for fluids sheared within the gaps (4° and 7°) of cone-and-plate rheometers. These measurements employ a specially constructed rheogoniometer, which fits within the NMR probe system. While the uniform shear rate assumption is verified in the case of simple Newtonian and non-Newtonian fluids, a range of anomalous behavior (apparent slip, shear banding, and fracture) is observed in other systems, including wormlike surfactants, semidilute solutions of 18 MDa polyacrylamide, and dispersed silica in silicone grease.

Experimental study of pure bitumens in tension, compression, and shear

Abstract: Two nominally identical pure bitumens, commonly employed for pavement construction in the United Kingdom, were tested in uniaxial tension, compression, and shear, over a wide range of temperatures, stresses, and strain rates. The bitumens were found to exhibit linear viscous behavior at low stress levels, and power-law creeping behavior at higher stress levels. The temperature dependence was found to follow the Arrhenius relationship at temperatures immediately above the glass transition and the Williams, Landel, and Ferry (WLF) equation at higher temperatures. Below the glass transition temperature, the Eyring plasticity model was found to hold. Constitutive models that reflect the physical mechanisms of steady state and transient deformation are proposed. The fracture properties of the bitumens are also discussed.

Interfacial phenomena in the capillary extrusion of metallocene polyethylenes

Abstract: Metallocene catalysts are known to produce homogeneous random polyolefin copolymers with narrow molecular weight distribution and controlled long/short-chain branching. Two such linear low-density polyethylenes were studied by using both constant-stress and capillary rheometry, in order to assess their rheological and processing behavior, as well as to identify critical conditions for the onset of flow instabilities. It was found that these polymers are thermorheologically complex liquids, apparently due to the presence of long-chain branching. Compared with conventional linear low-density polyethylenes, these metallocene polyethylenes exhibit quite unusual behavior in capillary flow, not previously reported to our knowledge. Specifically, we have encountered long transients in start-up of capillary experiments, and in some cases, the capillary reservoir had to be loaded several times before a steady-state pressure was obtained. In addition, we found that these polymers slip at shear stresses higher than about 0.05 MPa. This critical value is much smaller than the critical value of 0.10 MPa reported for other conventional polyethylenes. A simple qualitative model is proposed to rationalize these findings. It suggests that the molecular mechanism of slip is dominated by a competition between flow-induced chain detachment from the wall and disentanglement of the chains in the bulk from those attached to the wall.

Rheological properties and domain structures of immiscible polymer blends under steady and oscillatory shear flows

Abstract: Rheological properties of three immiscible polyisoprene (PI)/polydimethylsiloxane (PDMS) blends (PI:PDMS=1:9 by weight) having viscosity ratios ηd/ηm=0.155, 0.826, and 4.02, where ηd and ηm are the viscosities of the droplet and matrix, were investigated by directly measuring the droplet size distributions with a video microscope. Steady state measurements were performed by sequentially stepping up the shear rate γ (step-up) and step decrease (step-down) of the shear rate. Dynamic frequency sweep measurements were also performed immediately following cessation of steady shear flow. In the step-up measurements for blends with ηd/ηm=0.155 and 0.826, stresses were proportional to γ and the droplet sizes were well regulated by the flow and inversely proportional to γ for γ⩾0.553 s−1, in good agreement with the Doi–Ohta model [J. Chem. Phys. 95, 1242–1248 (1991)]. The results of step-down measurements were consistent with dynamic moduli measurements, because the droplet sizes were almost unchanged during these measurements. The results are in accord with Palierne’s model [Rheol. Acta 29, 204–214 (1990)]. For the ηd/ηm=4.02 blend, however, the droplet sizes were insensitive to γ, resulting in worse agreement between the experimental and theoretical results than for the other blends.Rheological properties of three immiscible polyisoprene (PI)/polydimethylsiloxane (PDMS) blends (PI:PDMS=1:9 by weight) having viscosity ratios ηd/ηm=0.155, 0.826, and 4.02, where ηd and ηm are the viscosities of the droplet and matrix, were investigated by directly measuring the droplet size distributions with a video microscope. Steady state measurements were performed by sequentially stepping up the shear rate γ (step-up) and step decrease (step-down) of the shear rate. Dynamic frequency sweep measurements were also performed immediately following cessation of steady shear flow. In the step-up measurements for blends with ηd/ηm=0.155 and 0.826, stresses were proportional to γ and the droplet sizes were well regulated by the flow and inversely proportional to γ for γ⩾0.553 s−1, in good agreement with the Doi–Ohta model [J. Chem. Phys. 95, 1242–1248 (1991)]. The results of step-down measurements were consistent with dynamic moduli measurements, because the droplet sizes were almost unchanged during t...

The effect of die materials and pressure-dependent slip on the extrusion of linear low-density polyethylene

Abstract: The flow of linear low-density polyethylene through stainless-steel slit dies occurred at shear rates approximately 12% higher than in identical α-brass dies at the same wall shear stresses, indicating near-wall slip. The flow curves were independent of gap spacing. We show through the slip theory of Hill and co-workers [J. Rheol. 34, 891–918 (1990)] that a measurable gap dependence of the flow curve is not a necessary consequence of wall slip; the flow curves for both stainless steel and α-brass dies can be fit with the same rheological parameters, with a difference in the work of adhesion accounting for the differences in the flow curves. X-ray photoelectron spectroscopy revealed differences in the chemistry of brass surfaces with different pretreating, corresponding to small differences in flow curves. Fluorocarbon-coated die surfaces showed no more slip than stainless steel, while the flow curve with gold-coated surfaces followed stainless steel at intermediate stress and brass at high stress.

From dynamic moduli to molecular weight distribution: A study of various polydisperse linear polymers

Abstract: The linear viscoelastic behavior of various polydisperse linear polymers in the melt is used to predict their average molecular weights and polydispersity index. The method is based on simplified molecular dynamics and has been previously shown to enable a correct description of the dynamic moduli of polypropylenes from the knowledge of their molecular weight distribution (MWD). This so-called forward calculation only requires a few parameters, namely the scaling law for the zero-shear viscosity of narrow fractions η0=f(M), the plateau modulus GN0, and the value of the molecular weight between entanglements Me. The main goal of the present work is to find a solution to the “inverse” problem. To avoid the problem of becoming ill-posed, the shape of the MWD has to be prescribed. Using the assumption of a typical logarithmic bell-shaped Wesslau MWD, the method has been proven to be successful for the recovery of the weight average molecular weight and of the polydispersity index of many linear polymers in a ...

Dynamic viscosity of macroscopic suspensions of bimodal sized solid spheres

Abstract: In this paper, we present experimental measurements for the dynamic viscosity of macroscopic (non-Brownian and noncolloidal) suspensions of bimodal sized spheres when submitted to an oscillating plane Couette flow. The measured viscosity is what we call the dynamic viscosity at finite frequency. Concerning the viscosity of such systems, numerous experimental studies have been done under steady flow conditions, i.e., at zero frequency, but few studies concern the dynamic case. Our measurements have been performed for different values of the three relevant parameters, namely the size ratio λ, the fraction ξ of small spheres to total solids, and the total solid volume fraction Φ. Our results show a viscosity reduction upon mixing, which increases as the total solid volume fraction Φ is increased. We analyze our results by a model that takes into account the volume fraction Φ and the maximum volume fraction Φm, which depends on the two parameters λ and ξ. On the other hand, we compare our experimental results...

Experimental characterization of sharkskin in polyethylenes

Abstract: The sharkskin defect appearing during the capillary extrusion of three low-density polyethylene resins with different molecular structures has been characterized. Using complementary techniques, profilometry, optical microscopy, and observation of cross sections, the amplitude and the period of the defects have been measured accurately. The influence of flow rate, temperature, and die geometry has been quantified. The specific behavior of the orifice die has been put in evidence. It shows that, if the role played by the stress field is evident, the wall shear stress is not the unique determinant of the sharkskin process. The influences of molecular structure and elongational behavior on sharkskin are discussed. It appears that resins exhibiting long chain branching and strain hardening are less sensitive to sharkskin.

A Smoluchowski theory with simple approximations for hydrodynamic interactions in concentrated dispersions

Abstract: In the literature there exist many theories that provide predictions for the rheological properties of concentrated colloidal suspensions, some in excellent agreement with existing experimental data. However, the manner in which hydrodynamic interactions are included differs greatly among the various approaches. Here we incorporate hydrodynamic interactions into the formalism developed previously to account for many body thermodynamic interactions in concentrated suspensions. A conservation equation involving two particle conditional averages of the hydrodynamic functions is derived and solved with different approximations for the functions, including the dilute limit, the rescaling due to Brady, and a lubrication approximation from our previous work. Through our previous calculations without hydrodynamic interactions we have a well characterized approximation for the thermodynamic couplings in concentrated suspensions. The addition of approximations for the hydrodynamic interactions allows prediction for...

Shear induced gelation of colloidal dispersions

Abstract: Nanometric silica particles, dispersed in water, have been bridged into long necklaces by adsorbing macromolecules of poly(ethylene oxide). At high coverage of particles by macromolecules the necklaces repel each other and the dispersions are homogeneous solutions; at low coverage the necklaces bind to each other and a concentrated phase separates from excess water. It is reported here that shear induced gelation and shear induced flocculation are observed near the boundary of the phase separation region. The structures of these dispersions under shear have been observed through small angle neutron scattering. It has been found that, above a critical shear rate, the necklaces connect to each other to form threadlike objects which align along the velocity. At higher shear these objects associate sideways to form three-dimensional flocs.

Melt elongation and recovery of polymer blends, morphology, and influence of interfacial tension

Abstract: Blends of polystyrene (PS) and poly(methylmethacrylate) (PMMA) have been investigated in elongation at 170 °C. The melts of the pure PS and PMMA have only a small difference in their viscosities. Consequently, with 8–12–16–20 weight % PS in PMMA, the elongational viscosities of the melts show only small differences and are similar to that of PMMA. However, when the maximum elongation of 3.5 Hencky units (corresponding to a stretch λ=33 and obtained at a strain rate of 0.1 s−1) is followed by recovery, the recoverable strain strongly depends on the PS concentration and shows very large values with a maximum that corresponds to a recoverable stretch of λR=14.5 (for the 20% PS blend). The morphology obtained by quenching the samples shows that the originally spherical PS droplets in the continuous PMMA are stretched into ellipsoids and finally into long needles. During recovery, the viscoelastic deformation (molecular orientation) recovers and the stretched needles change backwards into spheres. But these tw...

Nuclear magnetic resonance imaging of particle migration in suspensions undergoing extrusion

Abstract: Nuclear magnetic resonance imaging was used to measure fluid velocity and fluid fraction in suspensions flowing into an abrupt four-to-one contraction in pipe diameter, through a section of smaller diameter pipe, and out of an abrupt expansion back to the original pipe size. Suspensions of 50% by volume of particles in a Newtonian liquid were forced to flow by a plunger moving at a constant, slow velocity. Two sizes (100 and 675 μm diameter) of suspended spheres were studied. Conditions were such that buoyant, inertial, Brownian, and surface forces could be assumed to be negligibly small. Little change in particle concentration was seen in the region of the contraction until the plunger was within about one pipe diameter of the contraction. The particles in the small diameter section of pipe migrated toward the pipe axis, the region of lowest shear rate. Particle concentration varied downstream of the pipe expansion, especially in a suspension of the larger particles. Over time, particles were partially s...

Rheological properties and fiber orientations of short fiber-reinforced plastics

Abstract: The effect of fiber orientation on the rheological properties of short glass fiber-reinforced composites was investigated by dynamic oscillatory shearing with parallel plate fixtures. As an oscillatory shear amplitude and frequency applied to fiber-reinforced composites increased, more fibers in the composites were aligned in the flow direction, thus the complex viscosity gradually decreased. This phenomenon was confirmed by observing the fiber orientations using optical photographs. The complex viscosity depended upon the strain amplitude, and pre-oscillatory shearing frequency, and shearing time. Experimental results for fiber orientations and complex viscosity were compared with predictions available at the present time. The predictions of the dependence of fiber orientation upon strain amplitudes and fiber volume fractions are in qualitative agreement with experimental data. However, there were effects of the magnitude of frequency and oscillatory shearing time on fiber orientation, thus complex viscosity could not be predicted successfully although these effects were clearly demonstrated by the experiment.

Electrorheological effect in immiscible polymer blends

Abstract: A new model of the electrorheological (ER) effect for immiscible polymer blends is proposed on the basis of microscopic observations. In the absence of an electric field the polymer with high viscosity is dispersed as droplets in the other polymer with low viscosity, while in the presence of an electric field, the droplets stretch and coalesce to form bridges between electrodes, resulting in the increase of the macroscopic viscosity. A few experimental results are given to confirm the validity of this model.

Rheology and phase separation in a model upper critical solution temperature polymer blend

Abstract: The viscoelasticproperties of a model binary polymer blend exhibiting an upper critical solution temperature phase diagram were investigated by utilizing small amplitude oscillatory and steady shear measurements A mixture of unentangled monodisperse polystyrene and poly(phenyl methyl siloxane), exhibiting Newtonian shear viscosity, was used, and its phase diagram was established by turbidity and dynamic light scattering measurements In the miscible region, the concentration dependence of the viscosity was adequately described by a mixing rule accounting for the surface fractions instead of volume fractions Near the phase separation temperature and far from the glass transition, critical concentration fluctuations dominated the linear viscoelastic response and were responsible for the observed thermorheological complexity An appropriate quantitative account of these fluctuations resulted in the accurate rheological determination of both the binodal and spinodal temperatures, extending thus the applicability of the relevant procedure earlier applied to lower critical solution temperature blends involving higher molecular weight entangled polymers In the phase separated regime, the normal stress of the dispersed phase undergoing spinodal decomposition followed a recent scaling proposed for molecular mixtures with large viscosity difference

Three-dimensional constitutive viscoelastic laws with fractional order time derivatives

Abstract: In this article the three-dimensional behavior of constitutive models containing fractional order time derivatives in their strain and stress operators is investigated. Assuming isotropic viscoelastic behavior, it is shown that when the material is incompressible, then the one-dimensional constitutive law calibrated either from shear or elongation tests can be directly extended in three dimensions, and the order of fractional differentiation is the same in all deformation patterns. When the material is viscoelastically compressible, the constitutive law in elongation involves additional orders of fractional differentiation that do not appear in the constitutive law in shear. In the special case where the material is elastically compressible, the constitutive laws during elongation and shear are different; however the order of fractional differentiation remains the same. It is shown that for an elastically compressible material, the four-parameter fractional solid—the rubbery, transition, and glassy model,...

Direct measurement of adhesion between viscoelastic polymers: A contact mechanical approach

Abstract: We have measured the surface energies of several viscoelastic polymers, using contact mechanics methods. The materials studied were diblock copolymers of poly(ethylene)-poly(ethylene-propylene) (PE-PEP) having different PE volume fractions and molecular weights. The materials exhibit differing viscoelastic relaxation functions in the bulk. Surface energies were determined from contact mechanics experiments in the loading mode, analyzed by incorporating the bulk viscoelastic effects into the Johnson–Kendall–Roberts theory of adhesive elastic contacts. The samples were made of spherical caps of the materials formed by first melting, and then cooling to room temperature. The measured values of the surface energies are close to the reported value for that of the surface-active PEP block, based on prior contact angle measurements, indicating that reliable surface energy values can be extracted from contact mechanics experiments, suitably analyzed to account for viscoelastic behavior. We believe that this metho...

Viscosity equation for concentrated suspensions of charged colloidal particles

Abstract: Concentrated suspensions of charged stabilized colloidal particles exhibit very large viscosity at low shear rate, a strong shear-thinning behavior at intermediate shear rate, and a constant second Newtonian viscosity at high shear rate. This type of non-Newtonian behavior is affected by many factors such as the particle volume fraction φ, the particle diameter, the surface electric potential ψ0, salt concentration, etc. The generalized equation for the viscosity η of this system is proposed by applying Eyring’s transition state theory. The surface electric potential ψ0 and the thickness of the electric double layer κ−1 are determined by applying the theory to experimental data. Systematic experiments of η of the model colloidal dispersion systems are carried out as the function of φ and shear rate and the results are satisfactorily reproduced by the present theory. The effects of hydrodynamic diameter dh and ψ0 of the colloidal particle on η are also quantitatively explained.

A viscoelastic model for dense granular flows

Abstract: In traditional kinetic theory for a granular flow, it is usually assumed that particle interactions are instantaneous and binary. For a dense granular system, these assumptions are usually invalid. In this paper, we use an ensemble averaging technique to examine the effects of finite particle interaction time and multiparticle collisions. The main objectives of this paper are to develop a method and to provide a tool to study dense granular materials. As an example, we study flows of granular particles coated with thin layers of resin. To model particle elasticity and resin viscosity, the force between a pair of particles is approximated by a serial connection of a linear spring and a dashpot. Subsequently, a viscoelastic model is developed from the averaging method. In order to determine coefficients in the constitutive model, direct numerical simulations are performed. When the particle concentration is relatively low, the shear stress is quadratically proportional to the shear rate, in agreement with k...