Search

We all instinctively poke, bounce, scoop, and observe materials to understand rheological properties quickly. Yet, these observations are rarely analyzed quantitatively. To address this, here we introduce the paradigm of protorheology: approximate quantitative inference from simple observations. Several case studies demonstrate how protorheology is an inclusive entry to rheology for a broad range of practitioners and strengthens the confidence and interpretation of accurate laboratory measurements. We survey a range of creative tests according to which rheological phenomenon is revealed. Some new working equations are derived, and all working equations are summarized for convenient reference and comparison across different methods. This establishes a framework to enable increased use of photos, videos, and quantitative inference and to support the increasing interest in digital image analysis, inverse methods, and high-throughput characterization being applied to rheological properties. [ABSTRACT FROM AUTHOR]

Thixotropy, antithixotropy, and viscoelasticity are three types of time-dependent dynamics that involve fundamentally different underlying physical processes. Here, we show that the three dynamics exhibit different signatures in hysteresis by examining the fingerprints of the simplest thixotropic kinetic model, a new antithixotropic model that we introduce here, and the Giesekus model. We start by showing that a consistent protocol to generate hysteresis loops is a discrete shear-rate controlled ramp that begins and ends at high shear rates, rather than at low shear rates. Using this protocol, we identify two distinguishing features in the resulting stress versus shear rate loops. The first is the direction of the hysteresis loops: clockwise for thixotropy, but counterclockwise for viscoelasticity and antithixotropy. A second feature is achieved at high ramping rates where all responses lose hysteresis: the viscoelastic response shows a stress plateau at low shear rates due to lack of stress relaxation, whereas the thixotropic and antithixotropic responses are purely viscous with minimal shear thinning or thickening. We establish further evidence for these signatures by experimentally measuring the hysteresis of Laponite suspensions, carbon black suspensions, and poly(ethylene oxide) solutions, each representing a historically accepted example of each class of material behavior. The signatures measured in experiments are consistent with those predicted by the three models. This study reveals different fingerprints in hysteresis loops associated with thixotropy, antithixotropy, and viscoelasticity, which may be helpful in distinguishing the three time-dependent responses. [ABSTRACT FROM AUTHOR]

We report a detailed experimental study of peculiar thixotropic dynamics of carbon black (CB, Vulcan XC-72) suspensions in mineral oil, specifically the observation of sequential stress increase then decrease at a fixed shear rate in a step-down test. We verify that such dynamics, though peculiar, come from a true material response rather than experimental artifacts. We also reveal how this long-time stress decay is associated with antithixotropic structural change rather than viscoelastic stress relaxation by using orthogonal superposition (OSP) rheometry to probe viscoelastic moduli during the step-down tests. The orthogonal storage and loss moduli are present, showing this two-timescale recovery then decay response, which demonstrates that this response is antithixotropic, and it involves shear-induced structuring. We further show a mechanical anisotropy in the CB suspension under shear using OSP. Based on the rheological results, a microstructural schematic is proposed, considering qualitatively thixotropic structure build-up, antithixotropic densification, and anisotropic structure evolution. Our observation for these CB suspensions is outside the standard paradigm of thixotropic structure-parameter models, and the elastic response provides us with new insight into the transient dynamics of CB suspensions. [ABSTRACT FROM AUTHOR]

There is no universal model for thixotropy, and comparing thixotropic effects between different fluids is a subtle yet challenging problem. We introduce a generalized (model-insensitive) framework for comparing thixotropic properties based on thixotropic spectra. A superposition of exponential stress modes distributed over thixotropic time scales is used to quantify buildup and breakdown times and mode strengths in response to step-change input. This mathematical framework is tested with several experimental step-shear rate data on colloidal suspensions. Low-dimensional metrics based on moments of the distribution reveal characteristic average thixotropic properties, which are visualized on Ashby-style diagrams. This method outlines a framework for describing thixotropy across a diverse range of microstructures, supporting scientific studies as well as material selection for engineering design applications. [ABSTRACT FROM AUTHOR]

We describe the structure and outcomes of a course project for do-it-yourself (DIY) rheometry. Although the project was created in response to the shelter-in-place orders of the COVID-19 pandemic, the student learning outcomes were so positive that we have continued implementing the project even when students have access to laboratory rheometers. Students select an interesting complex fluid, collect qualitative visual evidence of key rheological phenomena, and then produce their own readily available flows that they quantitatively analyze to infer rheological properties, such as yield stress, extensional viscosity, or shear viscosity. We provide an example rubric, present example student project outcomes, and discuss learning outcomes that are achieved with DIY measurements. [ABSTRACT FROM AUTHOR]

We show that Hertzian particle contacts are the underlying cause of the as-yet-unexplained noninteger power laws in weakly nonlinear rheology. In the medium amplitude oscillatory shear (MAOS) region, the cubic scaling of the leading order nonlinear shear stress (σ 3 ∼ γ 0 m 3 , m 3 = 3) is the standard expectation. Expanding on the work by Natalia et al. [J. Rheol. 64, 625–635 (2020)], we report an extensive data set of noncubical, noninteger power law scalings m 3 for particle suspensions in two immiscible fluids with a capillary attractive interaction, known as capillary suspensions. Here, we show that distinct power law exponents are found for the storage and loss moduli and these noninteger scalings occur at every secondary fluid concentration for two different contact angles. These compelling results indicate that the noninteger scalings are related to the underlying microstructure of capillary suspensions. We show that the magnitude of the third harmonic elastic stress scaling m 3 , elastic originates from Hertzian-like contacts in combination with the attractive capillary force. The related third harmonic viscous stress scaling m 3 , viscous is found to be associated with adhesive-controlled friction. These observations, conducted for a wide range of compositions, can help explain previous reports of noninteger scaling for materials involving particle contacts and offers a new opportunity using the variable power law exponent of MAOS rheology to reveal the physics of particle bonds and friction in the rheological response under low deformation instead of at very high shear rates. [ABSTRACT FROM AUTHOR]

We derive new analytical solutions for the nonaffine Johnson–Segalman/Gordon–Schowalter (JS/GS) constitutive equation with a general relaxation kernel in medium-amplitude oscillatory shear (MAOS) deformation. The results show time-strain separable (TSS) nonlinearity, therefore providing new physically meaningful interpretation to the heuristic TSS nonlinear parameter in MAOS [Martinetti and Ewoldt, Phys. Fluids 31, 021213 (2019)]. The upper-convected, lower-convected, and corotational Maxwell models are all subsets of the results presented here. The model assumes that the microscale elements causing stress in the material slip compared to the continuum deformation. We introduce a visualization of the nonaffine deformation field that acts on stress-generating elements to reinforce the physical interpretation of the JS/GS class of models. Finally, a case study is presented where previously published results, from fitting TSS models to MAOS data, can be reinterpreted based on the concept of nonaffine motion of the JS/GS framework. [ABSTRACT FROM AUTHOR]

We report viscous flow properties of a redox-active organic molecule, N-(2-(2-methoxyethoxy)ethyl)phenothiazine (MEEPT), a candidate for non-aqueous redox flow batteries, and two of its radical cation salts. A microfluidic viscometer enabled the use of small sample volumes in determining viscosity as a function of shear rate and concentration in the non-aqueous solvent, acetonitrile, both with and without supporting salts. All solutions tested show Newtonian behavior over shear rates of up to 30 000 s−1, which was rationalized by scaling arguments for the diffusion-based relaxation time of a single MEEPT molecule without aggregation. Neat MEEPT is flowable but with a large viscosity (412 mPa ⋅ s at room temperature), which is ∼1000 times larger than that of acetonitrile. MEEPT solutions in acetonitrile have low viscosities; at concentrations up to 0.5 M, the viscosity increases by less than a factor of two. From concentration-dependent viscosity measurements, molecular information was inferred from intrinsic viscosity (hydrodynamic diameter) and the Huggins coefficient (interactions). Model fit credibility was assessed using the Bayesian Information Criterion. It is found that the MEEPT and its charged cations are "flowable" and do not flocculate at concentrations up to 0.5 M. MEEPT has a hydrodynamic diameter of around 8.5 Å, which is almost insensitive to supporting salt and state of charge. This size is comparable to molecular dimensions of single molecules obtained from optimized structures using density functional theory calculations. The results suggest that MEEPT is a promising candidate for redox flow batteries in terms of its viscous flow properties. [ABSTRACT FROM AUTHOR]

We report the most complete experimental evidence to date of noninteger power-law expansions for nonlinear viscoelastic properties. Our observations, made with a capillary suspension of spherical poly(methyl methacrylate) (PMMA) particles, falsify the universality of the standard assumption where shear stress nonlinearities arise as the amplitude cubed, e.g., σ ∼ O (γ 0 3) in an oscillatory shear test at small amplitudes. In terms of material properties, our measurements require noninteger Taylor expansions about a linear reference state. Furthermore, distinct power-law exponents are found for the storage and loss signals. In this work, we conclusively demonstrate that the noninteger scaling is real and not an experimental artifact. Our experimental and data analysis methods ensure that we report the most complete result to date. While the noninteger scaling remains to be associated with underlying microstructural physics, the thorough experimental data reported herein demonstrate that such theoretical studies are worth pursuing. [ABSTRACT FROM AUTHOR]

We derive and study equations for the weakly nonlinear medium-amplitude oscillatory shear (MAOS) response of materials exhibiting time-strain separability. Results apply to constitutive models with arbitrary linear memory function m(s) and for both viscoelastic liquids and viscoelastic solids. The derived equations serve as a reference to identify which models are time-strain separable (TSS) and which may appear separable but are not, in the weakly nonlinear limit. More importantly, we study how the linear viscoelastic (LVE) relaxation spectrum, H(τ), affects the frequency dependence of the TSS MAOS material functions. Continuous relaxation spectra are considered that are associated with analytical functions (log-normal and asymmetric Lorentzian distributions), fractional mechanical models (Maxwell and Zener), and molecular theories (Rouse and Doi-Edwards). TSS MAOS signatures reveal much more than just the perturbation parameter A in the shear damping function small-strain expansion, h (γ) = 1 + A γ 2 + O γ 4 . Specifically, the distribution of terminal relaxation times is significantly more apparent in the TSS MAOS functions than their LVE counterparts. We theoretically show that this occurs because TSS MAOS material functions are sensitive to higher-order moments of the relaxation spectrum, which leads to the definition of MAOS liquids. We also show the first examples of MAOS signatures that differ from the liquid-like terminal MAOS behavior predicted by the fourth-order fluid expansion. This occurs when higher moments of the relaxation spectrum are not finite. The famous corotational Maxwell model is a subset of our results here, for which A = −1/6, and any LVE relaxation spectrum could be used. [ABSTRACT FROM AUTHOR]

We derive and demonstrate how analytical solutions for strain-controlled medium-amplitude oscillatory shear (MAOS) can be coupled with a continuous distribution of relaxation times. This applies generally to the vast library of existing MAOS models, including those that are not time-strain separable. The theoretical results are important for improved certainty in model parameters, since their number can be reduced dramatically compared to a discrete distribution of relaxation times. We exemplify this continuous spectrum MAOS approach by modeling experimental data for a transient network formed from an aqueous semidilute unentangled solution of poly(vinyl alcohol) and sodium tetraborate (Borax). The full frequency-dependent MAOS signatures are fit well by only five parameters: three linear parameters for a log-normal spectral distribution and two nonlinear parameters for the strength of the nonlinearity and its cutoff time scale. Remarkably, longer modes (τ > τ w ) are not activated in the asymptotically nonlinear regime. Although this may be compatible with the possible mechanisms for the shear nonlinearities, the reason for the hard cutoff of time scales is currently unknown. Our results also suggest that the sign change location for the third-harmonic viscous nonlinearity may be sensitive to large-scale structural features (such as molar mass distribution or long-chain branching) that control the polydispersity of terminal relaxation times. [ABSTRACT FROM AUTHOR]

We report experimental measurements of high extensibility of several yield-stress fluids, demonstrating a behavior completely outside the standard paradigm of model materials and constitutive equations for yield-stress fluids. We identify "highly" extensible materials using uniaxial tension tests, as materials with values of strain-to-break larger than predictions from the tensorial Herschel-Bulkley model, and larger strain-to-break than some of the most studied model materials including aqueous microgel particle suspensions (Carbopol), and aqueous clay suspensions (Laponite and Bentonite). Materials in commercial use, including a printing resin, chewing gums, and food products, have a yield stress but do not rupture until reaching extremely large extensional deformations. The purpose of this work is to introduce a method for characterizing the extensibility of yield-stress fluids, demonstrate the range of extensibility seen in real yield-stress fluids (commercial products, biomaterials), and introduce one possible model material for highly extensible yield-stress fluids: Silicone oil droplets emulsified at moderate volume fraction into an aqueous solution of polyvinyl alcohol cross-linked by sodium tetraborate. [ABSTRACT FROM AUTHOR]

Medium-amplitude oscillatory shear (MAOS) is asymptotically nonlinear and a valuable tool for inferring structure from rheology. However, a drawback of conventional MAOS is the time and material intensive nature of experiments. Many strain amplitude sweeps, and typically multiple sample loadings, are required to obtain frequency-dependent properties. Here, we propose a new MAOS methodology that is much faster (fewer data points) and cheaper (fewer material loadings): The frequency-sweep MAOS. Similar to conventional frequency-sweep SAOS (small-amplitude oscillatory shear), we use only a frequency-sweep. A key challenge is that measurable MAOS data lie within a narrow and frequency-dependent domain between the instrument resolution (at small strain amplitudes) and nonlinear behavior beyond asymptotic nonlinearity (at large strain amplitudes). This necessitates a nonconstant strain amplitude c0(x) for the frequency-sweep instead of a single constant strain amplitude. We provide guidelines for finding this c0(x) trajectory for frequency-sweep MAOS. Full characterization of all four MAOS measures requires two frequency-sweeps: One frequency-sweep for the third-harmonic measures and two frequency-sweeps (at different input strain amplitudes) for the first-harmonic measures. We propose criteria to validate this MAOS data taken at a single strain amplitude based on ratios of stress harmonics; a similar idea is extended to validate frequency-sweep SAOS data as well. The proposed method of frequency-sweep MAOS is demonstrated for a polyvinyl alcohol-borax hydrogel. This new, faster, and material economical MAOS approach will be particularly beneficial for precious samples with very limited availability such as model polymers with well-defined architectures. [ABSTRACT FROM AUTHOR]

We construct a microstructure-based constitutive model that successfully predicts experimental rheology signatures that no other model has previously described. The experimental observations are the low-dimensional descriptions of asymptotically nonlinear oscillatory shear [Ewoldt and Bharadwaj, Rheol. Acta 52, 201-209 (2013)], also known as medium-amplitude oscillatory shear, characterized by four frequency-dependent material measures: [e1]ω, [e3]ω, [υ1]ω and [υ3]ω. These slightly nonlinear rheological measurements are the systematic step beyond linear viscoelastic characterization. The material is a transiently crosslinked polymeric hydrogel of aqueous polyvinyl alcohol cross-linked by sodium tetraborate (borax) [Bharadwaj and Ewoldt, J. Rheol. 59, 557-592 (2015)], which shows nonlinear elastic stiffening inferred from [e1]ω>0. Here, we hypothesize that the appropriate physical model is a transient network of strain-stiffening elastic elements. We rationalize that all nonlinearities are driven by the instantaneous stretch magnitude Q between junctions, either through strain-stiffening of network elements or through deformation-assisted network structuring. These two physical aspects are embedded into a single nonlinear parameter that successfully captures both elastic energy storage ([e1]ω and [e3]ω]) and viscous energy dissipation ([υ1]ω and [υ3]ω), including frequency-dependent sign changes. Analytical results are derived for all four asymptotic nonlinearities. The quantitative agreement provides fit parameters that are related to molecular features and network architecture. While the work here is focused on a specific polymeric system, it represents the broad potential contribution of asymptotic, leading-order nonlinearities to enable structurerheology insight, constitutive model development, and model selection for soft materials in general. [ABSTRACT FROM AUTHOR]

Many recent studies on the viscoelasticity of individual cells link mechanics with cellular function and health. Here, we introduce a measurement of the viscoelastic properties of individual human colon cancer cells (HT-29) using silicon pedestal microelectromechanical systems (MEMS) resonant sensors. We demonstrate that the viscoelastic properties of single adherent cells can be extracted by measuring a difference in vibrational amplitude of our resonant sensor platform. The magnitude of vibration of the pedestal sensor is measured using a laser Doppler vibrometer (LDV). A change in amplitude of the sensor, compared with the driving amplitude (amplitude ratio), is influenced by the mechanical properties of the adhered cells. The amplitude ratio of the fixed cells was greater than the live cells, with a p-value<0.0001. By combining the amplitude shift with the resonant frequency shift measure, we determined the elastic modulus and viscosity values of 100 Pa and 0.0031 Pa s, respectively. Our method using the change in amplitude of resonant MEMS devices can enable the determination of a refined solution space and could improve measuring the stiffness of cells. [ABSTRACT FROM AUTHOR]

The more parameters in a rheological constitutive model, the better it tends to reproduce available data, though this does not mean that it is necessarily better justified. Good fits to data are only part of model selection. We develop a Bayesian inference approach that rigorously balances closeness to data against both the number of model parameters and their a priori uncertainty. The analysis reflects a basic principle: Models grounded in physics will enjoy greater generality and perform better away from where they are calibrated. In contrast, relatively empirical models can provide comparable fits, but their a priori uncertainty is penalized. We demonstrate the approach by computing the best-justified number of modes for a multimode Maxwell model (MMM) to describe the dynamic shear moduli G'(m), G"(w) of a synthetic polymer network with transient crosslinks (polyvinyl alcohol with sodium tetra-borate). It is shown that a corresponding array of spring-pots, arranged as a parallel array of fractional-Maxwell model elements, is less credible. In contrast, for a biopolymer gluten dough we show that the MMM, irrespective of number of modes, is far less credible than a critical-gel/Rouse model (CGRM), which with its firmer physical basis provides a more credible model. This is true even though the MMM provides a closer fit to the data than the CGRM for the gluten system. Though quantitative, this formulation does not fully supplant user judgment. However, unlike most model fitting/selection approaches, it requires specific, quantifiable, and potentially debatable quantification of this judgment, and thus it provides a rigorous, repeatable assessment of model viability. Models are supported (or not) given numerical input, not vague assertions. [ABSTRACT FROM AUTHOR]

Yield-stress fluids, including gels and pastes, are effectively fluid at high stress and solid at low stress. In liquid-solid impacts, the fluid motion can be halted by the yield stress at different points of the event, and these fluids can therefore stick and accumulate where they impact, motivating several applications of these rheologically complex materials. Here, we use high-speed imaging to experimentally study liquid-solid impact of yield-stress fluids on pre-coated horizontal surfaces. With a pre-coating of the same material, we can observe large long-lifetime ejection sheets with redirected momentum which extend away from the impact location. Under critical splash conditions, sheet breakup occurs and ejected droplets can be nonspherical and threadlike due to the inability of capillary stresses to deform material above a certain lengthscale. By varying the droplet size, impact velocity, surface coating thickness, and rheological material properties, we develop appropriate dimensionless parameters and present a low-dimensional regime map of impact behaviors. [ABSTRACT FROM AUTHOR]

Rheologists have expectations for signatures of linear viscoelastic properties, such as shapes of G'(ω) and G?(ω). Medium amplitude (or asymptotically-nonlinear) oscillatory shear (MAOS) provides additional nonlinear rheological information with low dimensional, well-defined material functions [Ewoldt and Bharadwaj, Rheol. Acta 52, 201-209 (2013)]. Here, we develop expectations of signatures (or fingerprints) for the four asymptotically-nonlinear material functions associated with MAOS, [e1](ω), [e3](ω), [v1](ω), [v3](ω). Although the linear fingerprints may be identical for different models, the asymptotically-nonlinear fingerprints may be different in magnitude, frequency-scaling, curve shapes, and sign changes. To perform the analysis, we collect/translate a library of available analytical strain-controlled MAOS fingerprints for seven different constitutive models. Using this library, we identify general trends and highlight key differences of asymptotic-nonlinear viscoelasticity. Asymptotic nonlinearities for all models considered here obey the terminal regime inter-relations and frequency scaling predicted by Bharadwaj and Ewoldt [J. Rheol. 58, 891-910 (2014)]. Unlike the positive linear viscoelastic measures, at least one of the four asymptotic nonlinearities changes signs with Deborah number (De). Following sign interpretations of Ewoldt and Bharadwaj [Rheol. Acta 52, 201-209 (2013)], we show that nonlinearities tend to be driven by strain-rates at small De, and by strains at large De, a trend observed for nearly all the constitutive models studied here, the exception being the model for dilute rigid dumbbell suspensions of Bird et al. [J. Chem. Phys. 140, 074904 (2014)]. Some constitutive models exhibit multiple sign changes at intermediate De and there may be no universal behavior of asymptotically-nonlinear fingerprints in this regime. Therefore, frequency-dependent signatures can be material-specific. This will allow inverse problems to infer structure, select models, and fit model parameters using asymptotically-nonlinear signatures. To illustrate this aspect, we demonstrate a fingerprint matching exercise with experimental measurements on a transiently cross-linked hydrogel system. We find that currently available model fingerprints can match the qualitative magnitudes and frequency dependence, but not the signs of the experimental transient network response. [ABSTRACT FROM AUTHOR]

We use a fourth-order fluid expansion to make general predictions for asymptotically nonlinear material functions in oscillatory shear, a characterization protocol sometimes known as mediumamplitude oscillatory shear. The calculation applies to any viscoelastic fluid in the terminal regime defined by the limit of Deborah number much less than one. Two viscous nonlinearities appear at third order, with shear stress scaling as ω³, and are interrelated by a constant multiplicative factor. Two elastic nonlinearities appear at fourth order, with shear stress scaling as ω4, and are also interrelated by a constant multiplicative factor. These nonlinear measures are decoupled from the linear material functions G' and G" because they depend on different expansion coefficients. Experimental measurements are presented for all four asymptotic shear material functions using a linear and well-entangled homopolymer of polyisoprene. The experimental observations are consistent with both the predicted frequency scaling and the predicted interrelations in the terminal regime. Signs and magnitudes cannot be universally predicted, leaving these as free parameters that depend on the specifics of the material microstructure or constitutive model. These general results explain previous observations involving different materials and constitutive models, and provide an important reference for future experiments, analytical results, and numerical computations of these rheological fingerprints. [ABSTRACT FROM AUTHOR]

Chewing gum provides an excellent everyday example of viscoelastic behavior, and understanding its rheological properties is important for application purposes. Here, we compare the rheological behavior of selected commercial chewing gums and bubble gums. Small amplitude oscillatory shear, shear creep, and steady shear demonstrated that both chewing and bubble gums behave like power-law critical gels in the linear regime. Nonlinear viscoelastic behavior was investigated using large amplitude oscillatory shear, shear creep, and start-up flows (in shear and uniaxial extension). Bubble gums showed more pronounced strain hardening and greater stresses to break in start-up of steady uniaxial extension than chewing gums. We argue that this combination of rheological signatures is sufficient to provide a new robust definition of chewing gum that is independent of specific molecular composition. There are potentially many different formulations and design routes that can achieve this distinctive rheological fingerprint. [ABSTRACT FROM AUTHOR]

For rotational rheometers, surface tension results in a torque that should not occur in an ideal, rotationally symmetric geometry. This paper identifies and explains the effect, which is due to surface tension and contact line traction forces, and not surface rheology at the liquid-air interface. For water, we show that this torque can be more than two orders of magnitude larger than the viscous torque. In steady shear flow, the effect appears as a constant torque independent of rate, which would appear inaccurately as apparent shear thinning of water. In oscillatory tests, this may appear inaccurately as an elastic modulus. This surface tension torque is sensitive to wetting conditions and contact line asymmetries and cannot be deterministically corrected in experimental measurements. It therefore raises the lower bound of the instrument low-torque limit. The surface tension torque is reduced by maximizing rotational symmetry of the contact line, minimizing evaporation and the migration of the contact line, reducing the radial location of the contact line, and lowering the surface tension. Identifying and eliminating the surface tension torque is critical for low viscosities, intrinsic viscosities, soft materials, subdominant viscoelastic components, small gaps, and any circumstance where the low-torque limit is experimentally important. [ABSTRACT FROM AUTHOR]

Large amplitude oscillatory shear (LAOS) is used as a tool to probe the nonlinear rheological response of a model elasto-viscoplastic material (a Carbopol microgel). In contrast to most recent studies, these large amplitude measurements are carried out in a stress-controlled manner. We outline a descriptive framework of characterization measures for nonlinear rheology under stress-controlled LAOS, and this is contrasted experimentally to the strain-controlled framework that is more commonly used. We show that this stress-controlled methodology allows for a physically intuitive interpretation of the yielding behavior of elasto-viscoplastic materials. The insight gained into the material behavior through these nonlinear measures is then used to develop two constitutive models that prescribe the rheological response of the Carbopol microgel. We show that these two successively more sophisticated constitutive models, which are based on the idea of strain decomposition, capture in a compact manner the important features of the nonlinear rheology of the microgel. The second constitutive model, which incorporates the concept of kinematic hardening, embodies all of the essential behaviors exhibited by Carbopol. These include elasto-viscoplastic creep and time-dependent viscosity plateaus below a critical stress, a viscosity bifurcation at the critical stress, and Herschel-Bulkley flow behavior at large stresses. [ABSTRACT FROM AUTHOR]

Material functions underlie our understanding of rheology. They form the descriptive language of rheologists and require clear definitions. Here, it is shown that the definitions of oscillatory material functions depend on how the oscillating input is mathematically referenced, as a sine or cosine. Depending on this seemingly arbitrary trigonometric reference choice, the (3rd, 7th, 11th, etc.) Fourier coefficients of a nonlinear shear response change sign. Additionally, the even harmonic coefficients of a shear normal stress response are transposed. This impacts large-amplitude oscillatory shear (LAOS) characterization in both shear strain-control (LAOStrain) and shear stress-control (LAOStress) modes. It is important to resolve this issue, because it involves the leading-order nonlinearities and the signs of these higher harmonics convey important information. This paper provides a resolution, in two parts. First, it is shown that the deformation-domain Chebyshev coefficients are immune to the arbitrary trigonometric reference in the time domain, and therefore the Chebyshev-coefficient material functions can be used and interpreted without risk of inconsistency. Second, this paper proposes the convention of referencing to a sine input for strain-control tests (currently the typical convention) and using a cosine input for stress-control (where there is not currently a convention). Finally, clarity is brought to the practical issue of data processing a digital signal, which is required for numerical simulations and every instrument that performs oscillatory characterization. [ABSTRACT FROM AUTHOR]

We demonstrate that field-responsive magnetorheological fluids can be used for variable-strength controllable adhesion. The adhesive performance is measured experimentally in tensile tests (a.k.a. probe-tack experiments) in which the magnetic field is provided by a cylindrical permanent magnet. Increasing the magnetic field strength induces higher peak adhesive forces. We hypothesize that the adhesion mechanism arises from the shear resistance of a yield stress fluid in a thin gap. This hypothesis is supported by comparing the experimentally measured adhesive performance to the response predicted by a lubrication model for a non-Newtonian fluid with a field-dependent yield stress. The model predictions are in agreement with experimental data up to moderate field strengths. Above a critical magnetic field strength the model over-predicts the experimentally measured values indicating non-ideal conditions such as local fluid dewetting from the surface. [ABSTRACT FROM AUTHOR]

In a previous paper [T. S. K. Ng and G. H. McKinley, J. Rheol. 52(2), 417-449 (2008)], we demonstrated that gluten gels can best be understood as a polymeric network with a power-law frequency response that reflects the fractal structure of the gluten network. Large deformation tests in both transient shear and extension show that in the absence of rigid starch fillers these networks are also time-strain factorizable up to very large strain amplitudes (γ*>5). In the present work, we further explore the nonlinear rheological behavior of these critical gels by considering the material response obtained in large amplitude oscillatory shear over a wide range of strains and frequencies. We use a Lissajous representation to compare the measured material response with the predictions of a network theory that is consistent with the proposed molecular structure of gluten gels. In the linear viscoelastic regime, the Lissajous figures are elliptical as expected and can be quantitatively described by the same power-law relaxation parameters determined independently from earlier experiments. In the nonlinear regime, the Lissajous curves show two prominent additional features. First is a gradual softening of the network indicated by the rotation of the major axis of the stress ellipse. This feature is accounted for in the model by the inclusion of a simple nonlinear network destruction term that reflects the reduction in network connectivity as the polymer chains are increasingly stretched. Second, a distinct upturn in the viscoelastic stress is discernable at large strains. We show that this phenomenon can be modeled by considering the effects of finitely extensible segments in the elastic network. We use this model to quantitatively predict the material response in other large amplitude transient flows such as the start-up of steady shear and transient uniaxial extension up until the onset of strongly nonlinear unsteady phenomena such as edge fracture in shear and sample rupture during extension. [ABSTRACT FROM AUTHOR]

Characterizing purely viscous or purely elastic rheological nonlinearities is straightforward using rheometric tests such as steady shear or step strains. However, a definitive framework does not exist to characterize materials which exhibit both viscous and elastic nonlinearities simultaneously. We define a robust and physically meaningful scheme to quantify such behavior, using an imposed large amplitude oscillatory shear (LAOS) strain. Our new framework includes new material measures and clearly defined terminology such as intra-/intercycle nonlinearities, strain-stiffening/softening, and shear-thinning/thickening. The method naturally lends a physical interpretation to the higher Fourier coefficients that are commonly reported to describe the nonlinear stress response. These nonlinear viscoelastic properties can be used to provide a “rheological fingerprint” in a Pipkin diagram that characterizes the material response as a function of both imposed frequency and strain amplitude. We illustrate our new framework by first examining prototypical nonlinear constitutive models (including purely elastic and purely viscous models, and the nonlinear viscoelastic constitutive equation proposed by Giesekus). In addition, we use this new framework to study experimentally two representative nonlinear soft materials, a biopolymer hydrogel and a wormlike micelle solution. These new material measures can be used to characterize the rheology of any complex fluid or soft solid and clearly reveal important nonlinear material properties which are typically obscured by conventional test protocols. [ABSTRACT FROM AUTHOR]