Quantifying the errors due to overfilling for Newtonian fluids in rotational rheometry

The errors on rheological measurements due to overfilling of Newtonian fluids using parallel plate and cone-plate setups in rotational rheometry are quantified. Overfilled sample causes an additional drag force, thereby increasing the measured viscosity, especially when the sample wets the geometry rim. This can cause errors up to 30% in standard experimental setups such as parallel plates with a gap height of 1 mm. This viscosity error increases proportionally with the ratio of gap height to radius of the geometry. By developing a scaling relation that captures the main effects of the geometrical parameters on the viscosity error due to overfilling, a master curve was constructed for the viscosity error as a function of the amount of overfilling. Our systematic analysis of the viscosity error due to overfilling can be utilized to correct for this error during rheological measurements in which overfilling is known but unavoidable or desired.KeywordsOverfilling Edge effects Rotational rheometry Shear viscosit

Direct ink writing of particle-based multiphase materials:From rheology to functionality

Direct ink writing (DIW) allows to produce complicated geometries by extruding material from a nozzle. The ink has to meet certain material requirements during and after printing for the object to be successfully produced. Meanwhile, the functionality requirements of the end-use application should be met. This paper attempts to provide the rheological basis and critical view to understand the material requirements for DIW inks and to help making the bridge between the rheology and printability of particle-based multiphase DIW inks while meeting the functional demands of the end product. Colloidal suspensions and Pickering emulsions are often used material classes for DIW. Some of the most important and noteworthy applications are described for both material classes. Thereafter, a more novel particle based multiphase system for DIW, namely capillary suspensions, is briefly discussed

In-situ experimental investigation of fiber orientation kinetics and rheology of polymer composites in shear flow

In this study, we experimentally investigate the fiber orientation kinetics and rheology of fiber-filled polymer melts in shear flow. A novel setup is designed with custom-built bottom and top geometries that are mounted on a conventional rotational rheometer. Shear flow between parallel sliding plates is applied by vertical movement of the top geometry. The axial force measurement data of the rotational rheometer are used to determine the shear stress growth coefficient. The fiber orientation kinetics are measured in situ with this setup using small angle light scattering. We consider a non-Brownian experimental system with short glass fibers for the suspended phase ( L / D = 8 - 15 ) and different polyethylene based materials for the matrix phase. The fiber orientation kinetics are investigated as a function of fiber volume fraction ( ϕ = 1 % , 5%, and 10%) and as a function of the shear rate ( γ ˙ = 0.03 , 0.55 , and 5 s − 1 ). Within the studied range, these parameters do not influence the fiber orientation kinetics, and a multiparticle model, based on Jeffery’s equation for single particles, can describe these kinetics. Our results show that, up to the concentrated regime ( ϕ ≈ D / L ), fiber-fiber interactions do not influence the fiber orientation in shear flow. Finally, we investigate the shear stress growth coefficient of these composites and demonstrate that a simple rheological model for fiber composites, which assumes a constant, isotropic orientation distribution of the fibers, is able to describe the shear stress growth coefficient of the short fiber composite samples.</p

In situ experimental investigation of fiber orientation kinetics during uniaxial extensional flow of polymer composites

The demand for fiber-filled polymers has witnessed a significant upswing in recent years. A comprehensive understanding of the local fiber orientation is imperative to accurately predict the mechanical properties of fiber-filled products. In this study, we experimentally investigated the fiber orientation kinetics in uniaxial extensional flows. For this, we equipped a rheometer with a Sentmanat extensional measurement device and with an optical train that allows us to measure the fiber orientation in situ during uniaxial extension using small angle light scattering. We investigated an experimental system with glass fibers for the suspended phase ( L / D = 8 − 15 ), and for the matrix either low density polyethylene, which shows strain hardening in extension, or linear low density polyethylene, which shows no strain hardening. For these two polymer matrices, the fiber orientation kinetics were investigated as a function of fiber volume fraction ( ϕ = 1 % , 5%, and 10%) and Weissenberg number (by varying the Hencky strain rate, ϵ ˙ H = 0.01 − 1 s − 1 ). We found that all these parameters did not influence the fiber orientation kinetics in uniaxial extension and that these kinetics can be described by a multiparticle model, based on Jeffery’s equation for single particles. Our results show that, in uniaxial extension, fiber orientation is solely determined by the applied strain and that, up to the concentrated regime ( ϕ ≈ D / L ), fiber-fiber interactions do not influence the fiber orientation. The extensional stress growth coefficient of these composites, which is measured simultaneously with the orientation, shows high agreement with Batchelor’s equation for rodlike suspensions.</p

Numerical simulation of fiber orientation kinetics and rheology of fiber-filled polymers in uniaxial extension

During processing of fiber composites, the fiber-induced stresses influence the local flow fields, which, in turn, influence the stress distribution and the fiber orientation. Therefore, it is crucial to be able to predict the rheology of fiber-filled polymer composites. In this study, we investigate the fiber orientation kinetics and rheological properties of fiber composites in uniaxial extensional flow by comparing direct numerical finite element simulations to experimental results from our previous study [Egelmeers et al., “In-situ experimental investigation of fiber orientation kinetics during uniaxial extensional flow of polymer composites,” J. Rheol. 68, 171-185 (2023)]. In the simulations, fiber-fiber interactions only occur hydrodynamically and lubrication stresses are fully resolved by using adaptive meshing. We employed a 7-mode and a 5-mode viscoelastic Giesekus material model to describe the behavior of, respectively, a strain hardening low-density polyethylene (LDPE) matrix and a non-strain hardening linear LDPE matrix, and investigated the influence of the Weissenberg number, strain hardening, and fiber volume fraction on the fiber orientation kinetics. We found that none of these parameters influence the fiber orientation kinetics, which agrees with our experimental data. The transient uniaxial extensional viscosity of a fiber-filled polymer suspension is investigated by comparing finite element simulations to a constitutive model proposed by Hinch and Leal [“Time-dependent shear flows of a suspension of particles with weak Brownian rotations,” J. Fluid Mech. 57(4), 753-767 (1973)] and to experimental results obtained in our previous study [Egelmeers et al., “In-situ experimental investigation of fiber orientation kinetics during uniaxial extensional flow of polymer composites,” J. Rheol. 68, 171-185 (2023)]. The simulations describe the experimental data well. Moreover, high agreement is found for the transient viscosity as a function of fiber orientation between the model and the simulations. At high strains for high fiber volume fractions, however, the simulations show additional strain hardening, which we attribute to local changes in microstructure.</p

Toughening Brittle Poly(ethylene Furanoate) with Linear Low-Density Polyethylene via Interface Modulation Using Reactive Compatibilizers

Among various biorenewable polymers, poly(2,5-ethylene furandicarboxylate) (PEF) has a large potential to replace fossil-based poly(ethylene terephthalate) (PET) for different applications. However, despite showing better gas barrier properties compared to PET, the inferior mechanical properties of PEF hinder its potential applications. This study reports the toughening of PEF with linear low-density polyethylene (PE) via melt blending by reactive compatibilization at the polymer-polymer interface and benchmarking against similar PET/PE blends. The wettability and spreading coefficient predictions indicate a preferable location of the ternary component (styrene-ethylene/butylene-styrene-graft-maleic anhydride (SEBS-g-MA) or polyethylene-graft-maleic anhydride (PE-g-MA)) along the PEF/PE interface. The interfacial ternary component (concentration and type) exhibited substantial effects on the PEF/PE morphology, altering it from a very coarse incompatible structure to a dispersed morphology for SEBS-g-MA, and fibrillar and cocontinuous morphologies for PE-g-MA. The morphology change in the blends is attributed to reactive compatibilization between the anhydride group of the compatibilizer and the hydroxyl end-group in PEF at the interface. The SEBS-g-MA compatibilized blends exhibited enhanced ductility, as the elongation at break substantially increased with increasing compatibilizer loading, resulting in an 800% increment in the elongation at break and 250% in the tensile toughness compared to those of the neat PEF. These improvements may open new applications of biobased PEF flexible materials for the packaging industry.</p

The interplay between nucleation and patterning during shear-induced crystallization from solution in a parallel plate geometry

Cooling crystallization of small organic molecules from solution is an important operation for the separation and purification of drug products. In this research, shear-induced nucleation from a supersaturated solution is studied in a parallel plate geometry. Under conditions of shear and small gap sizes, narrow mesoscale circular bands of small crystals appeared spontaneously and reproducibly on the plate's surface. We have investigated the connection between nucleation and the emergence of these circular patterns. Our results show that nucleation occurs preferably in zones with high local shear rate (located at the outer edges of the plates), compared to zones with low local shear rate (at the center of the plates). The time before nucleation occurs decreases significantly for increasing mean shear rate and time. The circular crystalline patterns appear at the plate's surface, where heterogeneous nucleation first occurs. Multiple hypotheses are explored to understand the pattern formation in crystallization. Since no satisfactory explanation is found, a new mechanism is proposed. This hypothesis involves crystals initially forming on the surface of the plates and undergoing stick-slip motion, which influences the local nucleation kinetics. This results in an interplay between (secondary) nucleation and stick-slip motion at the start of the crystallization process. By modifying the surface of the plates, their ability to act as a heterogeneous nucleation site can be altered, allowing control over the formation of patterns.</p

Design of lossy dielectric polymer nanocomposite alternating A-B multilayers for absorption-dominated EMI shielding in the X-band regime

The role of permittivity mismatch and of individual layer thickness on the electromagnetic interference (EMI) shielding performance of lossy dielectric polymer nanocomposite multilayer shields is explored via a combined theoretical and experimental approach. The A-B multilayer shields comprise of two or more alternating layers of low and high permittivity facilitated by alternating the filler concentration. A parametric study based on a transfer matrix model is conducted making use of permittivity measurement data on nanocomposites of poly(methyl methacryclate) (PMMA) with carbon nanotubes (CNTs). Theoretical insights are confirmed via an experimental and numerical finite element case study on PMMA-CNT A-B multilayer stacks in a waveguide. The case study highlights the trade-off between absorption-based shielding and high shielding effectiveness. For a given total shield thickness, the shielding performance becomes independent of the number of layers when the individual layer thickness is less than the skin depth of the composite material in the high permittivity layer. Moreover, to obtain absorption-based shielding, less but thicker layers, like a bilayer, can be advantageous. For instance, we demonstrate that a 4 mm thick bilayer with a 1 wt% to 7 wt% CNT concentration mismatch exhibits absorption-based shielding with a shielding effectiveness close to 40 dB within the X-band frequency regime