Morton Denn

City College of New York | CCNY · Levich Institute

The phenomenon of shear-induced jamming is a factor in the complex rheological behavior of dense suspensions. Such shear-jammed states are fragile, i.e., they are not stable against applied stresses that are incompatible with the stress imposed to create them. This peculiar flow-history dependence of the stress response is due to flow-induced micro...

Simulations are used to study the steady shear rheology of dense suspensions of frictional particles exhibiting discontinuous shear thickening and shear jamming, in which finite-range cohesive interactions result in a yield stress. We develop a constitutive model that combines yielding behavior and shear thinning at low stress with the frictional s...

Normal stresses in complex fluids lead to new flow phenomena because they can be comparable to, or even larger than, the shear stress. In addition, they are of paramount importance for formulating and testing constitutive equations for predicting nonviscometric flow behavior. Very little attention has thus far been paid to the normal stresses of yi...

The phenomenon of shear-induced jamming is a factor in the complex rheological behavior of dense suspensions. Such shear-jammed states are fragile, i.e., they are not stable against applied stresses that are incompatible with the stress imposed to create them. This peculiar flow-history dependence of the stress response is due to flow-induced micro...

Simulations are used to study the steady shear rheology of dense suspensions of frictional particles exhibiting discontinuous shear thickening and shear jamming, in which finite-range cohesive interactions result in a yield stress. We develop a constitutive model that combines yielding behavior and shear thinning at low stress with the frictional s...

In many instances, sprays are formed from the breakup of liquid jets or sheets. We investigate the different parameters that determine the characteristic drop size in the breakup of sheets. We vary both the spraying parameters, such as the pressure and geometry of the nozzle, and the fluid parameters, such as viscosity and surface tension. The comb...

Normal stresses in complex fluids lead to new flow phenomena because they can be comparable to or even larger than the shear stress itself. In addition, they are of paramount importance for formulating and testing constitutive equations for predicting non-viscometric flow behavior. Very little attention has so far been paid to the normal stresses o...

There is an ongoing discussion in the literature about the flow behavior of the widely used model yield-stress fluid Carbopol. Some papers show that it is indeed the simple model yield-stress fluid that many people believe it to be. However, other authors report rheological hysteresis in the flow curve, transient shear banding that persists for a v...

Discrete particle simulations are used to study the shear rheology of dense, stabilized, frictional particulate suspensions in a viscous liquid, toward development of a constitutive model for steady shear flows at arbitrary stress. These suspensions undergo increasingly strong continuous shear thickening (CST) as the solid volume fraction ϕ increas...

Shear thickening is a phenomenon in which the viscosity of a suspension increases with increasing stress or shear rate, sometimes in a discontinuous fashion. While the phenomenon, when observed in suspensions of corn starch in water, or Oobleck, is popular as a science experiment for children, shear thickening is actually of considerable importance...

Nanosized filler particles enhance the mechanical properties of polymer composites in a size-dependent fashion. This is puzzling, because classical elasticity is inherently scale-free, and models for the elasticity of composite systems never predict a filler-size dependence. Here, we study the industrially important system of silica-filled rubbers,...

It is now 30 years since Barnes and Walters published a provocative paper in which they asserted that the yield stress is an experimental artifact. We now know that the situation is far more complicated than understood at the time, and that the mechanics of the solid material prior to yielding must be considered carefully. In this paper, we examine...

Dynamic particle-scale numerical simulations are used to study the variation of microstructure with shear stress during shear thickening in dense non-Brownian suspensions. The microscale information is used to characterize the differences between the shear thickened (frictional) and non-thickened (lubricated, frictionless) states. Here, we focus on...

Yield stress materials are ubiquitous, yet the best way to obtain the value of the yield stress for any given material has been the subject of considerable debate. Here we compare different methods of measuring the yield stress with conventional rheometers that have been used in the literature on a variety of materials. The main conclusion is that,...

Steady shear measurements on three yield-stress fluids, a foam and two types of emulsions, were carried out using a smooth torsional parallel-plate geometry, with different gap spacings to assess the effect of wall slip on the normal stress differences. The classical Mooney analysis gave slip-corrected flow curves that agreed with data from a rough...

Dynamic particle-scale numerical simulations are used to show that the shear thickening observed in dense colloidal, or Brownian, suspensions is of a similar nature to that observed in non-colloidal suspensions, i.e., a stress-induced transition from a flow of lubricated near-contacting particles to a flow of a frictionally contacting network of pa...

We have measured the viscometric functions of mono- and bimodal noncolloidal suspensions of poly (methyl methacrylate) spheres in a density-matched aqueous Newtonian suspending fluid using parallel plate and cone-and-plate rheometry for particle volume fractions in the range of 0.20–0.50. Cone-and-plate normal stress measurements employed the metho...

Particulate fillers are often used to enhance the properties of soft materials; polymer composites often contain nanometer-sized particles to improve reinforcement, for example. The rationale for using nanometer-sized particles remains unclear, however, and classical micromechanical models cannot account for a scale dependent reinforcement. The sys...

The flow of dry and wet granular media is investigated in a Couette geometry using magnetic resonance imaging in order to test the applicability of the “fluidity model” for nonlocality in these materials. Local volume fraction measurements show that the systems become heterogeneous during flow. We find that the nonlocal rheology of suspensions can...

We present a comprehensive review of the physical behavior of yield stress materials in soft condensed matter, which encompasses a broad range of soft materials from colloidal assemblies and gels to emulsions and non-Brownian suspensions. All these disordered materials display a nonlinear response to an external mechanical forcing, which results fr...

Numerous materials, from biopolymers to filled rubbers, exhibit strain softening at high strain amplitudes during a strain sweep in oscillatory rheology: The modulus decreases with increasing deformation. On the other hand, if the nonlinear elastic response is analyzed within a single oscillation cycle (described by a Lissajous curve), these system...

Numerous materials, from biopolymers to filled rubbers, exhibit strain softening at high strain amplitudes during a strain sweep in oscillatory rheology: The modulus decreases with increasing deformation. On the other hand, if the nonlinear elastic response is analyzed within a single oscillation cycle (described by a Lissajous curve), these system...

The discontinuous shear thickening (DST) of dense suspensions is a remarkable phenomenon in which the viscosity can increase by several orders of magnitude at a critical shear rate. It has the appearance of a first order phase transition between two `"states" that we have recently identified as Stokes flows with lubricated or frictional contacts, r...

Particles suspended in a Newtonian fluid raise the viscosity and also generally give rise to a shear-rate dependent rheology. In particular, pronounced shear thickening may be observed at large solid volume fractions. In a recent article (R. Seto, R. Mari, J. F. Morris, and M. M. Denn., Phys. Rev. Lett., 111:218301, 2013) we have considered the min...

In a recent article (R. Seto, R. Mari, J. F. Morris, and M. M. Denn, Phys. Rev. Lett., 111:218301, 2013) we found frictional contact forces to be essential for reproducing the shear thickening behavior of non-Brownian suspensions. Although the introduction of frictional contact to a Stokesian Dy- namics simulation is speculative and requires the ex...

Particles suspended in a Newtonian fluid raise the viscosity and also generally give rise to a shear-rate dependent rheology. In particular, pronounced shear thickening may be observed at large solid volume fractions. In a recent article (R. Seto, R. Mari, J. F. Morris, and M. M. Denn., Phys. Rev. Lett., 111:218301, 2013) we have considered the min...

Suspensions of non-Brownian particles are commonly encountered in applications in a large number of industries. These suspensions exhibit nonlinear flow behavior, even in Newtonian suspending fluids under conditions where inertial effects can be ignored and linearity would normally be expected. We review the observed rheological behavior, emphasizi...

Discontinuous shear thickening (DST) observed in many dense athermal suspensions has proven difficult to understand and to reproduce by numerical simulation. By introducing a numerical scheme including both relevant hydrodynamic interactions and granularlike contacts, we show that contact friction is essential for having DST. Above a critical volum...

We have incorporated a configuration-dependent friction coefficient (CDFC) in the full-chain stochastic tube model for entangled melts and solutions developed by Xu, Denn, and Schieber [J. Rheology 50, 477-494 (2006)] to account for microstructural change of the tube away from equilibrium. The simulation results from the modified model significantl...

We have modified the full-chain stochastic tube (XDS) model developed by Xu [J. Rheol. 50, 477-494 (2006)] to simulate the rheology of entangled melts and solutions of linear monodisperse polymers. The XDS model, which has a single adjustable parameter that is equivalent to the Rouse time, successfully describes steady and transient shear and norma...

Yield-stress liquids are materials that are solid below a critical applied stress and flow like mobile liquids at higher stresses. Classical descriptions of yield-stress liquids, which have been the basis for asymptotic and computational studies for five decades, are inadequate to describe many recent experimental observations, and it is clear that...

Polymer melt processing requires an integration of fluid mechanics and heat transfer, with unique issues regarding boundary conditions, phase change, stability and sensitivity, and melt rheology. Simulation has been useful in industrial melt processing applications. This brief overview is a personal perspective on some of the issues that arise and...

We are surrounded in everyday life by yield stress fluids: materials that behave as solids under small stresses but flow like liquids beyond a critical stress. For example, paint must flow under the brush, but remain fixed in a vertical film despite the force of gravity. Food products (such as mayonnaise), other consumer products (such as toothpast...

A computational framework for computing taper and twist profiles for chiral fibers fabricated from heated bi-component drawn glass filaments is developed, with full coupling between the momentum and temperature fields in both the glass and the surrounding air. A method for solving the inverse problem, in which the takeup holder velocity is varied w...

Gel propulsion systems combine the best characteristics of solid and liquid propellants. The gel system stores like solid propellant, but flows like a liquid when pressurized, enabling throttle and restart capability similar to liquid propellants. In addition, gels have a lower vapor pressure compared to their liquid counterparts. Consequently, the...

Rutherford Aris was at the forefront of the movement to utilize mathematical rigor to understand the foundations of chemical engineering practice. The impact of Aris, his Minnesota colleagues, and their students is felt throughout the chemical engineering curriculum. One place that impact has not been felt, however, is in the introductory course in...

We compute the surface-induced morphology and the free-energy pathways as a cylindrical liquid crystalline filament with preferred homeotropic (orthogonal) interface orientation passes through a sequence of growing sinusoidal perturbations and breaks up into droplets. Liquid crystalline morphology is determined using a simulated annealing algorithm...

We study the buoyancy-driven motion of two-dimensional bubbles and droplets in a Bingham fluid using a regularization method. The finite-element computations are carried out using the method of level sets to track the interface. We find that multiple bubbles and droplets can move in a body force field under conditions where a single bubble or dropl...

The stochastic chain model described by Xu et al. [J. Rheology50, 477–494 (2006)] provides fundamental insight into the mechanism of apparent wall slip in entangled polymer melts.Apparent slip is shown to be a consequence of a rapid decrease in the entanglement density for chains in the region of the wall. There is good agreement between model pred...

Orientation distributions in droplets of liquid crystals with homeotropic anchoring are computed with a simulated annealing algorithm that minimizes the free energy of the Oseen-Frank continuum theory. The droplets exhibit multiple orientational steady states that are separated by finite energy barriers over the entire range of the dimensionless ra...

We seek to understand the way in which a cylindrical liquid crystalline dispersed phase with preferred homeotropic (orthogonal) interface orientation can break up into droplets. Our motivation is the observation by Inn and Denn [1] that a dispersion of the biphenylcarbonitrile 8CB in its nematic state in polydimethylsiloxane has both locally stable...

This report summarizes the issues discussed during a Symposium of the International Union of Theoretical and Applied Mechanics, entitled "Interactions for Dispersed Systems in Newtonian and Viscoelastic Fluids," which was held in March 2006 in Guanajuato, Mexico.

We describe a full-chain stochastic tube model for entangled melts and solutions of linear polymers. The model incorporates two forces that result from chain confinement: a tensile force along the chain that prevents chain collapse and a transverse force that keeps the chain within the virtual tube. The model tracks conjugate constraint pairs and u...

We describe dielectric spectroscopy measurements on dispersions of two thermotropic liquid crystals (5CB and 8CB) in a poly(dimethylsiloxane) matrix. 5CB exhibits nematic and isotropic phases, while 8CB exhibits smectic, nematic, and isotropic phases. The spectra of the dispersions exhibit a temperature-dependent dielectric relaxation in the interv...

Journal of Rheology 49(6), i (2005) DOI: http://doi.org/10.1122/1.4757288

The effects of edge fracture on constant torque rheometry of entangled polymer solutions were analyzed. The analysis were carried out on the 10 and 7.5 wt % solution of 1,4-polybutadiene dissolved in phenyl-terminated oligomeric butadiene. It was observed that the power-law region for the 7.5% solution is about 0.085. It was also observed that the...

The no-slip boundary condition is a valuable empiricism derived from 19^th Century experiments on low molar-mass liquids. Data that suggest deviations from the no-slip condition have long been available, but convincing evidence came only through experiments with entangled molten polymers, where the molecular scale over which slip might occur is lar...

Rheology is the study of the deformation and flow of matter. Interest in paints, plastics, ceramic pastes, lubricants, and other industrial fluids led to the founding of the Society of Rheology in December, 1929. Flow is essential to the measurement of many properties of a nonlinear or time-dependent fluid, so non-Newtonian fluid dynamics and rheol...

Synopsis The scaling theory for rupture of entangled polymeric liquids, which is based on a critical recoverable strain Joshi, Y. M., and M. M. Denn, J. Rheol. 47, 291–298 2003, is extended to include the viscoelastic regime, where the total strain at rupture is a decreasing function of the elongation rate. Rupture can be predicted in both the visc...

A model is a quantitative abstraction of a physical process, in which the description of the process is represented by the solution to a set of mathematical equations. The mathematical formulation enables the model to be used for a variety of purposes, including design, control, and exploration of operating strategies; the effects of changes in pro...

nt transient that may be many orders of magnitude longer than time scales associated with instrument inertia. Intrinsic material time scales can be identified with the dynamics of the macro­ molecular chains. Shaping processes for polymeric materials are usually carried out in the liquid state, often over times that are rapid relative to those asso...

Inertialess planar contraction flow with a Navier (linear) slip boundary condition has been studied for Newtonian and inelastic non-Newtonian fluids. For Newtonian fluids there is a region of curved streamlines as the flow adjusts from an upstream no-slip region with radial streamlines to a downstream slip-flow region that also has radial streamlin...

The effect of a chemically heterogeneous surface pattern on the microscopic phase separation of a thin film composed of a binary polymer blend has been studied using a three-dimensional self-consistent field method and a Monte Carlo method based on the “bond fluctuation model.” The surface motif can be transferred from the surface into the polymer...

The interfacial tension between two low-molar-mass biphenylcarbonitriles that exhibit liquid crystalline phases (4‘-pentyl-4-biphenylcarbonitrile and 4‘-octyl-4-biphenylcarbonitrile) and poly(dimethylsiloxane) was measured as a function of temperature using pendant drop tensiometry enhanced by video-image digitization. Interfacial tensions are incr...

The interactions of two identical rigid spheres of radius R translating in a Bingham material in creeping flow along their line of centers are calculated at various sphere separations using the finite element method. The yield surfaces are determined by an extrapolation using a regularized constitutive model. Two spheres falling in a line interact...

Synopsis Polymer melts and concentrated solutions rupture at high rates of elongation in a manner that is reminiscent of the cohesive failure of solids. We propose a simple molecular picture of rupture of a polymer filament, in which catastrophic failure occurs when the frictional force on an entangled chain can no longer balance the tension in the...

The properties of the interfacial region between melt phases of a liquid crystalline polymer and a flexible polymer were studied using the three-dimensional bond fluctuation model. The far-field orientation has a strong effect on the magnitude of the interfacial tension and the structure of the “interphase”. The interfacial tension is an increasing...

Recent progress in modeling and simulation of the flow of nematic liquid crystals is presented. The Leslie-Ericksen (LE) theory has been successful in elucidating the flow of low molar-mass nematics. The theoretical framework for the flow of polymeric nematic liquid crystals is still evolving; extensions of the Doi theory capture qualitative featur...

Creeping flow around a solid sphere is solved numerically using two regularized constitutive equations that approximate a Bingham material. The yield surface cannot be easily established from contours of the yield stress obtained with finite values of the regularization parameter due to numerical constraints. The outer yield surface can be estimate...

Enhanced throughput and the elimination of sharkskin were observed over an extended temperature range when linear low-density polyethylenes having the same molecular weight, polydispersity, and melt index, one with and one without commercial additives, were extruded through α-brass dies. The additives appear to play a significant role in the detail...

The structure of the interface between a liquid crystalline polymer (LCP) and a flexible polymer was studied using the three-dimensional bond fluctuation model in Monte Carlo simulations. Orientation in both phases in the neighborhood of the interface is sensitive to the far-field nematic orientation. The more diffuse interface caused by a homeotro...

Key Words melt fracture, sharkskin, slip-stick, adhesion, extrudate swell s Abstract Polymer melts exhibit extrusion instabilities at sufficiently high levels of stress, and they appear to exhibit wall slip. I explore the evidence for slip, the possible mechanisms of slip, and the relation between slip and extrusion instabilities.

Abstract Polymer melts exhibit extrusion instabilities at sufficiently high levels of stress, and they appear to exhibit wall slip. I explore the evidence for slip, the possible mechanisms of slip, and the relation between slip and extrusion instabilities.

We have studied the initial deformation and shape recovery of droplets of a flexible polymer and a liquid crystalline polymer in a flexible thermoplastic matrix. The LCP droplets exhibit a size dependence that is not observed for the flexible polymer droplets, possibly because of the effect of orientational elasticity in the liquid crystalline poly...

We have solved the equations of transient shear flow for a model of a liquid crystalline polymer that contains a long-range interaction term in the nematic potential. The model exhibits a rich set of dynamics when the structure and momentum equations are coupled, including a periodic vorticity ‘burst’ near the shearing surface and very large gradie...

We employ rheological measurements of polyethylene blends in the melt and solid state, together with thermal analysis, to infer phase behavior. Partial-miscibility in the melt is characterized by use of the double-reptation model to define the complex modulus of the continuous phase for input into the emulsion model for the blend; this approach int...

Surface mobility and wall slip of entangled polybutadiene melts were studied with attenuated-total-reflectance infrared spectroscopy at stresses characteristic of the sharkskin, spurt, and melt-fracture regimes. Small-scale slip, accompanied by an apparent decrease in transverse mobility, occurs in the sharkskin regime, but at a stress above the vi...

The steady-shear viscosity and first normal stress difference and the dynamic storage and loss moduli have been measured for a blend consisting of a thermotropic liquid crystalline polymer dispersed in a thermoplastic fluoropolymer matrix. The components are immiscible and nonreacting. Consistency with the Palierne emulsion theory for viscoelastic...

The general solution for squeeze flow between closely-spaced plates of arbitrary shape includes an in-plane potential flow whose components satisfy the Cauchy–Riemann conditions, and the velocity field and the pressure are both determined by the boundary conditions at the edge of the plates. In contrast, the velocity field for the infinite-plate li...

The addition of 10% of an aqueous lyotropic liquid crystalline dispersed phase to an isotropic polydimethylsiloxane melt increases the steady shear and normal stresses and the dynamic moduli, but the increase caused by the liquid crystalline dispersed phase is much less than expected from the Palierne and Doi–Ohta theories of blends. In contrast, t...

We have extruded a linear low-density polyethylene through capillary dies fabricated from stainless steel and brass. We confirm a result first reported by Ramamurthy [Ramamurthy, A. V., {open_quotes}Wall Slip in Viscous Fluids and Influence of Materials of Construction,{close_quotes} J. Rheol. {bold 30}, 337â357 (1986)]: sharkskin can be eliminated...

A kinetic equation for polydomains is used to develop a formal structure for a mesoscopic constitutive theory of textured liquid crystalline polymers. The lowest-order approximation for the texture terms leads to the Larson and Doi (1991) theory, but with a different meaning for the model parameters. Predicted macroscopic order parameters are in th...

The Leslie-Ericksen equations in the tumbling regime were solved for developing flow between parallel plates to study the two-dimensional spatial development of director orientation. Calculations were carried out using the physical properties of 8CB. Texture development is governed by the number of director rotations required to reach the equilibri...

Fully-aromatic thermotropic liquid crystalline polymers (LCP) containing 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA) were studied with 1H NMR. A two- or three-parameter nematic director distribution in molten or nearly molten samples was obtained via rigorous simulation of wideline spectral lineshapes. This methodology was furt...

Using attenuated total-reflectance infrared spectroscopy (ATR/FTIR), the concentration of deuterated polybutadiene near the surface of a flat zinc selenide crystal was followed as it was replaced by ordinary polybutadiene by flow and diffusion. Experiments were performed in the melt, both below (M ∼ 1,500) and above (M ∼ 15,000) the entanglement th...

Coalescence of electrolytically-generated, 50- to 600– μm-diameter gas bubbles was observed using an optical technique that employs a linear photodiode array to detect interface movement with a resolution of 10−6s. When two bubbles coalesce, the surface energy that is released causes interface velocities of 2 to 4 m/s; these are followed by large-a...

Flow visualization experiments have been used to characterize the fluid dynamics of a thermotropic liquid crystalline polymer, Vectra A, in the barrel and conical entry of a capillary rheometer. The displacement profiles indicate shear-thinning in the straight section, with a power law index much lower than that measured rheometrically. The contrac...

The flow of linear low-density polyethylene through stainless-steel slit dies occurred at shear rates approximately 12% higher than in identical alpha-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 (...

The Chilton−Colburn j-factor is used to analyze the relation between published drag and heat-transfer correlations in fiber spinning, and a proposal of consistent correlations for transport coefficients is presented.

The isothermal crystallization kinetics of poly(ethylene terephthalate) (PET) in blends with a fully aromatic liquid crystalline copolyester (Vectra A) were studied with differential scanning calorimetry. PET crystallization rates decreases with increasing Vectra fractions in the blends, and the percentage of PET that is crystalline also decreases...

A fully aromatic thermotropic liquid crystalline polymer, poly(4-hydroxybenzoic acid/6-hydroxy-2-naphthoic acid) or p-HBA/HNA, was studied at elevated temperatures with ^1H NMR. The wideline ^1H spectral lineshapes of molten p-HBA/HNA reflect the orientation distribution of the (NMR timescale averaged) polymer long axis or director, and (2) the dis...

The solid-phase rheology of a thermotropic polyester which is liquid crystalline in the melt (Vectra A) was studied for one-dimensional finite-amplitude deformations, including step-strain and recovery, step-stress and recovery, and step-strain followed by small-amplitude oscillations. The rheology is complex, and cannot be described by existing mo...

The influence of functional groups on the dynamics of exchange between a flowing bulk melt and the channel wall was studied for oligomeric polyolefins (up to C-16) using attenuated total reflectance Fourier transform infrared spectroscopy. Surface transport is dominated by adsorbed layers whose structure is determined by functional groups and chain...

The viscosities of liquid dairy foods were measured in a slit rheometer with gap spacings as small as 34 μm. Large positive deviations from the true flow curve and an approach to a constant stress were observed below a critical rate in each material studied. This behavior is indicative of probable agglomeration and“bridging”, most likely in the ent...

The dynamics of chain exchange between flowing bulk melt and the channel wall were studied for oligomeric polyolefins using attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FTIR). The dynamics are dominated by a surprisingly slow first-order process which depends on flow rate and materials of construction of the channel wal...

A stable island'' has been discovered in the slip-stick region of a linear low-density polyethylene. Within this region, which exists only in a narrow temperature range, pressure oscillations cease, the pressure drop falls by a substantial amount, and the extrudate becomes reasonably smooth. Unstable oscillating flow persists at throughputs on both...

The dynamics of a viscoelastic polymer melt spinline are analyzed by solving the linearized perturbation equations in the frequency domain using a realistic constitutive equation for the melt rheology. The computed frequency response is in reasonably good agreement with experiments on the spinning of poly(ethylene terephthalate), but the agreement...

The onset of sharkskin and wavy extrudate distortions was studied in a linear low-density polyethylene which was diluted up to 40 wt.% with octadecane in order to vary the plateau modulus. The ratio of critical wall stress to plateau modulus remained constant for the onset of sharkskin. The transition to wavy flow occurred at a constant wall stress...

The flow of a non-tumbling nematic liquid crystal through a planar 4 : 1 contraction was simulated using the Leslie-Ericksen (L-E) continuum theory ; this theory accounts for fluid anisotropy and elastic stresses resulting from spatial distortion of the "director", which is a vector field describing average local molecular orientation. Calculations...