Complex Rheological Behavior Research Articles

FingerMap: a new approach to predict soft material 3D objects printability

Soft material 3D printing through liquid deposition modelling (LDM) is a challenging manufacturing process where yield stress control is mandatory. Indeed, the higher the yield stress value, the more complex the 3D printed structure can be. In a bid to go one step further, this report proposes a new approach enabling the prediction of soft material 3D printability as a function of the material’s properties and shape. The prediction consists in numerical simulation to anticipate, in silico, the collapse of a voxelised 3D design, called FingerMap. To do so, a calibration of the program using three silicone formulations (with increasing yield stress value) was first performed to define a printability domain according to mass/surface ratio, overhang angle and the z-position of each voxel. Then, two anatomical 3D models (ear and aortic valve) were used to demonstrate the capacity of the tool to predict printability. Good correlations between theoretical and experimental results were obtained. The proposed in silico simulation tool was then proven to be useful for LDM, even if some limitations were identified, particularly in the case of materials exhibiting complex rheological behaviours such as time-dependent rheological properties.

Nonlinear responses of carbon black-filled polymer solutions to forced oscillatory shear

Systematic rheometric measurements are carried out to explore the nonlinear responses of carbon black-filled polyisoprene/squalene solutions with the polymer concentrations ϕ in the matrix ranging from the coil overlapping state (ϕ =ϕ*) to the melt state (ϕ =1). At the coil overlapping state, when ϕ <<1, the system typically shows the classic nonlinearity, where the storage modulus G’ decreases as strain amplitude γ0 increases and the resulting stress responses are distorted and deviated from normal sinusoidal waves. At the melting state when ϕ = 1, the system displays a new nonlinearity, where the stress responses to forced oscillatory shear are surprisingly sinusoidal regardless the reduction of G’ as the strain amplitude is increased. Between the two states when ϕ*< ϕ <1, the system exhibits an anomalous and complex nonlinear rheological behavior throughout a wide range of polymer concentrations. The ratio of the third to the first harmonics (I3/I1) in the stress responses first decreases rapidly reaching a minimum when ϕ ≈ 0.1, then increases to a maximum when ϕ ≈ 0.3, after which it decreases again as ϕ approaches the melt state. Lowering the test frequency shifts horizontally the minimum and the maximum in I3/I1 vs ϕ to the right. Increasing strain amplitude intensifies this anomalous behavior. Remarkably, the restoration of broken filler network in the polymer/solvent media also displays the corresponding minimum and maximum values. The quasi-sinusoidal or linear dynamic response in a system that has a strain-dependent modulus is apparently due to the restoration of broken filler network requiring longer than the time scale of a typical dynamic perturbation.

Effect of pre-shearing on the steady and dynamic rheological properties of mud sediments

Mud sediments can exhibit a complex rheological behaviour particularly a thixotropic character or structural recovery after breakup due to the presence of organic matter/biopolymer. Such biopolymers can lead towards the development of flocculated structures having multiple length scales which are sensitive to shearing rate and history. In this study, the extent and rate of structural recovery of mud sediments was studied by measuring the storage modulus as a function of time using small amplitude oscillatory tests after a destructive steady shearing. This linear viscoelastic response of the sediments was further investigated as a function of several parameters including pre-shear rate, pre-shear time, measuring geometry, mud density and organic matter content. The equilibrium storage modulus (G∞') and the characteristic time (tr) for the structural recovery of the sediment matrix were estimated by fitting the experimental data to a stretched exponential function. The normalized storage modulus, G'/G0' (i.e., structural parameter) was used to relate it with the yield stresses of mud sediments. The results showed that the recovery of structure after shearing was instantaneous (tr being of the order of seconds), however, the extent of recovery was highly dependent on the studied parameters. The extent of recovery was higher for the samples with lower density and lower organic matter content. The effect of the shearing time on tr and G∞' was almost negligible, which implies that the destruction of the structure was achieved within seconds. Using vane geometry, the extent of recovery was higher than using Couette geometry which is linked with the distribution of shear stresses within the cell for each geometry. Yield stresses showed a strong dependency on structural parameter, until it reaches very small values. At low values of structural parameter, the yield stresses were constant as the structural recovery was even faster than the time required to perform the amplitude sweep tests. This study provides an extensive knowledge about the structural recovery in mud sediments under different shearing conditions which can be useful for sediment management.

Numerical modelling of solid hydrogen flow through an extruder die

Harnessing energy by nuclear fusion has progressed into technological developments as a result of sufficient progress made in the theoretical investigations. Fueling a tokamak by pellet injection is reported to be most efficient among other fueling techniques. It is thus important to produce a solid hydrogen filament by extrusion process using an extruder-die setup. It is required to arrive at an optimum configuration of the die as the pressure drop in the die decides the load on the extruder. The present analysis focuses on developing a CFD model of a die taking into account the complex rheological behavior of solid hydrogen. A systematic parametric analysis is performed to study the influence of die geometry on the flow behavior. The un-yielded regions are identified from the simulation results. It has been found that the un-yielded region decreases as the contraction ratio and contraction angle is decreased. The un-yielded zone is predominantly present in the region before contraction. However, at lower flow rates, the un-yielded zone appears even after the contraction.

Viscoelastic model with complex rheological behavior (VisCoR): incremental formulation

A thermo-rheologically complex linear viscoelastic material model, accounting for temperature and degree of cure (DoC), is developed starting with series expansion of the Helmholtz free energy and systematically implementing simplifying assumptions regarding the material behavior. In addition to the temperature and DoC dependent shift factor present in rheologically simple models, the derived novel model contains three cure and temperature dependent functions. The first function is identified as the rubbery modulus. The second is a weight factor to the transient integral term in the model and reflects the current temperature and cure state, whereas the third function is under the sign of the convolution integral, thus affecting the “memory” of the material. An incremental form of this model is presented which, due to improved approximation inside the time increment, has better numerical convergence than most of the similar forms. Parametric analysis is performed simulating stress development in a polymer, geometrically constrained in the mold during curing and cool-down. The importance of using proper viscoelastic model is shown, and the role of parameters in the model is revealed and discussed.

Is density enough to predict the rheology of natural sediments?

Mud is a cohesive material which contains predominantly clay minerals, water, organic matter and some amounts of silt and sand. Mud samples can have complex rheological behaviour, displaying viscoelasticity, shear-thinning, thixotropy and yield stress. In this study, influence of organic matter on the rheological behaviour of different mud samples having similar densities is investigated. Four samples, collected from different locations and depths of Port of Hamburg (Germany) were selected. Two samples with the density of about 1210 kg/m3 and two samples with the density of about 1090 kg/m3 were analysed by different rheological tests, including stress ramp-up tests, flow curves, thixotropic tests, oscillatory amplitude and frequency sweep tests. Two yield stress regions (with two yield stress values stated as “static” and “fluidic” yield stresses) were identified for all the samples, and these regions, corresponding to a structural change of the samples were significantly different from sample to sample due to the differences in organic matter content. For lower density samples, the ratio of fluidic to static yield stress increased from 3 to 4.4 while it increased from 4.4 to 5.2 in case of higher density samples, by increasing the organic matter content. The thixotropic studies showed that the mud samples having lowest organic matter content (VH and KBZ) exhibit a combination of thixotropic and anti-thixotropic behaviours. The results of frequency sweep tests revealed the solid-like character of the mud within the linear viscoelastic regime. Mud samples having higher organic matter content (RV and RT) had a higher complex modulus (417 Pa and 7909 Pa) than the ones with lower organic matter content (13 Pa and 1774 Pa), for a given density. This study demonstrated that the density only is not a sufficient criterion to predict the rheology of different mud. Furthermore, even small amounts of organic matter content change significantly the mud rheological behaviour.

A smoothed particle hydrodynamics (SPH) formulation of a two-phase mixture model and its application to turbulent sediment transport

A Smoothed Particle Hydrodynamics (SPH) formulation and implementation of the classical two-phase mixture model are reported, with a particular focus on the turbulent sediment transport and the sediment disturbances generated by moving equipment operating near or on the seabed. In the mixture model, the fluid-particle system is considered to be an equivalent medium whose evolution is described by a set of equations for the mixture continuity and momentum conservation, with the particle volume fraction being tracked by a transport equation. The governing equations are adapted to a Lagrangian, weakly-compressible SPH framework, the turbulence is modeled by a Reynolds-averaged Navier-Stokes approach, and adaptive boundary conditions for shear stress and turbulent quantities are implemented to account for laminar or turbulent local flow conditions. The complex rheological behavior of clay sediment/water mixtures is modeled using a volume fraction, shear rate-dependent viscosity which accounts for the existence of a yield stress. Hence, the proposed work encompasses several challenging modeling aspects: turbulence, non-Newtonian fluid behavior, sediment transport, and fluid-structure interactions. It is then illustrated on diverse cases of interest: a fluid-particle mixture column release, its subsequent turbulent transport and return to a hydrostatic equilibrium, the settling of particle clouds and two cases of particle-driven gravity currents, and their comparisons with available results. Finally, SPH simulation results for the disturbance of a bed of clay sediment/water mixture induced by a moving plate are reported and compared with experiments performed in our laboratory. The proposed SPH two-phase mixture model agrees well with the existing results considered in this study.

Shear jamming and fragility in dense suspensions

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 microstructures. To examine jammed states realized under constant shear stress, we perform dynamic simulations of non-Brownian particles with frictional contact forces and hydrodynamic lubrication forces. We find clear signatures that distinguish these fragile states from the more conventional isotropic jammed states.

Complex dynamical interplay between solid particles and flow in driven granular suspensions

Granular materials immersed in a fluid are ubiquitous in daily life, industry, and nature. They include food processing, pastes, cosmetics, paints, concretes, cements, muds, wet sands, snows, landslides, and lava flow. They are known to exhibit a rich variety of complex rheological behavior, but the role of a fluid component in such behavior has remained poorly understood due to the nonlocal and many-body nature of hydrodynamic interactions between solid particles mediated by the fluid. We address this fundamental problem by comparing the microrheological response of athermal granular suspensions with and without hydrodynamic interactions to an externally driven probe particle by numerical simulations. We find that the presence of the fluid drastically increases the drag coefficient of the probe particle by more than one order of magnitude near the jamming transition. We reveal that this is a consequence of the nontrivial long-range nature of hydrodynamic interactions, which originates from unlimited cumulative transmission of near-field hydrodynamic interactions due to the incompressibility of both fluid and solid particles. Force chain formation of solid particles is dynamically coupled with hydrodynamic flow, leading to strong spatiotemporal fluctuations of flow pattern and nonlinear rheological response. Our study reveals essential roles of hydrodynamic interactions in complex rheological behavior of dense granular suspensions under an external drive.

Using Coupled Rheometer-FBRM to Study Rheological Properties and Microstructure of Cemented Paste Backfill

Rheological properties, such as the yield stress, viscosity, and thixotropy, are related to the microstructure of cemented paste backfill (CPB). To highlight the relationship, two instruments were combined to measure the changes in the microstructure and the rheological properties of CPB simultaneously. In this way, the particle/agglomerate size distribution characterized by the focused beam reflectance measurement (FBRM) and the rheological factors measured by the rheometers could be directly linked. The results show that when under shearing, the intrinsic network structure of CPB responds to the shear-induced stresses with the interference of interparticle forces, leading to changes in the rheological behavior. Shear thinning can be found in CPB suspensions with a microstructure that is either loose interconnection or random. With an increase in the shear rate, random collisions among particles become organized in the flow, lowering the yield stress and viscosity. However, when the shear rate exceeds a certain threshold value, the rheological parameters change as a result of shear thickening. The results of this study contribute to better understanding of the complex rheological behavior of CPB.

Functional Epithelium Remodeling in Response to Applied Stress under In Vitro Conditions.

Mathematical modeling is often used in tissue engineering in order to overcome one of its major challenges: transformation of complex biological and rheological behaviors of cells and tissue in a mathematically predictive and physically manipulative engineering process. The successive accomplishment of this task will greatly help in quantifying and optimizing clinical application of the tissue engineering products. One of the problems emerging in this area is the relation between resting and migrating cell groups, as well as between different configurations of migrating cells and viscoelasticity. A deeper comprehension of the relation between various configurations of migrating cells and viscoelasticity at the supracellular level represents the prerequisite for optimization of the performance of the artificial epithelium. Since resting and migrating cell groups have a considerable difference in stiffness, a change in their mutual volume ratio and distribution may affect the viscoelasticity of multicellular surfaces. If those cell groups are treated as different phases, then an analogous model may be applied to represent such systems. In this work, a two-step Eyring model is developed in order to demonstrate the main mechanical and biochemical factors that influence configurations of migrating cells. This model could be also used for considering the long-time cell rearrangement under various types of applied stress. The results of this theoretical analysis point out the cause-consequence relationship between the configuration of migrating cells and rheological behavior of multicellular surfaces. Configuration of migrating cells is influenced by mechanical and biochemical perturbations, difficult to measure experimentally, which lead to uncorrelated motility. Uncorrelated motility results in (1) decrease of the volume fraction of migrating cells, (2) change of their configuration, and (3) softening of multicellular surfaces.

Slurry rheology in mineral processing unit operations: A critical review

ABSTRACTKnowledge of slurry rheology in mineral processing is essential for the optimization of wet‐grinding, dewatering, transport, and tailings management and it is becoming important in flotation due to the increasing need to process complex ores with a high clay mineral content. Slurries of these ores often display complex rheological behaviour even at low solids concentration, and traditional rheology measurements in mineral processing, such as yield stress and apparent viscosities, may not provide the critical information needed for process optimization. The objective of this paper is to provide a comprehensive review of some of the previous rheology studies performed at the bench, pilot plant, and industrial scale for different unit operations and to discuss the potential for alternative rheology measurements to provide a better insight into colloidal particle interactions in slurries with clay minerals. Firstly, the fundamentals of slurry rheology and the influence of colloidal particles are presented. Secondly, rheology measurements for suspensions are described briefly. Thirdly, a review of rheology studies in grinding, classification, flotation, dewatering, and tailings management is done, outlining possible ways of determining types of particle associations in slurries. Finally, recommendations for future research directions are provided.

Motility and cell shape roles in the rheology of growing bacteria cultures.

Cell shape, size and self-motility appear as determinant intrinsic cell factors in the rheological behavior of living bacterial cultures during the growth process. In this work three different species were considered due to their differences on these intrinsic characteristics: two different strains of Staphylococcus aureus - strain COL and its isogenic cell wall autolysis mutant, RUSAL9 - both non-motile and Escherichia coli and Bacillus subtilis - both presenting intrinsic motility. In situ real-time rheology, was used to characterize the activity of growing bacteria, under steady-shear conditions, in particular the viscosity growth curve was measured, for a constant shear flow rate, presenting for all studied cultures, different and rich flow curves. These complex rheological behaviors are a consequence of two coupled effects: the cell density continuous increase and its changing interacting properties, where cell size and shape and intrinsic motility are major players.

Rheology of ore suspensions with fibrous minerals and its impact on flotation performance

Flotation separation and recovery of value minerals from ores containing fibrous silicate minerals are known to be quite challenging. Earlier studies have established that the metallurgical challenges (poor selectivity and recovery, high energy consumption, etc.) arise from the impact of the fibrous minerals (with high aspect ratio) on pulp rheology, which was found be rather complex. The complex rheological behavior of ore suspensions containing particles with high aspect ratio and its influence on flotation outcome are investigated in this study. Flotation tests and rheological measurements were carried out with both ores (containing fibrous minerals) and a model system comprising nylon fibers (chemically inert) added to a copper ore that had no fibrous mineral. These studies allowed the determination of contributions of physical properties of the fiber to pulp rheological behavior. Optical microscopy images of the fiber pulp indicated that the fibers entangled to form two levels of microstructures: two-dimensional (2D) aggregates under semi-dilute ( 30% solids. The yield stress (σ) values determined for the fiber pulp were logarithmically related to the volume fraction of the 2D/3D structures. The relationships developed here for different ARs showed that the yield stress of a flotation pulp changes with respect to shape, size, and the concentrations of the microstructures. Implications to the effects of such entangle networks on selectivity of flotation separation are discussed.

Extrudability analysis of drug loaded pastes for 3D printing of modified release tablets

The rheological characteristics of pastes for 3D printing of tablets may not be described fully by the traditional rheological tests generally used for other pastes. In the present study, extrudability testing of carbopol based 3D printing pastes was performed to establish a constitutive rheological model for micro-extrusion. This model was developed for pastes that exhibit a non-linear plasto-viscoelastic behavior and follow the generalized Herschel–Bulkley flow rule. An analytical model was applied to extrudability data obtained by micro-extrusion through nozzles of 0.4 and 0.6 mm diameters. For this purpose, nineteen pastes were prepared per a fractional factorial design using various concentrations of the active ingredient and soluble and insoluble excipients. Critical material parameters (σ0, k and n) of the pastes were then calculated by analyzing extrudability data using a constitutive equation relating flow rate, nozzle and cartridge diameters, printing pressure and slip-flow angle. The accuracy of the constitutive model to predict paste yield stress, consistency and flow indices was evident by low RMSE values of 0.0691 bar, 0.034 and 6.3 bar/sn, respectively. Yield stress, flow and consistency parameters of the pastes were significantly affected by percentages of soluble and swellable excipients. The nozzle diameter had significant effect on flow index (n) but not on the consistency index (k). Hence, this study provides a mechanistic model to characterize the complex rheological behavior of pastes for 3D printing of tablets by a micro-extrusion process.

A non-linear fluid suspension model for blood flow

Motivated by the complex rheological behaviors observed in small/micro scale blood vessels, such as the Fahraeus effect, plasma-skimming, shear-thinning, etc., we develop a non-linear suspension model for blood. The viscosity is assumed to depend on the volume fraction (hematocrit) and the shear rate. The migration of the red blood cells (RBCs) is studied using a concentration flux equation. A parametric study with two representative problems, namely simple shear flow and a pressure driven flow demonstrate the ability of this reduced-order model to reproduce several key features of the two-fluid model (mixture theory approach), with much lower computational cost.

How different nanoparticles affect the rheological properties of aqueous Wyoming sodium bentonite suspensions

Clay suspensions present complex microstructures in different environments and deep understanding of such microstructures is crucial to control their flow properties. Their rheological profile is closely linked with the structural association (3-D network) of bentonite particles. Nanomaterials are considered very good candidates for smart fluids formulation which can improve the performance of conventional drilling fluids. Their incorporation in water-bentonite suspensions endow complex microstructures and hence complex rheological behavior, which is still under investigation. This study aims to explore the micro-mechanisms involved on shaping this rheological behavior with samples of 7 wt% water-sodium bentonite suspensions containing 0.5 wt% each of, commercial Fe3O4, commercial SiO2 NP and custom-made (bare or citric acid coated) Fe3O4 NP at alkaline pH. We tried to achieve this by combining macroscopic measurements (rheological measurements) with microscopic measurements (i.e. TEM). A comprehensive physico-chemical characterization of the materials and suspensions was performed using X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), N2 adsorption-desorption isotherms and Fourier-transform infrared spectroscopy (FTIR). An effective drying process was adopted using freeze-granulation and freeze-drying (FG-FD) techniques in order to capture as accurately as possible the evolved microstructures of these aqueous bentonite suspensions at the different temperatures (25–60 °C). The results indicated that all samples exhibited a yield stress followed by a shear thinning behavior. The three parameter Herschel-Bulkley model provided excellent fit of the experimental data for all samples. HR-TEM images revealed that the association of the nanoparticles with bentonite particles in different configurations plays a crucial role in their rheological characteristics with the charge and the coating of the added nanoparticles being important factors in determining the magnitude of the effects observed. We hypothesize that attractive magnetic forces between the magnetite nanoparticles may suppress the electrostatic repulsions and thus they may play a key role in promoting the observed aggregation of the nanoparticles which in turn plausibly affected their rheological profile. A thorough examination and understanding of the evolution of such complex inter-particle structures may lead towards an optimal rheology control of such suspensions in a wide range of applications.

INJECTABLE NANOCURCUMIN–FORMULATED CHITOSAN-G-PLURONIC HYDROGEL EXHIBITING A GREAT POTENTIAL FOR BURN TREATMENT

Burns is one of the popular accidents today and usually leaves serious physical and mental damage. For long time represent acute wounds, burns are evolution into chronic wounds, if inadequately managed. Up to now, there have been abundancy of natural and synthetic products for burn healing. In the study, we fabricated a thermosensitive nanocomposite hydrogel in which incorporated dual active curcumin and chitosan. Beside of a well-known characteristic of chitosan for wound healing, curcumin has been a lot of interest in burn wound healing application due to ability in depleting the action of oxidative radicals and stimulation of fibroblast cells. In order to enhance the therapeutic efficacy of curcumin, we introduced the new method to synthesize nanocurcumin in the a thermosensitive chitosan-g-Pluronic F127 copolymer solution under ultrasonication. The rheology of aqueous solutions of this material is studied as a function of temperature. The solutions of this material undergo a transition to a gel at higher temperature, above which a complex rheological behavior is observed. In addition, a minimum inhibitory concentration of this material was determined for a variety of bacterial and was compared to that of curcumin. It was found that the aqueous dispersion of this material was much more effective than curcumin against both positive and negative gram bacterial. In the third degree burn models, the nCur-CP hydrogel performed a higher burn healing rate as compared to Silvirin-treated burn. These data suggest that the nanocomposite hydrogel may be great potential for burn treatment.

Recycled clay/PET nanocomposites evaluated by novel rheological analysis approach

Clay-polymer nanocomposites exhibit complex rheological behavior due to physical and also possibly chemical interactions between individual phases. Up to now, rheology of clay-polymer nanocomposites has been usually described by evaluation of complex viscosity curve (shear thinning phenomenon), storage modulus curve (G´ secondary plateau) or plotting parameters characterizing damping behavior (e.g. Van Gurp-Palmen-plot, Cole-Cole plot). On the contrary to evaluation of damping behavior, new approach – based on evaluation of rigidity behavior – was tested, where the values of cot δ were calculated and called as „storage factor“, analogically to broadly used loss factor. Afterwards, values of storage factor were integrated over measured frequency range and called as “cumulative storage factor”. In this contribution, clay-PET nanocomposites with different organoclays have been prepared and characterized by both conventional as well as novel analysis approach. Rheological results have been supported by AFM micrographs.

Ways to Transfer Oil and Oil Products in Heating Conditions and Methods for Their Enhancement

This paper discusses the methods of oil transportation through pipelines. Particular attention is paid to the processes of complex rheological behavior of high-paraffin crude oils. Optimal methods applicable for transportation of heavy oil with the help of intensifiers are revealed.