Showing papers in "Rheologica Acta in 2020"

Rheological properties of sodium alginate solutions in the presence of added salt: an application of Kulicke equation

Abstract: Mark–Houwink parameters are determined for sodium alginate with low (SA LV) and medium (SA MV) molecular mass via a rheological approach. Firstly, salt screening concentration and polyelectrolyte nature of the polymers are investigated. Despite both alginates behaving as neutral polymer in θ solvent, differences are observed between them with the strong associative nature of SA MV highlighted by an abnormal high viscosity concentration dependence compared with the predicted values. The polysaccharide-specific hydrodynamic volume (i.e., intrinsic viscosity) is determined via Fedors approach, which is proved to provide accurate results applicable to a wide concentration range. A viscosity–molecular mass–concentration relationship is finally derived to calculate Mark–Houwink parameters via a non-linear regression according to Kulicke equation; the obtained values are consistent with those reported in literature, confirming the semi-flexible nature of SA in the investigated environment and proving the effectiveness of the proposed approach for polyelectrolytes with added salt ions.

Phase state and rheology of polyisobutylene blends with silicone resin

Abstract: Silicone resins are hyperbranched macromolecules, which are nearly spherical in shape, nanometer in size, and covered with numerous terminal organic groups. For this reason, they can play a role of functionalized silica nanoparticles that are miscible with a polymer medium. In this paper, mixtures of linear polyisobutylene (PIB) with methyl-terminated silicone MT resin are considered. The phase states of mixtures were studied by laser interferometry method accompanied with rheological tests. It turned out that the solubility of the silicone resin in PIB matrix does not exceed a few percent even at high temperatures. Nevertheless, the addition of the resin significantly reduces the effective viscosity of PIB due to the shear-induced interlayer slip. To describe the concentration dependence of the blend viscosity, various empirical equations were considered, and their combination to describe the viscosity of blends in different phase states by one equation was proposed.

Stability of bubbles in wax-based oleofoams: decoupling the effects of bulk oleogel rheology and interfacial rheology

Abstract: Oleofoams are dispersions of gas bubbles in a continuous oil phase and can be stabilized by crystals of fatty acids or waxes adsorbing at the oil-air interface. Because excess crystals in the continuous phase form an oleogel, an effect of the bulk rheology of the continuous phase is also expected. Here, we evaluate the contributions of bulk and interfacial rheology below and above the melting point of a wax forming an oleogel in sunflower oil. We study the dissolution behaviour of single bubbles using microscopy on a temperature-controlled stage. We compare the behaviour of a bubble embedded in an oleofoam, which owes its stability to both bulk and interfacial rheology, to that of a bubble extracted from the oleofoam and resuspended in oil, for which the interfacial dilatational rheology alone provides stability. We find that below the melting point of the wax, bubbles in the oleofoam are stable whereas bubbles that are only coated with wax crystals dissolve. Both systems dissolve when heated above the melting point of the wax. These findings are rationalized through independent bulk rheological measurements of the oleogel at different temperatures, as well as measurements of the dilatational rheological properties of a wax-coated oil-air interface.

Bulk rheometry at high frequencies: a review of experimental approaches

Abstract: High-frequency rheology is a form of mechanical spectroscopy which provides access to fast dynamics in soft materials and hence can give valuable information about the local scale microstructure. It is particularly useful for systems where time-temperature superposition cannot be used, when there is a need to extend the frequency range beyond what is possible with conventional rotational devices. This review gives an overview of different approaches to high-frequency bulk rheometry, i.e. mechanical rheometers that can operate at acoustic (20 Hz–20 kHz) or ultrasound (> 20 kHz) frequencies. As with all rheometers, precise control and know-how of the kinematic conditions are of prime importance. The inherent effects of shear wave propagation that occur in oscillatory measurements will hence be addressed first, identifying the gap and surface loading limits. Different high-frequency techniques are then classified based on their mode of operation. They are reviewed critically, contrasting ease of operation with the dynamic frequency range obtained. A comparative overview of the different types of techniques in terms of their operating window aims to provide a practical guide for selecting the right approach for a given problem. The review ends with a more forward looking discussion of selected material classes for which the use of high-frequency rheometry has proven particularly valuable or holds promise for bringing physical insights.

Analysis of the flow behavior of electrorheological fluids containing polypyrrole nanoparticles or polypyrrole/silica nanocomposite particles

Abstract: A four-parameter model (Seo-Seo model) was used to analyze the flow behavior of some electrorheological (ER) fluids containing polypyrrole (PPy) nanoparticles, nanocomposite particles of conductive polypyrrole confined in mesoporous silica (MCM-41), and core-shell-structured SiO2/polypyrrole nanoparticles. The static yield stress predictions by the model were compared with the experimental data and dynamic yield stress obtained from the Bingham model and/or Cho-Choi-Jhon (CCJ) model. The static yield stress values were larger than the dynamic yield stress values. It was also found that the static yield stress of the polypyrrole suspension had a quadratic dependence on the electric field strength as predicted by the electric polarization model whereas those of the nanocomposite suspensions showed 1.5 power-law dependency. A master curve describing the yield stress data dependence on the electric field strength was obtained using a single-parameter scaling function to interpret the underlying mechanism of ER activity. A simple method for evaluating the activity mechanism criterion has been proposed and applied to the ER response of those three kinds of suspension. The results show that the critical electric field strength should be checked before the conduction mechanism is asserted, even if the yield stress plot shows 1.5 power-law dependence on the electric field strength.

A constitutive analysis of nonlinear shear flow

Abstract: We analyse shear stress and normal stress data obtained by cone-partitioned-plate (CPP) shear rheometry in recent years. The data sets of Schweizer et al. (Rheol. Acta 47, 943–957, 2008) and Costanzo et al. (Macromolecules 49, 3925–3935, 2016; & Fluids 4, 28, 2019) on nearly monodisperse polystyrene melts and solutions are considered to be among the most reliable shear data available. The Doi-Edwards independent alignment (DEIA) model (J. Chem. Soc., Faraday Transactions 2: Molecular and Chemical Physics 74, 1802–1832, 1978a,b) allows for quantitative description of the steady-state values of shear viscosity $$ \eta \left(\dot{\gamma}\right) $$ and first normal stress coefficient $$ {\psi}_1\left(\dot{\gamma}\right) $$ , while it underpredicts the stress overshoot of the stress growth coefficient of the shear stress, η+(t), and fails in predicting a stress overshoot of the stress growth coefficient of first normal stress difference, $$ {\psi}_1^{+}(t) $$ . On the other hand, the extended interchain pressure (EIP) model (J. Rheol. 64, 95–110, 2020) provides an excellent prediction of the stress overshoots of both shear stress and first normal stress difference, while overpredicting the steady-state shear viscosity and the first normal stress coefficient. We demonstrate that the shear stress overshoot is the result of a combination of orientational stress overshoot and stretch overshoot, while the normal stress overshoot depends solely on the overshoot of the stretch. Based on these considerations, we propose a novel constitutive approach consisting of a combination of the DEIA and the EIP model, and predictions of this approach are found to be in quantitative agreement with the data sets of Schweizer et al. and Costanzo et al. within experimental accuracy.

Anisotropic behaviour analysis of silicone/carbonyl iron particles magnetorheological elastomers

Abstract: We report the microscopic, magnetic and rheological properties of magnetorheological elastomers (MRE) with carbonyl iron magnetic particles (CIP) dispersed into silicone in the concentration range 5–30% volume content. The samples have been fabricated under the action of a magnetic field (anisotropic A-MRE) or without it (isotropic I-MRE). For the A-MRE samples and at low particle concentration, the anisotropy is evident in the microstructure and the magnetic properties. However and at high particle concentration, the microstructural and magnetic anisotropy is much less noticeable and makes difficult to distinguish between isotropic and anisotropic state. The rheological characterization shows changes in the storage modulus G′ when CIP content is from 5 to 30% volume and I-MRE (72% change) and A-MRE (70% change) character of the samples. However, this influence is remarkable in the loss modulus G″ with big changes when considering CIP content from 5 to 30% volume and I-MRE (114% change) and A-MRE (142% change). We have also determined that the anisotropic samples with high particle content present the maximum magnetorheological effect of about 31% at low frequency (1–2 Hz).

Steady and dynamic shear rheology as a toolfor evaluation of the interactions between egg white albumin and basil seed gum

Abstract: In the current study, the interactions between egg white albumin (EWA; 0 and 4% w/v) and basil seed gum (BSG; 0 to 0.5% w/v) were investigated using the rheological analyses in a solution system. The Herschel–Bulkley model was able to efficiently describe the flow behavior data. Increasing BSG concentration resulted in a significant increase in the apparent viscosity and yield stress, besides a significant decrease in the flow behavior index. According to amplitude sweep data, the structural strength of the EWA–BSG mixtures improved with the increase in BSG concentration. Power-law model efficiently described the frequency dependence of the experimental mixtures. Overall, the rheological data confirmed some synergistic interactions between EWA and BSG in the case of the mixture solutions containing 4% EWA and 0.3% BSG. This information may be of substantial use where the mixtures of proteins and polysaccharides are used for the stabilization of various food products.

Microstructure, microrheology, and dynamics of laponite® and laponite®-poly(ethylene oxide) glasses and dispersions

Abstract: We utilize dynamic light scattering (DLS)-based passive microrheology to probe the dynamics and structural evolution of laponite® and laponite®-polymer glasses and dispersions at the microscale. The results reveal an increase in the dynamic heterogeneity of laponite® dispersions with an increase of laponite® concentration and aging time. In neat laponite® dispersions, the degree of stiffness is enhanced and the dynamics are retarded at higher laponite® concentration due to the formation of a repulsive glass. In the presence of PEO with a moderate molecular weight of 20 kg/mol, the microviscoelastic properties of 2 wt% laponite® dispersions show non-monotonic effects with PEO concentration upon aging, which agrees with the results obtained previously from bulk rheology. However, the magnitudes of the viscoelastic moduli (G’ and G”) of dispersions beyond the gel point obtained from DLS-microrheology is lower than that obtained from conventional rheology. Our results suggest that the DLS-microrheology can be used to qualitatively study dynamic transitions and the microviscoelastic properties of gels and soft solids.

Optimal conditions for pre-shearing thixotropic or aging soft materials

Abstract: Pre-shearing is widely recognized as a necessary step to guarantee repeatability in rheological studies of thixotropic or aging soft materials. When one-directional pre-shear protocols are used, unrecovered elastic strain which leads to biased material states that are not always relaxed because of the build-up of structure during the relaxation process. We propose a way of guaranteeing unbiased material states by incorporating recovery steps, consisting of steps of strain opposing the initial direction of shearing, into any pre-shear protocol. Using such a multi-step pre-shear protocol, we show that it is possible to produce identical results from shearing in the positive and negative directions for the same magnitude of rate after pre-shearing. We further show how this idea of unbiased material states can be used to obtain unbiased results for other fundamental rheological experiments such as flow curves and frequency sweeps. By performing the new pre-shear protocol for every single measurement point of a flow curve or frequency sweep, it is possible to obtain data which is not affected by previous data collection, which leads to material responses with simple and clear shear histories.

Amplitude sweep tests to comprehensively characterize soil micromechanics: brittle and elastic interparticle bonds and their interference with major soil aggregation factors organic matter and water content

Abstract: Rheometry ever since made part of soil physical characterization, but only since the availability of highly sensitive rheometers, amplitude sweep tests (AST) and thixotropy tests are conducted on structured and homogenized soil, mostly one the base of few or one rheological parameter Comprehensive and simultaneous analysis of different parameters has not been done yet, though it offers valuable clues to the highly complex soil processes and their multidimensional (spatial, temporal, and stress-rate-dependent) interferences, especially with regard to the role of the most important soil aggregation factors, ie, soil organic matter (SOM) and water content Consequently, we conducted a strain-controlled AST under varying normal stresses σn on different soil aggregate fractions of either high or low SOM content and at different water contents We determined shear stress τ, loss and storage modulus G″ and G′, respectively, and their ratio, tan δ, at the end of the linear viscoelastic range (LVR) and the yield point (YP) as well as the strain at which they occurred In addition, we analyzed tan δ curves for dilatancy, and classified shear failure behavior (plastic or brittle) Viscoelasticity parameters were mostly affected by SOM (ie, electrostatical attractive forces) and strain (spatial impact), while shear stress parameters altered with SOM, σn, and water content (friction and particle coordination) Based on integrated interpretation of the AST and its resulting parameters, we developed a conceptual framework to be used in future soil (micro)mechanical characterization, where rheological parameters serve as input data to model, eg, erosion, landslides, or soil deformation under shear

Characterizing long-chain branching in commercial HDPE samples via linear viscoelasticity and extensional rheology

Abstract: It is well established that polymer chain architecture and the distribution of molecular weight play a key role in the flow behavior (processing) and performance of a given polymer material. Long-chain branching (LCB) in particular is known to strongly affect the processability and the material performance of polymers. Often branching is a result of the polymerization process and therefore must be quantified in every sample. We study four commercial high-density polyethylene (HDPE) samples with unknown degrees of polydispersity and LCB. We first use size-exclusion chromatography and linear shear rheology to identify differences in molecular weight, polydispersity, and LCB. Each material is then tested in constant rate and constant stress uniaxial extension using a filament stretching rheometer to quantify extensional viscosity and strain hardening. Correlations between nonlinear extensional rheology, LCB and polydispersity are discussed. We show that the combination of the van Gurp-Palmen plot and extensional rheology allows for a full characterization of the LCB fraction and their effect on extensional rheology.

Elongational behavior of silica nanoparticle-filled low-density polyethylene/polylactic acid blends and their morphology

Abstract: We investigated the rheological behavior of linear low-density polyethylene (LLDPE)/polylactic acid (PLA) blends in the presence of modified and non-modified silica nanoparticles in extensional flow. Characterization methods were used as Fourier transform infrared spectroscopy, scanning electron microscopy, and rheometric measurements under shear and uniaxial extensional flows. The rheology behavior of LLDPE significantly was changed by the addition of PLA and silica nanoparticles. Extensional results showed that the elongational viscosity of the blends intensified by the incorporation of silica nanoparticles. Strain hardening was observed for LLDPE containing 2 wt.% of the unmodified silica nanoparticles, which disappeared by enhancement of the unmodified silica from 2 to 8 wt.%. Furthermore, elongation thinning was observed for the filled blends at high loading, which was more sensitive to the strain rate by increasing of PLA. Surface modification of silica was demonstrated in different elongational behavior. Indeed, a fracture took place by loading the modified silica nanoparticles in LLDPE; however, the intensity of this behavior dramatically increased for filled blends with a high loading of modified silica.

An efficient algorithm of solution for the flow of generalized Newtonian fluid in channels of simple geometries

Abstract: In this paper a problem of stationary flow of generalized Newtonian fluid in a thin channel is considered. An efficient algorithm of solution is proposed that includes a flexible procedure for a continuous approximation of the apparent viscosity by means of elementary functions combined with analytical integration of the governing equations. The algorithm can be easily adapted to circular or elliptic conduits. The accuracy and efficiency of computations are analyzed using an example of the Carreau fluid. The proposed computational scheme proves to be highly efficient and versatile providing excellent accuracy of solution at a very low computational cost.

The influence of fumed silica content, dispersion energy, and humidity on the stability of shear thickening fluids

Abstract: Shear thickening fluids (STFs) are smart materials that change from liquid to solid reversibly when undergoing critical stresses. These materials are good alternatives to improve applications where energy dissipation is important, for example, in the fabrication of liquid body armor and shock absorbing protective gear. However, as much as it is known about the effect of several variables on their properties, such as particle concentration and medium viscosity, the stability of these colloidal dispersions over time and over shearing is not yet well understood. The development and design of new applications depend on predicting for how long the material will keep its properties. In this project, we studied the influence of fumed silica content, ultrasonication energy used during dispersion of the silica particles, and humidity during storage to analyze the changes in properties of STFs. The influence of shearing magnitude on their properties was also studied. STFs with higher amounts of silica and produced using less dispersion energy showed the highest viscosity peak on initial tests, but they were also the least stable over time, due to flocculation of the particles. In stable samples, water absorption led to a large loss of maximum viscosity. The presence of humidity on samples diminished the overall viscosity, but did not prevent the sample from becoming a gel if the parameters used resulted in an unstable STF. Shearing the STF reduced its maximum viscosity, being more evident in samples with higher viscosity.

Rheological properties for fresh cement paste from colloidal suspension to the three-element Kelvin–Voigt model

Abstract: In the present work, rheological behaviors of fresh cement paste are studied based on multi-disciplinary approaches, i.e., colloidal suspension using attractive van der Waals force, rheology using the Bingham model and Bingham–Papanastasiou model, which are able to describe the behavior of cement pastes before and after yield stress, and finally the continuum mechanics based on Maxwell and Kelvin–Voigt models. To achieve this, the fresh cement paste with different water-to-cement ratios of 0.3 up to 0.6 is prepared. The attractive van der Waals forces are estimated based on the distances between solid cement particles, which vary at every single water-to-cement ratio. The rheology experiments of all water-to-cement ratios are performed using a rheometer. According to our experimental outcomes, the Bingham and Bingham–Papanastasiou models are applied in the modeling of the experimental curves and determination of yield stress and viscosity. Maxwell and the Kelvin–Voigt models are utilized in describing solid-like behavior before yield stress and fluid-like behavior beyond yield stress. It is observed that the increase of water generates a decrease in the viscosity, yield stress, and packing concentration of solids. It also increases the distances between two cement particles in the cement pastes. According to the modeling results, the Bingham–Papanastasiou model is well adapted for the cement paste flow due to its additional modeling parameter, which is known as m. The role of m is understood and described by linking the van der Waals interaction, rheology, and three-element Kelvin–Voigt model as a whole in function of water-to-cement ratio. m is understood as a key parameter in which the distance between particles affects the rheological behavior of fresh cement pastes. Lastly, the two-phase flow simulations have been successfully achieved and compared with the experiments. The conclusion and outlooks are summarized and discussed at the end of the paper.

Collective viscosity model for shear thinning polymeric materials

Abstract: This work presents a framework for collectively modeling shear viscosities of grouped polymeric materials. The viscosity model has been derived from the multi-modal White-Metzner constitutive equation. Simplification to the multi-modal viscosity has resulted in a viscosity model that controls gradual transition between two conventional viscosity models. It facilitates mathematical representation of multiple sets of viscosity data at the same time. A conventional shear viscosity function, which is common to the group, is multiplied by a material-specific function with one or two constants to form the collective viscosity model. The proposed framework has been applied to several polymeric systems such as polymers with varying molecular weight, polymer solutions with different concentrations, polymers with different filler loadings, and polymer blends with various composition ratios. It has been shown that the K-index in the proposed viscosity model and the variable in the material system such as concentration or compounding ratio can be correlated with each other to predict the viscosities of untested cases.

Radial flow velocity profiles of a yield stress fluid between smooth parallel disks

Abstract: In rock grouting, idealized 2D-radial laminar flow of yield stress fluids (YSF) is a fundamental flow configuration that is used for cement grout spread estimation. A limited amount of works have presented analytical and numerical solutions on the radial velocity profiles between parallel disks. However, to the best of our knowledge, there has been no experimental work that has presented measured velocity profiles for this geometry. In this paper, we present velocity profiles of Carbopol (a simple YSF), measured by pulsed ultrasound velocimetry within a radial flow model. We describe the design of the physical model and then present the measured velocity profiles while highlighting the plug-flow region and slip effects observed for three different apertures and volumetric flow rates. Although the measured velocity profiles exhibited wall slip, there was a reasonably good agreement with the analytical solution. We then discuss the major implications of our work on radial flow.

Relation between structure and stability of toothpaste with two-step yielding

Abstract: Rheological measurements are performed to examine the yielding behavior of different commercial and lab-made toothpaste. Several toothpaste are reported to yield via a two-step process which is noted in dynamic oscillatory strain sweeps. The two-step yielding is best seen as two plateaus for storage modulus in a typical plot of storage modulus as a function of applied strain amplitude. Such behavior in structured materials is usually attributed to rupture of the network at lower strains which is followed by the breakup of aggregates at higher strains. In the case of toothpaste, the network and aggregates are formed by particles, such as silica, mediated by polymers, such as xanthan gum and carboxymethyl cellulose. The toothpaste in which such two-step yielding is more pronounced are shown to be less stable, i.e., lose their ability to retain shape when deposited on a toothbrush.

Relaxation spectra using nonlinear Tikhonov regularization with a Bayesian criterion

Abstract: Nonlinear Tikhonov regularization within a Bayesian framework is incorporated into a computer program called pyReSpect, which infers the continuous and discrete relaxation spectra from oscillatory shear experiments. It uses Bayesian inference to provide uncertainty estimates for the continuous spectrum h(τ) by propagating the uncertainty in the regularization parameter λ. The new algorithm is about 6–9 times faster than an older version of the program (ReSpect) in which the optimal λ was determined by the L-curve method. About half of the speedup arises from the Bayesian formulation by restricting the window of λ explored. The other half arises from the nonlinear formulation for which the spectrum is a weak function of λ, allowing us to use a coarse mesh for λ. The program is tested and validated on three examples: a synthetic spectrum, a H-polymer, and an elastomer with a nonzero terminal plateau.

Nonlinear “oddities” at the percolation of 3D hierarchical graphene polymer nanocomposites

Abstract: The nonlinear rheology of a novel 3D hierarchical graphene polymer nanocomposites was investigated in this study. Based on an isotactic polypropylene, the nanocomposites were prepared using simple melt mixing, which is an industrially relevant and scalable technique. The novel nanocomposites stand out as having an electrical percolation threshold (≈0.94 wt%) comparable to solution mixing graphene-based polymer nanocomposites. Their nonlinear flow behavior was investigated in oscillatory shear via Fourier-transform (FT) rheology and Chebyshev polynomial decomposition. It was shown that in addition to an increase in the magnitude of nonlinearities with filler concentration, the electrical percolation threshold corresponds to a unique nonlinear rheological signature. Thus, in dynamic strain sweep tests, the nonlinearities are dependent on the applied angular frequency, potentially detecting the emergence of a weakly connected network that is being disrupted by the flow. This is valid for both the third relative higher harmonic from Fourier-transform rheology, I3/1, as well as the third relative viscous, v3/1, Chebyshev coefficient. The angular frequency dependency comprised non-quadratic scaling in I3/1 with the applied strain amplitude and a sign change in v3/1. The development of the nonlinear signatures was monitored up to concentrations in the conductor region to reveal the influence of a more robust percolated network.

Rheological behavior of water-clay suspensions under large amplitude oscillatory shear

Abstract: Mixtures of solid particles and Newtonian fluids such as water-clay suspensions are prepared for industrial usage and have many engineering applications Different methods, setups, and protocols are available to characterize rheological behavior of these aqueous suspensions In this study, large amplitude oscillatory shear (LAOS) protocols are considered as the main goal for the first time to examine non-linear rheological behavior of sepiolite suspensions Four types of sepiolite clays and a commercial bentonite clay as reference were used to prepare fresh water-clay suspensions Viscoelastic non-linearities of suspensions were studied as a function of strain and strain-rate using Lissajous–Bowditch curves and non-linear quantitative parameters Discovery Hybrid Rheometer (DHR-II) was used in oscillation sweep tests for measuring non-linear properties Impact of frequency on the evolution of LAOS properties was also investigated by constructing Pipkin diagrams Bentonite suspension an had ability of keeping its elastic characteristics at higher strains compared to sepiolite clays indicating that bentonite clay yielded more flexible and structurally stable suspension Sepiolite suspensions presented higher elastic stiffness (gel-strength) compared to bentonite suspension at all frequencies Strong correlation in viscous non-linear parameters indicated viscous dominant behavior for all clay suspensions in the non-linear regions It was also demonstrated that there is some tendency of shear-thickening behavior, particularly more notable for bentonite suspension under viscous deformation It is postulated that the reason is hidden behind the structural differences of bentonite (platelets like) and sepiolite (fiber like) clays Although both clays are members of smectite group, their structures and viscosity build mechanisms are quite different

Time-response functions of fractional derivative rheological models

Abstract: In view of the increasing attention to the time responses of complex fluids described by power-laws in association with the need to capture inertia effects that manifest in high-frequency microrheology, we compute the five basic time-response functions of in-series or in-parallel connections of two elementary fractional derivative elements known as the Scott-Blair (springpot) element. The order of fractional differentiation in each Scott-Blair element is allowed to exceed unity reaching values up to 2 and at this limit-case the Scott-Blair element becomes an inerter—a mechanical analogue of the electric capacitor that its output force is proportional only to the relative acceleration of its end-nodes. With this generalization, inertia effects may be captured beyond the traditional viscoelastic behavior. In addition to the relaxation moduli and the creep compliances, we compute closed-form expressions of the memory functions, impulse fluidities (impulse response functions) and impulse strain-rate response functions of the generalized fractional derivative Maxwell fluid, the generalized fractional derivative Kelvin-Voigt element and their special cases that have been implemented in the literature. Central to these calculations is the fractional derivative of the Dirac delta function which makes possible the extraction of singularities embedded in the fractional derivatives of the two-parameter Mittag-Leffler function that emerges invariably in the time-response functions of fractional derivative rheological models.

Rheological and physical analysis of oil-water emulsion based on enzymatic structured fat

Abstract: Structured triacylglycerols play an important role in determining the functional properties of fat-based emulsion products. The aim of the study was to evaluate the physical properties of the emulsion systems manufactured on the basis of enzymatically modified rabbit fat with pumpkin seed oil in the presence of sn-1,3 regioselective lipase. Emulsions containing variable contents of thickener and variable fat ratios were analyzed for rheological behavior and particle size changes during storage, and their stability was assessed using the Turbiscan test. The results showed that the emulsion containing the majority of rabbit fat and 1 wt% of carboxymethylcellulose was characterized by the highest stability. On the other hand, the emulsions containing higher amounts of pumpkin seed oil in a fatty base characterized the lowest resistance to destabilization processes. The research confirmed the possibility of producing structured fat which can be the basis for new emulsion systems proposed as a food, cosmetic, and pharmaceutical product.

A numerical study of droplet deformation and droplet breakup in a non-orthogonal cross-section

Abstract: In this work, we numerically investigate the deformation and breakup of a droplet flowing along the centerline of a microfluidic non-orthogonal intersection junction. The relevant boundary data of the velocity field is numerically computed by solving the depth-averaged Brinkman equation via a self-consistent integral equation using the boundary element method. The effect of the capillary number, droplet size, intersection angle, and ratio of outlet channel width to inlet channel width on maximum droplet deformation are studied. We study droplet deformation for the capillary numbers in the range of 0.08-0.3 and find that the maximum droplet deformation scales with the capillary number with power law with an exponent 1.10. We also investigate the effect of droplet size and intersection angle on the maximum droplet deformation and observe that the droplet deformation is proportional to droplet volume and square root of intersection angle, respectively. In continue, we study the droplet breakup phenomenon in an orthogonal intersection junction. By increasing the capillary number, the deformation of a droplet traveling in the cross-junction region becomes larger, until the droplet shape is no longer observed and droplet breakup takes place at a critical value of capillary number. We present a phase diagram for droplet breakup as a function of undeformed droplet radius.

Evolution of morphology and morphology stability in PP/PA6/EPDM-g-MA reactive ternary blends using viscoelastic measurements

Abstract: The relationship between the morphology and rheological properties of PP/PA6/EPDM-g-MA ternary blends, at relatively low rubber phase contents, was studied in detail. The results showed that the size of composite droplets of core-shell microstructures decreased and increased after passing through a minimum due to competitive effects of the emulsification role of EPDM-g-MA and viscosity ratio. This trend in morphology development was well reflected in the relaxation spectrums of the blends. The morphology of discrete core-shell particles altered to the large clusters of core-shell particles at higher rubbery phase contents which led to alteration of viscoelastic behavior from liquid-like to solid-like. The morphology of high EPDM-g-MA content ternary blends was unstable particularly at higher temperatures which was evaluated using time-sweep rheological experiments and confirmed via direct SEM analysis.

Investigation of thermoplastic melt flow and dimensionless groups in 3D bioplotting

Abstract: We investigate the key 3D bioplotting processing parameters, including needle diameter and dispensing pressure, on the shear rates, shear stresses, pressure drops, and swell ratios of extruded miscible polycaprolactone (PCL) blends having a range of viscosities. Assuming simple capillary flow, we construct flow curves and we estimate that the shear stresses inside the needle of the bioplotter range from 2500 to 20,000 Pa and the corresponding shear rates from 2 to 25 s−1, depending upon the viscosity of the blend. We further identify relevant dimensionless numbers that reflect the material rheological properties and processing conditions; these include the capillary number (Ca), Bond number (Bo), Weissenberg number (Wi), and elasticity number (El). At most processing conditions Ca > 1, whereas Bo < 1, suggesting that viscous forces dominated surface forces, except for needle diameters below 0.2 mm, where the flow approached micro-fluidic conditions. While Wi was below 1 at all conditions, El increased significantly with decreasing needle diameter. High El numbers at a needle internal diameter of 0.2 mm were associated with extrudate swell ratios above 2. Based on these results, we define ranges of operation in 3D bioplotting, which can serve as guidelines for process design. Even though this work is specific on the particular bioplotting equipment, the methodology described herein can be applied on any type of micro-extrusion equipment.

Micro rheology of Jeffrey nanofluid through cilia beating subject to the surrounding temperature

Abstract: The objective of this study is to discuss the micro rheology of mucus (Jeffrey nanofluid) which is a complex biological fluid that protects lungs from pollutants, bacteria, and allergens that can be inhaled during the breathing process To see the insight of pollutants and effect of surrounding temperature in the mucus, momentum, energy, and concentration equations are modeled with the help of metachronal wave formed by cilia beating The governing system of equations are modeled in the wave and fixed frame and simplified by the lubrication approach The velocity profile for recovery and effective stroke is compared and it is analyzed that effective stroke possess high magnitude of velocity when compared with the recovery stroke The flow of mucus with pollutants and surrounding temperature is calculated with homotopy perturbation method and software “MATHEMATICA” The results within the given domain are convergent under the different parameters appearing into the system of equations The physical interpretation of involved parameters is explained through graphs

Thermal fatigue and collapse of waxy suspensions

Abstract: Due to the existence of a continuous (percolating) network of weak interparticle bonds in a liquid, wax suspensions can behave as “soft breakable (brittle) solids”: under the action of either a large stress over a short time or oscillating low stress (fatigue test), the initially solid network of these materials is broken and dispersed in the liquid, which turns them abruptly (“collapse”) and irreversibly to a low viscous fluid. Here we show that the rheological behavior of these materials is not only impacted by the temperature but also by the history of the temperature. The elastic modulus and the yield stress increase when the temperature is decreased. The data for different concentrations (ranging from 7 to 50 wt% of wax in oil) and temperatures (as a function of the distance to the critical temperature) fall along a master curve, which shows some equivalence between temperature and concentration. More surprisingly, the elastic modulus in the linear regime and the yield stress are dependent on the minimum temperature the material has experienced during its preparation. As a consequence of these different characteristics, an original rheological behavior so far essentially observed with very different materials (metals) results, namely thermal fatigue: when the material is submitted to temperature cycling (small temperature amplitude test), the material progressively weakens during each elementary thermal cycle and can finally “collapse” after a sufficient number of cycles, i.e., the elastic modulus in the linear regime decreases from 106 to 103 Pa. These findings could have implications in the start-up flow of waxy oils in pipelines since with the help of this technique, the material strength (e.g., the yield stress) and consequently the pressure required to resume the flow can be reduced considerably just by imposing thermal cycles.

Decrease in non-linear viscosity of a polylactide nanocomposite with regard to the clay volume fraction

Abstract: The aim of this study was to assess the physical phenomena that influence the rheological behaviour of polylactide/organo-modified clay nanocomposites. A model was proposed to explain the decrease in non-linear viscosity at high clay contents. Several variables were studied, including shear rate, temperature, clay volume fraction, and polymer molar mass. When the effects of the molar mass and temperature were separated from that of the clay volume fraction, the results suggested a decrease in viscosity compared with that of neat polylactide, which was mostly assigned to molar mass degradation and partially assigned to enhanced temperature dependency. The time–temperature superposition principle was employed for this purpose and highlighted two distinct trends in the activation energies with the clay volume fraction. Ultimately, rheological measurements were corroborated by morphological observations to reveal a non-monotonic evolution of viscosity with the clay volume fraction. These results help provide insight into simulation of the flow behaviour of a nanocomposite based on layered silicate.