Showing papers in "Rheologica Acta in 2018"

Yield stress fluids and ageing

Abstract: Many high-concentration multiphase materials that are industrially and academically important do not reach thermodynamic equilibrium because of kinetic constraints originating from the structurally arrested microstructure. However, owing to thermal or elastic energy of the constituents, their microstructure progressively reorganizes to form thermodynamically more stable states. In this physical ageing process, the whole spectrum of relaxation times evolves that makes relaxation dynamics increasingly sluggish. In a process of rejuvenation, deformation field increases mobility, but non-trivially alters the shape of relaxation time spectrum. Interestingly, this class of materials also demonstrates yield stress, whose origin could be closely related to physical aging, and as a result, it depends on time and deformation history. In this review, we present an overview of experimental behaviors and theoretical advances that indicate a complex coupling between ageing and rejuvenation for this class of materials having diverse microstructures.

Slow flows of yield stress fluids: yielding liquids or flowing solids?

Abstract: Yield stress fluids (YSF) exhibit strongly non-linear rheological characteristics. As a consequence, they develop original flow features (as compared to simple fluids) under various boundary conditions. This paper reviews and analyzes the characteristics of a series of slow flows (just beyond yielding) under more or less complex conditions (simple shear flow, flow through a cavity, dip-coating, blade-coating, Rayleigh-Taylor instability, Saffman-Taylor instability) and highlights some of their common original characteristics: (i) a transition from a solid regime to a flowing regime which does not correspond to a true “liquid state,” the flow in this regime may rather be seen as a succession of solid states during very large deformation; (ii) a strong tendency to localization of the yielded regions in some small region of the material while the rest of the material undergoes some deformation in its solid state; (iii) the deformation of YSF interface with another fluid, in the form of fingers tending to penetrate the material via a local liquefaction process. Finally, these observations suggest that slow flows of YSF are a kind of extension of plastic flows for very large deformations and without irreversible changes of the structure. This suggests that the field of plasticity and the field of slow flows of YSF could benefit from each other.

Evaluating rheological models for human blood using steady state, transient, and oscillatory shear predictions

Abstract: The rheological characterization of human blood, through modeling and analysis of steady state, transient, and oscillatory shear flows, has made tremendous progress recently. Due to the aggregation of red blood cells at low shear rates, many recent models for blood rheology include a structural, thixotropic component with one of the most recent attempts unifying this approach with a viscoelastic formulation. We will show how these models, along with proposed modifications to another recent structural, kinetic thixotropy model, can improve modeling predictions. Results are compared to the Maxwell-like Bautista-Manero-Puig model, the Oldroyd-8 inspired viscoelastic Anand-Kwack-Masud model, a viscoelastic-thixotropic model from Blackwell and Ewoldt, and the Herschel-Bulkley model. We explore the weaknesses of the legacy blood models and then demonstrate the efficacy of the newly improved models for modeling human blood steady state and transient shear rheology to predict oscillatory shear flow.

Contemporary modeling and analysis of steady state and transient human blood rheology

Abstract: The rheological characterization of a human blood, through modeling and analysis of transient flows and large-amplitude oscillatory shear (LAOS) flow, has made tremendous progress recently. We show how various components, and modifications of two recent scalar, structural kinetic, thixotropic models, can offer several modeling and prediction improvements, and compare our results to the Maxwell-like Bautista-Manero-Puig (BMP) model, and a recent transient model based on the Herschel-Bulkley. We explore the weakness of the legacy blood models, and then, we apply this newly improved model to recently published data from the literature in order to demonstrate its efficacy in modeling steady state, transient, and oscillatory shear flow. Following this effort, we demonstrate a novel approach using the sequence of physical phenomena (SPP) to facilitate interpretation, characterization, mapping, and “fingerprinting” of transient blood data from the literature. We compare the SPP approach to other LAOS analysis techniques in the literature and show how our approach can function as a mechanical-property diagnostic blood analysis tool. The goal of this work is a deeper understanding of the microstructural basis and validity of structural thixotropic blood models, and transient flow analysis techniques and procedures.

Non-monotonic response of waxy oil gel strength to cooling rate

Abstract: The oil at high temperatures in the reservoir loses heat to the surroundings and is submitted to different shear stresses while it is produced and transported. Thermal and shear histories have great influence on the rheological characteristics of waxy oils at low temperature. Wax crystals precipitate during cooling, building up a percolated matrix that entraps the oil and consequently, forms a gel-like structure. One of the main parameters that affect the crystals’ morphology and therefore the gel strength is the cooling rate. Although the oil static cooling has been widely studied in the literature, many questions are still open. The current work analyzes the influence of the cooling rate on the gel strength and on the dynamic moduli (G′ and G″) of a waxy model oil. Microscopic images of wax crystals were obtained and a hypothesis to explain the non-monotonic response of the rheological parameters as a function of the cooling rate is proposed based on the crystals’ morphology.

Measuring and assessing first and second normal stress differences of polymeric fluids with a modular cone-partitioned plate geometry

Abstract: We propose a simple, robust method to measure both the first and second normal stress differences of polymers, hence obtaining the full set of viscometric material functions in nonlinear shear flow. The method is based on the use of a modular cone-partitioned plate (CPP) setup with two different diameters of the inner plate, mounted on a rotational strain-controlled rheometer. The use of CPP allows extending the measured range of shear rates without edge fracture problems. The main advantage of such a protocol is that it overcomes limitations of previous approaches based on CPP (moderate temperatures not exceeding 120 °C, multiple measurements of samples with different volume) and yields data over a wide temperature range by performing a two-step measurement on two different samples with the same volume. The method was tested with two entangled polystyrene solutions at elevated temperatures, and the results were favorably compared with both the limited literature data on the second normal stress difference and the predictions obtained with a recent tube-based model of entangled polymers accounting for shear flow-induced molecular tumbling. Limitations and possible improvements of the proposed simple experimental protocol are also discussed.

Rheological studies and optimization of Herschel-Bulkley flow parameters of viscous karaya polymer suspensions using GA and PSO algorithms

Abstract: Rheological characteristics of gum karaya suspensions which is proposed as a fracturing fluid were investigated with the main objective to determine the yield stress and other rheological parameters using various models. The flow hysteresis confirms the thixotropic behavior of fluid with increased structural breakdown at higher concentration and temperature. An empirical model developed for the studied samples accurately predicts the temperature and polymer concentration sensitivity of the apparent viscosity. The Herschel-Bulkley model showed the best fit to the experimental data; however, the yield stress obtained from some of the samples using nonlinear regression (NL) method reported physically insignificant, negative values. The proposed optimization technique, i.e., “Particle Swarm Optimization” offered the most realistic results with faster convergence over genetic algorithm making it a better choice. The oscillatory study provided more reliable yield stress values and revealed the thermogelation behavior of polymer suspensions making it suitable for high-temperature fracturing application.

Nonlinear viscoelasticity of fat crystal networks

Abstract: The rheology of fat crystal networks under linear shear deformations has been extensively studied due to its role in material functionality and sensory perceptions. In contrast, there has been limited focus on their viscoelastic response under large shear deformations imposed during processing and product use. We probed the nonlinear viscoelastic behavior of fats displaying mechanics akin to ductile and brittle solids using large amplitude oscillatory shear (LAOS). Using the FT-Chebyshev stress decomposition method, and local measures of nonlinear viscoelasticity, we obtained rheological properties relevant to bulk behavior. We found that ductile fats dissipate more viscous energy than brittle fats and show increased plastic deformation. Structural characterization revealed the presence of three hierarchy levels and layered microstructures in ductile fats in contrast to only two hierarchies and random microstructures in brittle fats. We suggest that these structural features account for increased hypothesize dissipation, which contributes to their ductile-like macroscopic behavior.

Rheological and sensory properties of toothpastes

Abstract: Complex rheological trends of several commercially available and lab-made prototype toothpastes are reported. The flow curves are generated using the rotational rheometers with a series of rheological procedures, comprising of stress ramps, creep-recovery, stepped-shear rates, and dynamic oscillatory strain sweeps performed on toothpastes. Intricacies due to the history and the effects of pre-conditioning of the samples are discussed. However, the main goal of this work was to identify the correlations between the rheological measurements and the consumer-perceived properties of toothpastes. Shape retention and stringiness are the main sensory properties of interest that were identified and evaluated by the panelists. A custom-built experimental setup is used to quantify shape retention of a toothpaste ribbon on a brush and on a flat surface in a test which resonates with the popular slump test. It is demonstrated that the degree of shape retention correlates with the yield stress and the instantaneous viscosity. A comparison of yield stresses obtained using different methods in relation to degree of shape retention is presented. An experimental setup designed to measure stringiness of toothpastes is delineated. The stringiness measured with this device correlates well with human perception and also with the slope of the flow curve, i.e., the higher the degree of shear thinning, the less stringy the pastes tend to be. For lab-made prototype toothpastes, basic structure-property relations are established in terms of correlations between the three formulation variables: thickening silica, Xanthan gum, and carboxymethyl cellulose (CMC).

Dynamic rheological properties of a fumed silica grease

Abstract: The thermo-rheological characteristics of a fumed silica lubricating grease in linear and nonlinear oscillatory experiments have been investigated. The material rheological behavior represents a soft solid being thermo-rheologically complex. There is an abnormal temperature dependency in the range of − 10 to 10 °C which is related to the phase transition of the base oil. The dynamic moduli data in linear viscoelastic envelop (LVE) have been modeled using mode-coupling theory (MCT) in the whole temperature range. Two main relaxation mechanisms can be identified through linear and nonlinear viscoelastic properties related to interaction of the primary particle and its neighbor particles as well as a slow relaxation process which represents the escape of this particle from its “cage”. Finally, it is demonstrated that the dominant yielding process in large amplitude oscillatory experiments can be explained based on either particle cage rupture (consistent with MCT framework) or particle “hopping” out of its cage proposed in soft glassy rheology (SGR) model. It will be discussed that the governing mechanism depends on the applied frequency.

On the oscillating shear rheometry of magnetorheological elastomers

Abstract: Critical issues of the oscillating shear rheometry with disc-shaped elastomer specimens are exemplified. A torsional oscillation method to characterise magnetorheological elastomers (MREs) using a rheometer is introduced and compared to the standard procedure with disc-shaped specimens. Advantages and disadvantages of the method are identified. It is shown that the rheometry with rod-like specimens provides more reliable data, which are easier to reproduce. The method also allows characterisation of MREs in a not pre-strained condition, an aspect which importance is demonstrated. Moreover, it opens an easy way to characterise permanently magnetised MREs with a complex composition using conventional rheometers.

A bootstrap mechanism for non-colloidal suspension viscosity

Abstract: The role of friction in non-colloidal suspensions is examined with a model which splits the viscosity into a frictionless component (τ*) plus a frictional component which depends on the ratio of the particle pressure (P) to the shear stress (τ). The model needs the input by computation of τ* and P and a suitable choice of particle friction coefficient (μ). It can be extended to elongational flows and cases where sphere roughness is important; volume fractions up to 0.5 are considered. It is shown that friction acts in a feedback or “bootstrap” manner to increase the suspension viscosity. The analysis is also useful for deducing the friction coefficient in suspensions from experimental data. It was applied to several sets of experimental data and reasonable correlations of the viscosities were demonstrated. An example of the correlation for spheres in a silicone oil is shown for volume fractions 0.1–0.5.

Large amplitude oscillatory shear behavior of graphene derivative/polydimethylsiloxane nanocomposites

Abstract: Rheological properties of three different nanocomposites, consisting of graphene oxide (GO), reduced graphene oxide (rGO), and polyhedral oligomeric silsesquioxane grafted reduced graphene oxide (rGO-POSS) as nanofillers and polydimethylsiloxane (PDMS), were investigated by large amplitude oscillatory shear (LAOS). The viscoelastic nonlinearity of the three nanofluids groups was studied by Lissajous curves, local nonlinear viscoelastic moduli of an oscillatory shear cycle, and Fourier transform rheology as a function of filler concentration and increasing and decreasing strain magnitude. The nonlinear behavior of the nanofluids was compared to understand the variation of internal microstructures. Firstly, GO/PDMS composites behave with higher moduli and smaller linear viscoelastic range comparing to that of other two composites. Secondly, the elastic stress Lissajous curves of these composites changed from elliptic to rectangular with round the corner with increasing the filler level and strain amplitude. Thirdly, all these three nanofluids exhibited intra-cycle strain stiffening with increasing strains and shear thickening at intermediate strain and then shearing thinning with increasing strain further. Fourthly, higher harmonic intensity of rGO/PDMS increased with increasing strain and came to a plateau, while that of other two nanofluids reached a maximum and then decreased. It suggested that different surface functionalization of nanoparticles will present different rheological behavior due to formed different network and LAOS could be used as a potential helpful method to characterize rheological properties of nanocomposites, especially at higher shear strain.

Linear viscoelastic analysis using frequency-domain algorithm on oscillating circular shear flows for bubble suspensions

Abstract: To achieve a stable evaluation of the linear viscoelasticity of bubble suspensions, which have difficulties for conventional rheometers from spatial distributions of rheological properties with bubble deformations, we proposed a novel rheometry based on spatio-temporal velocity data obtained by ultrasonic velocity profiling (UVP). A frequency-domain algorithm was adopted to overcome a critical influence of measurement noise on the rheological assessment, which is inferred from error propagation characteristics through the equations of motion in discretized form. Applicability and advantage of the present rheometry with the frequency-domain algorithm were verified by two kinds of fluids: high viscous oil as a Newtonian fluid and polyacrylamide aqueous solution as a shear thinning, viscoelastic fluid. The rheometry was finally adopted for bubble suspensions subject to high oscillatory shear, and it could validly extract elasticity-originated momentum transfer as a function of space.

Characterization of physical aging by time-resolved rheometry: fundamentals and application to bituminous binders

Abstract: Physical aging is a ubiquitous phenomenon in glassy materials and it is reflected, for example, in the time evolution of rheological properties under isothermal conditions. In this paper, time-resolved rheometry (TRR) is used to characterize this time-dependent rheological behavior. The fundamentals of TRR are briefly reviewed, and its advantages over the traditional Struik’s physical aging test protocol are discussed. In the experimental section, the TRR technique is applied to study physical aging in bituminous binders. Small-diameter parallel plate (SDPP) rheometry is employed to perform cyclic frequency sweep (CFS) experiments over extended periods of time (from one to 8.6 days). The results verify that the mutation of rheological properties is relatively slow during physical aging (mutation number N′mu << 1), thus allowing rheological measurements on a quasi-stable sample. The effects of temperature, crystallinity, and styrene-butadiene-styrene (SBS) polymer modification on the physical aging of bitumen are evaluated. The time-aging time superposition is found to be valid both for unmodified and for polymer-modified bitumen. Vertical shifts are necessary, in addition to horizontal time-aging time shifts, to generate smooth master curves for highly SBS-modified bitumen.

Formulation, characterisation and flexographic printing of novel Boger fluids to assess the effects of ink elasticity on print uniformity

Abstract: Model elastic inks were formulated, rheologically characterised in shear and extension, and printed via flexography to assess the impact of ink elasticity on print uniformity. Flexography is a roll-to-roll printing process with great potential in the mass production of printed electronics for which understanding layer uniformity and the influence of rheology is of critical importance. A new set of flexo-printable Boger fluids was formulated by blending polyvinyl alcohol and high molecular weight polyacrylamide to provide inks of varying elasticity. During print trials, the phenomenon of viscous fingering was observed in all prints, with those of the Newtonian ink exhibiting a continuous striping in the printing direction. Increasing elasticity significantly influenced this continuity, disrupting it and leading to a quantifiable decrease in the overall relative size of the printed finger features. As such, ink elasticity was seen to have a profound effect on flexographic printing uniformity, showing the rheological tuning of inks may be a route to obtaining specific printed features.

Entanglement properties of carboxymethyl cellulose and related polysaccharides

Abstract: Entanglement network of carboxymethyl cellulose (CMC) was characterized based on the dynamic viscoelasticity of the concentrated solutions in an ionic liquid. According to the concentration dependence of the molecular weight between entanglements (M e), M e for the molten state (M e,melt) for CMC was estimated to be 3.9 × 103 as a chain variable reflecting the chemical structure of the polysaccharide. Furthermore, relations between M e,melt and other chain variables were examined to elucidate the specificity in the entanglement properties of CMC and related polysaccharides. It was shown that the number of entanglement strands (P e), the ratio of the cube of the tube diameter, and the volume occupied by the entanglement strand, for CMC was 72 being significantly larger than the universal value of ca. 20 recognized for flexible polymers. Anomalous values of P e > 20 were also obtained for related polysaccharides such as cellulose and amylose.

Filling the gap between transient and steady shear rheology of aqueous graphene oxide dispersions

Abstract: Even though the rheological behavior of aqueous graphene oxide (G-O) dispersions has been shown to be strongly time-dependent, only few transient measurements have been reported in the literature. In this work, we attempt to fill the gap between transient and steady shear rheological characterizations of aqueous G-O dispersions in the concentration range of 0.004 < ϕ < 3.5 wt%, by conducting comprehensive rheological measurements, including oscillatory shear flow, transient shear flow, and steady shear flow. Steady shear measurements have been performed after the evaluation of transient properties of the G-O dispersions, to assure steady-state conditions. We identify the critical concentration ϕ c = 0.08 wt% (where G-O sheets start to interact) from oscillatory shear experiments. We find that the rheology of G-O dispersions strongly depends on the G-O concentration ϕ. Transient measurements of shear viscosity and first normal stress difference suggest that G-O dispersions behave like nematic polymeric liquid crystals at ϕ/ϕ c = 25, in agreement with other work reported in the literature. G-O dispersions also display a transition from negative to positive values of the first normal stress difference with increasing shear rates. Experimental findings of aqueous graphene oxide dispersions are compared and discussed with models and experiments reported for nematic polymeric liquid crystals, laponite, and organoclay dispersions.

Dissipative versus reversible contributions to macroscopic dynamics: the role of time-reversal symmetry and entropy production

Abstract: We discuss the time-reversal behavior of dynamic cross-couplings among various hydrodynamic degrees of freedom in liquid crystal systems. Using a standard hydrodynamic description including linear irreversible thermodynamics, we show that the distinct thermodynamic requirements for reversible and irreversible couplings lead to experimentally accessible differences. We critically compare our descriptions with those of existing standard continuum mechanics theories, where time-reversal symmetry is not adequately invoked. The motivation comes from recent experimental progress allowing to discriminate between the hydrodynamic description and the continuum mechanics approach. This concerns the dynamics of Lehmann-type effects in chiral liquid crystals and the dynamic magneto-electric response in ferronematics and ferromagnetic nematics, a liquid multiferroic system. In addition, we discuss the consequences of time-reversal symmetry for flow alignment of the director in nematics (or pretransitional nematic domains) and for the dynamic thermo-mechanical and electro-mechanical couplings in textured nematic liquid crystals.

An empirical constitutive model for complex glass-forming liquids using bitumen as a model material

Abstract: While extensive research efforts have been devoted to understand the dynamics of chemically and structurally simple glass-forming liquids (SGFLs), the viscoelasticity of chemically and structurally complex glass-forming liquids (CGFLs) has received only little attention. This study explores the rheological properties of CGFLs in the vicinity of the glass transition. Bitumen is selected as the model material for CGFLs due to its extremely complex chemical composition and microstructure, fast physical aging and thermorheological simplicity, and abundant availability. A comprehensive rheological analysis reveals a significant broadening of the glass transition dynamics in bitumen as compared to SGFLs. In particular, the relaxation time spectrum of bitumen is characterized by a broad distribution of long relaxation modes. This observation leads to the development of a new constitutive equation, named the broadened power-law spectrum model. In this model, the wide distribution of long relaxation times is described by a power-law with positive exponent and a stretched exponential cut-off, with parameter β serving as a measure of the broadness of the distribution. This characteristic shape of the bitumen spectrum is attributed to the heterogeneous freezing of different molecular components of bitumen, i.e., to the coexistence of liquid and glassy micro-phases. Furthermore, as this type of heterogeneous glass transition behavior can be considered as a general feature of complex glass-forming systems, the broadened power-law spectrum model is expected to be valid for all types of CGFLs. Examples of the applicability of this model in various complex glass-forming systems are given.

Comparative study of the electrorheological effect in suspensions of needle-like and isotropic cerium dioxide nanoparticles

Abstract: A novel approach to the analysis of the electrorheological effect is proposed, based on the expansion of dimensionless relative shear stress as function of electric field strength in the power series $$ {\tau}_{\mathrm{rel}}=\frac{\tau_E}{\tau }=1+\frac{\alpha }{\tau }E+\frac{\beta }{\tau }{E}^n $$ . The application of this approach to investigation of the electrorheological effect in suspensions of isotropic and needle-like CeO2 nanoparticles in polydimethylsiloxane has revealed that the polynomial coefficients can be judged as a measure of the efficiency of transformation of electrical energy into mechanical energy. The values of α and β coefficients depend on the shape and concentration of filler particles, as well as on the shear rate. The value and the sign of these coefficients determine both the magnitude of the electrorheological effect and the type of dependence of the shear stress (linear or power law) on the strength of the electric field. It has been shown that the values of α and β coefficients for the electrorheological fluids with needle-like particles are greater than for fluids with isotropic particles (at the same concentration of suspensions), which is associated with the different polarization of particles in the applied electric field.

Electrorheology of clay particle suspensions. Effects of shape and surface treatment

Abstract: We investigate the electrorheological (ER) properties of clay (montmorillonite, sepiolite, and laponite®). The selected clays allow to distinguish between planar particles of different sizes (montmorillonite and laponite®), and elongated ones (sepiolite). The effect of coating them with the surfactant CTAB improves dispersibility in the oil medium and favors the ER response, prticularly in the case of laponite®, whereas in the case of montmorillonite, microscopic observations show that the columnar structures are broken in places leading to a reduced yield stress. Both the static yield stress and the storage modulus grow faster with the field in sepiolite suspensions as compared to laponite®. When dealing with mixed systems, it is found that the field-induced montmorillonite structures are reinforced by the addition of either laponite® or sepiolite, whereas when the latter two are combined, it is laponite® that dominates the ER response.

Effects of anionic and cationic surfactants on the rheological properties and kinetics of bovine serum albumin hydrogel

Abstract: The effects of the anionic surfactant, sodium dodecyl sulfate (SDS), and of the cationic surfactant cetyltrimethylammonium bromide (CTAB) on the gelation kinetics of bovine serum albumin (BSA) hydrogel were investigated by rheological measurements using surfactant concentrations of 0–0.05 M, and BSA concentrations of 5, 7, and 10 wt%. It was found while an increase in CTAB concentration accelerated the rate of gelation of BSA solution under temperature jump and temperature ramp conditions, BSA solutions containing SDS exhibited a heat-dependent protective effect against thermal denaturation and gelation. Under temperature ramp conditions, inhibition of BSA gelation by SDS was diminished by increasing SDS concentration, while under temperature jump conditions, inhibition of BSA gelation increased with SDS concentration. That is, gel temperature (Tgel) under temperature ramp decreased with increasing CTAB and with SDS concentration, but under temperature jump the gel time (tgel) decreased with increasing CTAB concentration but increased with SDS concentration. Furthermore, BSA/CTAB solutions were found to gel more rapidly than BSA/SDS solutions, which was in line with the lower activation energy of BSA/CTAB gel. In support of experiments, molecular dynamics (MD) simulations and dynamic light scattering (DLS) revealed the faster rate of BSA denaturation in the presence of CTAB was responsible for the increased gelation rate of BSA/CTAB solutions, whereas BSA was found to be protected by SDS against thermal denaturation leading to the slower gelation rate of BSA/SDS solutions.

Linear viscoelastic behavior of bidisperse polystyrene blends: experiments and slip-link predictions

Abstract: The linear viscoelastic behavior of three well-entangled linear monodisperse polystyrene melts and their blends is investigated. The monodisperse melts are blended in a 1:1 weight ratio to obtain three polystyrene bidisperse blends for which the linear viscoelastic behavior is also measured. Special attention is paid to controlling sample size and solvent content, and checking for consistency in the high-frequency regime. We also attempt to estimate uncertainty quantitatively. The experimental results agree well with the discrete slip-link model, a robust mesoscopic theory that has been successful in predicting the rheology of flexible entangled polymer liquids and gels. Using recently developed analytic expressions for the relaxation modulus, predictions of the monodisperse samples are made. The parameters for the model are obtained from the low-frequency crossover of one experiment. Using this parameter set without adjustment, predictions over the fully accessible experimental frequency range are obtained for the monodisperse samples and their blends with very good agreement.

Droplet retraction in the presence of nanoparticles with different surface modifications

Abstract: We studied the influence of nanoparticles with different surface modifications on the interfacial tension and relaxation of model polymer blend after cessation of different strains. The droplet retraction experiments were carried out on a model system composed of polydimethylsiloxane (PDMS) as the suspending fluid and polyisobutylene (PIB) as droplet at room temperature in the presence of hydrophobic and hydrophilic nanosilica. Different weight fractions of particles were dispersed in the PIB droplet before forming a dispersed droplet by using a microsyringe in shear cell. We found that applied strain, nanoparticle concentration and their thermodynamically preferred localization affect both nominal interfacial tension and droplet retraction process. By addition of nanoparticles at a concentration as low as 0.2%wt, the nominal interfacial tension decreases from 3.12 ± 0.15 mN/m for neat PIB-PDMS interface depending on the surface characteristics of nanosilica. Hydrophilic nanosilica has the most effect on nominal interfacial tension and decreases it as low as 0.2 ± 0.21 mN/m at 1 wt.% loading under a strain of 7. The results show that the retraction process in this system is mainly controlled by interfacial phenomena rather than bulk rheological properties. Additionally, the shape evolution of droplets changes and the retraction rate slows down in the presence of nanoparticles.

Hydrodynamic properties of squirmer swimming in power-law fluid near a wall

Abstract: Hydrodynamic properties of squirmer swimming in power-law fluid near a wall considering the interaction between squirmer and wall are numerically studied with an immersed boundary-lattice Boltzmann method The power-law index, Reynolds number, initial orientation angle of squirmer, and initial distance of squirmer from the wall are all taken into account to investigate the swimming characteristics for pusher (β 0) (three kinds of swimmer types) near the no-slip boundary Four new kinds of swimming modes are found Results show that, for the pushers and pullers, the wall displays an increasing attraction with increasing power-law index n, which differs from the neutral squirmer who always departs from the wall after the first collision with the wall Both the initial orientation angle and initial distance from the wall only affect the moving situations rather than the moving modes of the squirmers However, the squirmers depart from the wall as the Reynolds number increases and chaotic orbits appear for some squirmers at Re = 5 Several typical flow fields are analyzed and the power consumption and torque for different kinds of flows are also studied It is found that, as the absolute value of β increases, the power consumption generally increases in shear-thinning (n = 04), Newtonian (n = 1), and shear-thickening (n = 16) fluids Moreover, the pushers (β 0) expend almost the same power if the absolute value of β remains the same In addition, the power consumption of the squirmers is highly dependent on the power-law index n

A quest for a model of non-colloidal suspensions with Newtonian matrices

Abstract: It would be convenient to have a model, albeit approximate, of particle-laden materials (suspensions) that would not need large amounts of computing and/or experimentation to implement for design purposes. There are now adequate models of the pure matrix fluid behaviour, but there are no such models for suspensions with large particles (non-colloidal suspensions). One of the obstacles has been the single-minded devotion to shearing motions of suspensions; experience with the matrix modelling has shown that it is not possible to formulate widely usable models if only shear is considered. Here some new results of axially symmetric elongational tests on suspensions are compared with shearing data. Some suggestions for modelling these and other observations based on using strain rate and strain in a modified Reiner-Rivlin constitutive equation are presented. The model generally works quite well, but it does not predict the positive storage modulus seen in small and medium amplitude oscillatory shear flows.

Simultaneous slit rheometry and in situ neutron scattering

Abstract: In situ measurement of fluid structure during flow, e.g., by neutron scattering, is key to understanding the relationship between structure and rheology. For some applications, structures at high shear rates previously unreachable are of particular interest. Here, we report development of a flow cell slit rheometer for neutron scattering (μRheoSANS). The devices were used to measure the structure of a semi-dilute surfactant solution of worm-like micelles during flow. Analysis of the rheometry and scattering data allows isolation of the scattering signal from the high-shear, near-wall region of the flow cell. The reported results agree with those from the existing Couette-based RheoSANS instrument. The worm-like micelles exhibit an alignment transition at Weissenberg number (Wi) ≈ 1, coinciding with the onset of shear thinning. This transition is followed by a peak in micelle alignment at a higher shear rate, after which the degree of alignment decreases moderately. This technique can achieve higher shear rates than existing RheoSANS techniques, expanding the ability to study the structure of complex fluids at elevated shear rates.

Newtonian, power law, and infinite shear flow characteristics of concentrated slurries using percolation theory concepts

Abstract: There is a strong interest today in concentrated particulate-filled dispersion and slurries in both polymeric and Newtonian fluids. This paper reviews and extends theoretical approaches using percolation theory concepts to characterize the rheological behavior of fluids filled with particulate solids. First, a previously proposed limiting, zero shear viscosity model based on percolation theory concepts is reviewed. This model has been primarily tested with rigid fillers in a Newtonian carrier and polymeric fluids. Second, all Newtonian fluid-based slurries that have a high concentration of filler become pseudoplastic, shear-thinning slurries at some threshold shear rate. A new theory is reviewed and new data are evaluated that correlate the power law constant, n, to cluster formation of the fillers suspended in the fluids in shear flow. Slurry systems reported here cover a size range from 58 nm to 200 μm. Third, this cluster percolation-based rheological analysis is then extended to a newly proposed model for the calculation of the ratio of infinite shear, η∞, to the zero shear viscosity, η0. Using literature data, it is demonstrated that measurements of the viscosity ratio, η∞/η0, correlate with the power law through the use of an energy dissipation-based model for Bingham rheological fluids.

Yielding characterization of waxy gels by energy dissipation

Abstract: The rheological behaviors of yield stress fluids are commonly interpreted from the energy perspective, yet this perspective has rarely been applied for waxy gel. In this study, we calculated the energy dissipated into the waxy gel caused by viscous strain during the deformation process with the aid of a well-tested elasto-viscoplastic thixotropic model. The energy dissipation shows a power-law dependence on the strain regardless of the stress ramping rate. The energy dissipation has been shown to be useful to characterize the ductility of waxy gel, that is, the larger the energy is dissipated during fracture, the more ductile the waxy gel is. When the waxy gel is induced to the same strain by different stress ramping rate, the energy dissipation during the deformation process shows little dependence on the stress ramping rate. However, if during the loading process the stress is increased to the same value by different ramping rate, a higher stress ramping rate causes lower energy dissipation. These findings are helpful to elucidate the widely observed experimental observation for waxy gels that the measured yield stress depends on the stress ramping rate whereas the measured yield strain shows little dependence.