arXiv subjects

Yogesh M. Joshi

Publications and source records attributed to Yogesh M. Joshi.

At least 19 recordsLinked to original sources

arXiv2026

When Spectroscopies Speak the Same Language: Unifying Rheology, Electrochemical Impedance, and Dielectrics

Spectroscopic techniques measure the dynamical response of physical systems subjected to oscillatory perturbations. For small perturbations around the equilibrium state, these spectroscopic methods are unified by the common mathematical framework of linear response theory. This work presents a unified perspective on rheological or mechanical spectroscopy, electrochemical impedance spectroscopy, and broadband dielectric spectroscopy through the lens of linear response theory. Subtle conceptual similarities and differences among these techniques are highlighted by analyzing their mapping to the linear response theory, basic building blocks, elementary models, and time and frequency domain response functions. Data validation and analysis approaches, including Kramers-Kronig relations, equivalent circuits, and multimode models are discussed. The shared fundamentals of different spectroscopies enable seamless exchange of ideas across domains.

cond-mat.soft↗

arXiv2026

Nano-Clay-Stabilized Water-in-Oil Colloidal Pickering Emulsions as Thixotropic Lubricant

The limitations of conventional mineral oil-based lubricants motivate the development of environmentally benign emulsions capable of providing lubrication and heat dissipation in demanding applications. In this study, nano-organoclay (Garamite 1958)-stabilized thixotropic water-in-oil Pickering emulsions are developed using sunflower oil as the base. The rheological and tribological properties of the emulsion system are systematically examined. Rheological findings reveal a pronounced increase in yield stress, shear thinning and thixotropic behavior on increasing Garamite loading percentage in the emulsion. The tribological performance is assessed against dry, water, and oil-lubricated conditions for a steel-steel interface under high contact pressure. The findings indicate that the tribological performance is significantly influenced by the microstructure and thixotropic behavior of the emulsions. The emulsion with the optimal nano-clay concentration demonstrates approximately 41\% and 84\% lower friction and approximately 80\% and 96\% lower wear than oil and water, respectively. The emulsion exhibits sensitivity to the sliding direction and displays load-responsive friction behavior with a memory effect owing to the reversible structuring of the clay-droplet network. This superior performance is attributed to the combined effects of thixotropy, anisotropic nanoclay morphology, and stable droplet armoring, which form a robust and adaptive interfacial film. This study advances the understanding of Pickering emulsions in metallic tribosystems by correlating the microstructure and rheology with tribological performance, thereby facilitating the design of high-performance, smart, and eco-conscious lubricants for metallic systems.

cond-mat.soft↗

arXiv2025

Rheological Behavior of Colloidal Silica Dispersion: Irreversible Aging and Thixotropy

In this work, we study the rheological behavior of colloidal dispersion of charge-screened nanoparticles of silica suspended in aqueous media that exhibits soft solid-like consistency. We observe that the system shows various characteristics of physical aging wherein it undergoes time evolution of rheological properties such as elastic modulus, relaxation time, and yield stress subsequent to shear melting of the same. Notably, the relaxation time increases more strongly than linearly with time, which is suggestive of hyper-aging dynamics. When considered along with the time-dependent yield stress, this behavior indicates the steady state shear stress-shear rate flow curve to be non-monotonic with a negative slope in a lower shear rate region. Performing shear melting on this system at a later date since the preparation of the dispersion (rest time) results in higher viscosity as well as yield stress, and the corresponding evolution of the elastic modulus shifts to lower times. This implies that physical aging in studied silica dispersion, while reversible over short time scales (of the order of hours), becomes irreversible over longer durations (days) owing to the inability of strong shear to break interparticle bonds that have strengthened over long durations. We also develop a thixotropic structural kinetic model within a time-dependent Maxwell framework that captures the experimentally observed rheological behavior well.

cond-mat.soft↗

arXiv2025

Plastic or Viscous? A Reappraisal of Yielding in Soft Matter

Many soft jammed materials, such as pastes, gels, concentrated emulsions, and suspensions, possess a threshold stress, known as yield stress, that must be exceeded to cause permanent deformation or flow. In rheology, the term plastic flow is commonly used to describe continuous flow (unbounded increase in strain with time) that a material undergoes above a yield stress threshold. However, in solid mechanics, plasticity refers to irreversible but finite, rate-independent deformation (strain that does not evolve with time). In addition, many soft materials exhibit viscosity bifurcation, a prominent thixotropic signature, which further complicates the definition and interpretation of yield stress. The threshold stress at which viscosity bifurcation occurs is also termed a yield stress, even though deformation below this threshold is not purely elastic, while above this threshold, the material flows homogeneously with a constant shear rate. This paper revisits these critical issues by analyzing the rheological and solid mechanics perspectives on plasticity. The insights presented here are intended to address certain terminological ambiguities for interpreting flow in soft jammed materials.

cond-mat.soft↗

arXiv2025

Nano-silica based Aqueous Colloidal Gels as Eco-friendly Thixotropic Lubricant

The environmental risks posed by traditional oil and grease-based lubricants can be significantly mitigated by adopting water-based alternatives engineered with superior rheological performance. In this work, we present a fundamentally new and environmentally sustainable aqueous thixotropic colloidal gel of silica nanoparticles formed in the presence of NaCl. We conducted a systematic and detailed investigation of their rheological and tribological characteristics. The tribological performance was evaluated against dry and water-lubricated conditions for steel-steel interface. Our experiments demonstrate that the tribological performance of the formulated nanoparticle gel can be optimized by tuning its rheological properties. A combination of super-low friction and negligible wear was observed. The friction coefficient reduced by up to 97.46% (from 0.63 to 0.016) compared to dry sliding, and by 97.04% (from 0.541 to 0.016) compared to water lubrication. Similarly, the specific wear rate decreased by up to 99.62% and 96.10% under dry conditions and water lubrication respectively. This performance is attributed to a thixotropic, chemically robust gel formed via van der Waals interactions between silica flocs, enabling self-repairing properties, continuous tribo-film formation, and a nano-bearing effect from silica nanoparticles. These attributes enable the gel to maintain and regain its structure during periods of non-shear while also forming a thin film with sufficiently low viscosity to slip into the interfacial contact zone and continuously replenish it with lubricant.

cond-mat.soft↗

arXiv2024

Harmonic Balance for Differential Constitutive Models under Oscillatory Shear

Harmonic balance (HB) is a popular Fourier-Galerkin method used in the analysis of nonlinear vibration problems where dynamical systems are subjected to periodic forcing. We adapt HB to find the periodic steady-state response of nonlinear differential constitutive models subjected to large amplitude oscillatory shear flow. By incorporating the alternating-frequency-time scheme into HB, we develop a computer program called FLASH (acronym for Fast Large Amplitude Simulation using Harmonic balance), which makes it convenient to apply HB to any differential constitutive model. We validate FLASH by considering two representative constitutive models, viz., the exponential Phan-Thien Tanner model and a nonlinear temporary network model. In terms of accuracy and speed, FLASH outperforms the conventional approach of solving initial value problems by numerical integration via time-stepping methods often by several orders of magnitude. We discuss how FLASH can be conveniently extended for other nonlinear constitutive models, which opens up potential applications in model calibration and selection, and stability analysis.

cond-mat.soft↗

arXiv2023

Ductile-to-brittle transition and yielding in soft amorphous materials: perspectives and open questions

Soft amorphous materials are viscoelastic solids ubiquitously found around us, from clays and cementitious pastes to emulsions and physical gels encountered in food or biomedical engineering. Under an external deformation, these materials undergo a noteworthy transition from a solid to a liquid state that reshapes the material microstructure. This yielding transition was the main theme of a workshop held from January 9 to 13, 2023 at the Lorentz Center in Leiden. The manuscript presented here offers a critical perspective on the subject, synthesizing insights from the various brainstorming sessions and informal discussions that unfolded during this week of vibrant exchange of ideas. The result of these exchanges takes the form of a series of open questions that represent outstanding experimental, numerical, and theoretical challenges to be tackled in the near future.

cond-mat.soft↗

arXiv2023

Rheological Investigation of The Network Structure in Mixed Gels of Kappa and Iota Carrageenan

Carrageenans comprise linear sulfated high molecular weight polysaccharides obtained from seaweeds and are routinely used in food and home/personal care industries. Various kinds of carrageenans differ from others based on the ester sulfate group location on the polysaccharide chains. Pure and mixed systems of Kappa Carrageenan and Iota Carrageenan undergo a three-dimensional gel network structure formation or dissociation with a change in temperature. During the sol-gel and gel-sol transitions, the Carrageenan systems pass through a unique critical gel state, where dynamic moduli are scale-invariant owing to the self-similar structure of the three-dimensional network. In this work, we obtain the critical gel state associated with pure and mixed systems of Kappa and Iota Carrageenan during cooling and heating by exploring the material behavior for a range of frequencies. Interestingly, on the one hand, the mixed gels show a higher critical sol-gel transition temperature compared to the pure systems at equal individual concentrations. On the other hand, the low temperature moduli of mixed gels are closer to that of Kappa Carrageenan when the concentration of the same is more than half in the mixture. The rheological measurements demonstrate that the Kappa Carrageenan strongly affects the nature of aggregation of double helices of Iota Carrageenan, but Iota Carrageenan does not have a significant influence on that of Kappa Carrageenan. These results suggest an associative, interactive network formation between Kappa and Iota Carrageenan in the mixture, such that the gel behavior is predominantly influenced by Kappa Carrageenan.

cond-mat.soft↗

arXiv2023

Transient shear banding during startup flow: Insights from nonlinear simulations

We study the dynamics of shear startup of the Johnson-Segalman and non-stretching Rolie-Poly models using nonlinear simulations. We consider cases where the startup is from zero shear rate to shear rates in both the monotonic and nonmonotonic regions of the constitutive curve. For the Johnson-Segalman model, which exhibits a shear stress overshoot during startup, our nonlinear simulations show that transient shear banding is absent regardless of whether the start-up shear rate is in the monotonic or nonmonotonic regions of the constitutive curve. In the latter case, while there is clearly an inhomogeneity en route to the banded state, the extent of shear banding is not substantially large compared to that of the eventual banded state. Marked inhomogeneity in the velocity profile is predicted for the non-stretching Rolie-Poly model only if the solvent to solution viscosity ratio is smaller than O(10^(-3), but its occurrence does not appear to have any correlation with the stress overshoot during startup. These inhomogeneities are also very sensitive to initial amplitude of perturbations and the magnitude of Reynolds number. Our nonlinear simulations show that the transient evolution during shear startup is quite sensitive to the Reynolds number when the solvent viscosity parameter is much smaller than unity for non-stretching Rolie-Poly model. However, the results of the Johnson-Segalman model are very robust for solvent to solution viscosity greater than O(10^(-3) and do not reveal any transient shear banding during shear startup.

cond-mat.soft↗

arXiv2023

The Method of Harmonic Balance for the Giesekus Model under Oscillatory Shear

The method of harmonic balance (HB) is a spectrally accurate method used to obtain periodic steady state solutions to dynamical systems subjected to periodic perturbations. We adapt HB to solve for the stress response of the Giesekus model under large amplitude oscillatory shear (LAOS) deformation. HB transforms the system of differential equations to a set of nonlinear algebraic equations in the Fourier coefficients. Convergence studies find that the difference between the HB and true solutions decays exponentially with the number of harmonics ($H$) included in the ansatz as $e^{-m H}$. The decay coefficient $m$ decreases with increasing strain amplitude, and exhibits a "U" shaped dependence on applied frequency. The computational cost of HB increases slightly faster than linearly with $H$. The net result of rapid convergence and modest increase in computational cost with increasing $H$ implies that HB outperforms the conventional method of using numerical integration to solve differential constitutive equations under oscillatory shear. Numerical experiments find that HB is simultaneously about three orders of magnitude cheaper, and several orders of magnitude more accurate than numerical integration. Thus, it offers a compelling value proposition for parameter estimation or model selection.

math.NA↗

arXiv2023

On the Nature of Flow Curve and Categorization of Thixotropic Yield Stress Materials

Thixotropy is a phenomenon related to time dependent change in viscosity in presence or absence of flow. The yield stress, on the other hand, represents the minimum value of stress above which steady flow can be sustained. In addition, the yield stress of a material may also change as a function of time. Both these characteristic features in a material strongly influence the steady state flow curve of the same. This study aims to understand the interrelation between thixotropy, yield stress and their relation with the flow curve. In this regard, we study five thixotropic materials that show yield stress. The relaxation time of all the five systems shows power-law dependence on aging time with behaviors ranging from weaker than linear, linear to stronger than linear. Furthermore, the elastic modulus and yield stress has been observed to be constant for some systems while time dependent for the others. We also analyze the experimental behavior through a viscoelastic thixotropic structural kinetic model that predicts the observed experimental behavior of constant as well as time-dependent yield stress quite well. These findings indicate that a non-monotonic steady-state flow curve in a structural kinetic formalism necessarily leads to time-dependent yield stress, while constant yield stress is predicted by a monotonic steady-state flow curve with stress plateau in the limit of low shear rates. The present work, therefore, shows that thixotropic materials may exhibit either monotonic or non-monotonic flow curves. Consequently, thixotropic materials may show no yield stress, constant yield stress or time-dependent yield stress.

cond-mat.soft↗

arXiv2022

Large Amplitude Oscillatory Shear Study of a Colloidal Gel at the Critical State

We investigate the nonlinear viscoelastic behavior of a colloidal dispersion at the critical gel state using large amplitude oscillatory shear (LAOS) rheology. The colloidal gel at the critical point is subjected to oscillatory shear flow with increasing strain amplitude at different frequencies. We observe that the first harmonic of the elastic and viscous moduli exhibits a monotonic decrease as the material undergoes a linear to nonlinear transition. We analyze the stress waveform across this transition and obtain the nonlinear moduli and viscosity as a function of frequency and strain amplitude. The analysis of the nonlinear moduli and viscosities suggests intracycle strain stiffening and intracycle shear thinning in the colloidal dispersion. Based on the insights obtained from the nonlinear analysis, we propose a potential scenario of the microstructural changes occurring in the nonlinear region. We also develop an integral model using the time-strain separable K-BKZ constitutive equation with a power-law relaxation modulus and damping function obtained from experiments. At low strain amplitudes, this model compares well with experimental data at all frequencies. However, a stronger damping function, which can be efficiently inferred using a spectral method, is required to obtain quantitative fits across the entire range of strain amplitudes and the explored frequencies.

cond-mat.soft↗

arXiv2022

Gel-Sol Transition of Thermoresponsive Poly(vinyl alcohol) Solution: Validation of the Universal Critical Scaling Relations

While undergoing gelation transition, a material passes through a distinctive state called the critical gel state. In the neighborhood of this critical gel state, how viscosity, equilibrium modulus, and relaxation times evolve are correlated by scaling relations, and their universality has been validated for materials undergoing the sol to gel transition. In this work, we extend this approach for the gel to sol transition of a thermoresponsive polymeric system of aqueous Poly(vinyl alcohol) (PVOH) gel that passes through the critical state upon increasing temperature. We observe that, in the neighborhood of the critical gel state, the equilibrium modulus and viscosity demonstrate a power law dependence on the relative distance from the critical state in terms of normalized temperature. Furthermore, the relaxation times in the gel and the sol state shows symmetric power law divergence near the critical state. The corresponding critical power law exponents and the dynamic critical exponents computed at the critical gel to sol transition state validate the scaling and hyperscaling relations originally proposed for the critical sol to gel transition very well. Remarkably, the dependence of complex viscosity on frequency at different temperatures shows a comprehensive mastercurve irrespective of the temperature ramp rate independently in the gel and the sol state. This observation demonstrates how the shape of relaxation time spectrum is independent of both the temperature as well as the ramp rate. Since sol to gel as well as the gel to sol transitions are opposite to each other, the applicability of the scaling relations validated in this work suggests broader symmetry associated with how the structure evolves around the critical state irrespective of the direction.

cond-mat.soft↗

arXiv2022

Viscoelasticity and Rheological Hysteresis

Rheological characterization of complex fluids subjected to cyclic shear-rate sweep often exhibits hysteresis. Since both viscoelastic and thixotropic materials show hysteresis loops, it is important to understand distinguishing features (if any) in the same shown by either. Lately, there has been substantial work that attempts to relate the area enclosed by the hysteresis loop with the manner in which shear rate is varied in the cycle, in order to infer thixotropic parameters of a material. In this work, we use the nonlinear Giesekus model to study its response to the application of cyclic shear-rate sweep. We find that this model produces each type of ualitatively similar hysteresis loop that has hitherto been ascribed to thixotropic materials. We also show that the area of the hysteresis loop for a viscoelastic material as a function of sweep rate shows bell-shaped/bi-modal curves as has been observed for thixotropic materials. This study illustrates that caution needs to be exercised while attributing hysteresis loops and associated features observed in a material exclusively to thixotropy. Another feature related to the hysteresis loop is the occurrence of shear banding instability. We find that viscoelastic hysteresis may not have any connection to shear banding instability.

cond-mat.soft↗

arXiv2022

Kramers-Kronig Relations for Nonlinear Rheology: 1. General Expression and Implications

The principle of causality leads to linear Kramers-Kronig relations (KKR) that relate the real and imaginary parts of the complex modulus $G^{*}$ through integral transforms. Using the multiple integral generalization of the Boltzmann superposition principle for nonlinear rheology, and the principle of causality, we derived nonlinear KKR, which relate the real and imaginary parts of the $n^\text{th}$ order complex modulus $G_{n}^{*}$. For $n$=3, we obtained nonlinear KKR for medium amplitude parallel superposition (MAPS) rheology. A special case of MAPS is medium amplitude oscillatory shear (MAOS); we obtained MAOS KKR for the third-harmonic MAOS modulus $G_{33}^{*}$; however, no such KKR exists for the first harmonic MAOS modulus $G_{31}^{*}$. We verified MAPS and MAOS KKR for the single mode Giesekus model. We also probed the sensitivity of MAOS KKR when the domain of integration is truncated to a finite frequency window. We found that that (i) inferring $G_{33}^{\prime\prime}$ from $G_{33}^{\prime}$ is more reliable than vice-versa, (ii) predictions over a particular frequency range require approximately an excess of one decade of data beyond the frequency range of prediction, and (iii) $G_{33}^{\prime}$ is particularly susceptible to errors at large frequencies.

cond-mat.soft↗

arXiv2022

Kramers-Kronig Relations for Nonlinear Rheology: 2. Validation of Medium Amplitude Oscillatory Shear (MAOS) Measurements

The frequency dependence of third-harmonic medium amplitude oscillatory shear (MAOS) modulus $G_{33}^{*}(ω)$ provides insight into material behavior and microstructure in the asymptotically nonlinear regime. Motivated by the difficulty in the measurement of MAOS moduli, we propose a test for data validation based on nonlinear Kramers-Kronig relations. We extend the approach used to assess the consistency of linear viscoelastic data by expressing the real and imaginary parts of $G_{33}^{*}(ω)$ as a linear combination of Maxwell elements: the functional form for the MAOS kernels is inspired by time-strain separability (TSS). We propose a statistical fitting technique called the SMEL test, which works well on a broad range of materials and models including those that do not obey TSS. It successfully copes with experimental data that are noisy, or confined to a limited frequency range. When Maxwell modes obtained from the SMEL test are used to predict the first-harmonic MAOS modulus $G_{31}^{*}$, it is possible to identify the range of timescales over which a material exhibits TSS.

cond-mat.soft↗

arXiv2022

Onset of transient shear banding in viscoelastic shear start-up flows: Implications from linearized dynamics

We analyze transient dynamics during shear start-up in viscoelastic flows between two parallel plates, with a specific focus on the signatures for the onset of transient shear banding using the Johnson-Segalman, non-stretching Rolie-Poly and Giesekus models. We explore the dynamics of shear start-up in monotonic regions of the constitutive curves using two different methodologies: (i) the oft-used `frozen-time' linear stability (eigenvalue) analysis, wherein we examine whether infinitesimal perturbations imposed on instantaneous stress components (treated as quasi steady states) exhibit exponential growth, and (ii) the more mathematically rigorous fundamental-matrix approach that characterizes the transient growth via a numerical solution of the time-dependent linearized governing equations, wherein the linearized perturbations co-evolve with the start-up shear flow. Our results reinforce the hitherto understated point that there is no universal connection between the overshoot and subsequent decay of shear stress in the base state and the unstable eigenvalues obtained from the frozen-time stability analysis. It may therefore be difficult to subsume the occurrence of transient shear banding during shear start-up within the ambit of a single model-independent criterion. Our work also suggests that the strong transients during shear start-up seen in earlier work could well be a consequence of consideration of the limit of small solvent viscosity in the absence of otherwise negligible terms such as fluid inertia.

physics.flu-dyn↗

arXiv2021

Distinguishing thixotropy from viscoelasticity

Owing to nonlinear viscoelasticity, materials often show characteristic features that resemble those of thixotropy. This issue has been debated in the literature over the past several decades, and several experimental protocols have been proposed to distinguish thixotropy from viscoelasticity. In this work, we assess these protocols by carrying out experiments using polymer solutions, thixotropic clay dispersions, and modeling their behavior, respectively, using the FENE-P constitutive equation and a viscoelastic aging model. We find that the criteria proposed in the literature, such as a step down jump in the shear rate and shear startup at different waiting times elapsed since preshear, are inadequate to distinguish thixotropy from viscoelasticity. In marked contrast, we show that the application of step-strain or step-stress after cessation of the preshear serves as a useful discriminant between thixotropy and viscoelasticity. In thixotropic materials, we observe that the application of step strain (or step stress) after cessation of the preshear eventually leads to slowing down of relaxation dynamics as a function of waiting time. However, for viscoelastic materials, the relaxation modulus (creep compliance) curve shifts to lower modulus (higher compliance) values as a function of waiting time until equilibrium is reached. While the proposed criterion offers a robust distinction between viscoelasticity and thixotropy for the systems studied here, further experimental investigations based on other systems are needed to establish its versatility and will lead to a greater insight into this long-standing issue in rheological categorization.

cond-mat.soft↗