arXiv subjects

Yogesh M. Joshi

Publications and source records attributed to Yogesh M. Joshi.

At least 37 records · Page 2Linked to original sources

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↗

arXiv2019

On the universality of the critical scaling exponents during sol-gel transition

The evolution of viscoelastic properties near the sol-gel transition is studied by performing oscillatory rheological measurements on two different types of systems: a colloidal dispersion and a thermo-responsive polymer solution under isothermal and non-isothermal conditions. While undergoing sol-gel transition, both the systems pass through a critical point. An approach to the critical point is characterized in terms of divergence of zero shear viscosity and the subsequent appearance of the low frequency modulus. In the vicinity of the critical gel state, both the viscosity and the modulus show a power-law dependence on relative distance from the critical point. Interestingly, the longest relaxation time has been observed to diverge symmetrically on both the sides of the critical point and also shows a power-law dependence on relative distance from the critical point. The critical (power-law) exponents of the zero-shear viscosity and modulus are observed to be related to the exponents of the longest relaxation time by the hyper scaling laws. The dynamic critical exponent has also been calculated from the growth of the dynamic moduli. Remarkably, the critical relaxation exponent and dynamic critical exponent predicted from the scaling laws precisely agree with the experimental values from the isothermal as well as non-isothermal experiments. The associated critical exponents show remarkable internal consistency and universality for different kinds of systems undergoing the sol-gel transition.

cond-mat.mtrl-sci↗

arXiv2019

Three-Dimensional Yielding in Anisotropic Materials: Validation of Hill Criterion

Yielding transition in isotropic soft materials under superposition of orthogonal deformation fields is known to follow von Mises criterion. However, in anisotropic soft materials von Mises criterion fails owing to preferred directions associated with the system. In this work we study a model anisotropic yield stress system: electrorheological (ER) fluids that show structure formation in the direction of electric field. We subject the ER fluids to superposition of orthogonal stress fields that leads to different yield stress values. We obtain a yielding state diagram by plotting normalized rotational shear stress against normalized radial shear stress corresponding to yield point for a given electric field. Remarkably, the state diagram validates the Hill yielding criterion, which is a general yielding criterion for materials having anisotropy along three orthogonal directions, originally developed for metallic systems. Validation of Hill criterion suggests the universality of its application to anisotropic systems including conventional anisotropic soft materials having yield stress.

cond-mat.soft↗

arXiv2019

Signatures of Physical Aging and Thixotropy in Aqueous Dispersion of Carbopol

In this work, we investigate signatures of physical aging in an aqueous dispersion of Carbopol that shows yield stress and weak enhancement in elastic modulus as a function of time. We observe that the creep curves, as well as strain recovery, show a significant dependence on waiting time elapsed since shear melting. The corrected strain, which is the strain in excess of the recovered strain, has been observed to show time waiting time superposition in the effective time domain, wherein time is normalized by time-dependent relaxation time that shows a power-law dependence. The corresponding power law exponent, which is close to unity in a limit of small stresses, decreases with stress and tends to zero as stress approaches the yield stress. For a range of stresses, the material shows time stress superposition suggesting the shape of the evolving relaxation time spectrum to be independent of the time as well as the stress. This work, therefore, suggests the presence of physical aging in an aqueous dispersion of Carbopol even though the elastic modulus shows only a weak enhancement. We also discuss Andrade type of creep behavior in aqueous Carbopol dispersion.

cond-mat.soft↗

arXiv2019

Analyzing Onset of Nonlinearity of a Colloidal Gel at the Critical Point

In this work we study onset of nonlinear rheological behavior of a colloidal dispersion of a synthetic hectorite clay, Laponite, at the critical gel state while undergoing sol-gel transition. When subjected to step strain in the nonlinear regime, the relaxation modulus shifts vertically to the lower values such that the deviation from linearity can be accommodated using a strain dependent damping function. We also perform creep-recovery and start-up shear experiments on the studied colloidal dispersion at the critical gel state and monitor deviation in response as the flow becomes nonlinear. A quasi-linear integral model is developed with the time-strain separable relaxation modulus to account for the effect of nonlinear deformation. Remarkably, the proposed model predicts the deviation from linearity in the creep-recovery and start-up shear experiments very well leading to a simple formulation to analyze the onset of nonlinear rheological behavior in the critical gels. We also analyze the energy dissipation during the nonlinear deformation and validate the Bailey criterion using the developed viscoelastic framework.

cond-mat.soft↗

arXiv2018

In Situ Ion Induced Gelation of Colloidal Dispersion of Laponite: Relating Microscopic Interactions to Macroscopic Behavior

Aqueous dispersion of Laponite, when exposed to carbon dioxide environment leads to in situ inducement of magnesium and lithium ions, which is, however absent when dispersion is exposed to air. Consequently, in the rheological experiments, Laponite dispersion preserved under carbon dioxide shows more spectacular enhancement in the elastic and viscous moduli as a function of time compared to that exposed to air. By measuring concentration of all the ions present in a dispersion as well as change in pH, the evolving inter-particle interactions among the Laponite particles is estimated. DLVO analysis of a limiting case is performed, wherein two particles approach each other in a parallel fashion a situation with maximum repulsive interactions. Interestingly it is observed that DLVO analysis explains the qualitative details of an evolution of elastic and viscous moduli remarkably well thereby successfully relating the macroscopic phenomena to the microscopic interactions.

cond-mat.soft↗

arXiv2018

Microstructure and Soft Glassy Dynamics of Aqueous Laponite Dispersion

Synthetic hectorite clay Laponite RD/XLG is composed of disk-shaped nanoparticles that acquire dissimilar charges when suspended in an aqueous media. Owing to their property to spontaneously self-assemble, Laponite is used as a rheology modifier in a variety of commercial water-based products. Particularly, aqueous dispersion of Laponite undergoes liquid - to - solid transition at about 1 volume % concentration. The evolution of the physical properties as dispersion transforms to solid state is reminiscent of physical aging in molecular as well as colloidal glasses. The corresponding soft glassy dynamics of an aqueous Laponite dispersion, including the rheological behavior, has been extensively studied in the literature. In this feature article we take an overview of recent advances in understanding soft glassy dynamics and various efforts taken to understand the peculiar rheological behaviors. Furthermore, the continuously developing microstructure that is responsible for eventual formation of soft solid state that supports its own weight against gravity has also been a topic of intense debate and discussion. Particularly extensive experimental and theoretical studies lead to two types of microstructures for this system: an attractive gel-like or repulsive glass like. We carefully examine and critically analyze the literature and propose a state diagram that suggests aqueous Laponite dispersion to be present in an attractive gel state.

cond-mat.soft↗

arXiv2018

Transient start-up dynamics and shear banding in aging soft glassy materials: Rate-controlled flow field

We study the transient start-up dynamics of a fluidity model which captures the rheological behavior of aging soft glassy materials, in a rectilinear shear flow upon application of step shear rate. We observe that when the steady state flow curve is non-monotonic the system shows transient and/or apparent steady state shear banding in close qualitative agreement with experimental observations. Due to a competetion between aging and rejuvenation during the start up, we show that there is an apparent steady state banding at large times even for shear rates such that the steady state flow curve allows for a homogeneous flow. Thus, for aging soft glassy materials, the shear rate for achieving homogeneous flow is not necessarily given by the steady-state flow curve. We also observe that the transient and apparent steady state shear banding behavior is not correlated to negative slope of the stress-strain dependence during the transient. This work also emphasizes that in order to have a realistic description of shear banding behavior in the aging (time dependent) soft glassy materials, consideration of inertia during the start-up dynamics is crucial.

cond-mat.soft↗

arXiv2015

Kinetics of the glass transition of fragile soft colloidal suspensions

Microscopic relaxation timescales are estimated from the autocorrelation functions obtained by dynamic light scattering experiments for Laponite suspensions with different concentrations ($C_{L}$), added salt concentrations ($C_{S}$) and temperatures ($T$). It has been shown in an earlier work [Soft Matter, 10, 3292-3300 (2014)] that the evolutions of relaxation timescales of colloidal glasses can be compared with molecular glass formers by mapping the waiting time ($t_{w}$) of the former with the inverse of thermodynamic temperature ($1/T$) of the latter. In this work, the fragility parameter $D$, which signifies the deviation from Arrhenius behavior, is obtained from fits to the time evolutions of the structural relaxation timescales. For the Laponite suspensions studied in this work, $D$ is seen to be independent of $C_{L}$ and $C_{S}$, but is weakly dependent on $T$. Interestingly, the behavior of $D$ corroborates the behavior of fragility in molecular glass formers with respect to equivalent variables. Furthermore, the stretching exponent $β$, which quantifies the width $w$ of the spectrum of structural relaxation timescales is seen to depend on $t_{w}$. A hypothetical Kauzmann time $t_{k}$, analogous to the Kauzmann temperature for molecular glasses, is defined as the timescale at which $w$ diverges. Corresponding to the Vogel temperature defined for molecular glasses, a hypothetical Vogel time $t^{\infty}_α$ is also defined as the time at which the structural relaxation time diverges. Interestingly, a correlation is observed between $t_{k}$ and $t^{\infty}_α$, which is remarkably similar to that known for fragile molecular glass formers. A coupling model that accounts for the $t_{w}$-dependence of the stretching exponent is used to analyse and explain the observed correlation between $t_{k}$ and $t^{\infty}_α$.

cond-mat.soft↗

arXiv2015

A Model for Aging under Deformation Field, Residual Stresses and Strains in Soft Glassy Materials

A model is proposed that considers aging and rejuvenation in a soft glassy material as respectively a decrease and an increase in free energy. The aging term is weighted by inverse of characteristic relaxation time suggesting greater mobility of the constituents induce faster aging in a material. A dependence of relaxation time on free energy is proposed, which under quiescent conditions, leads to power law dependence of relaxation time on waiting time as observed experimentally. The model considers two cases namely, a constant modulus when aging is entropy controlled and a time dependent modulus. In the former and the latter cases the model has respectively two and three experimentally measurable parameters that are physically meaningful. Overall the model predicts how material undergoes aging and approaches rejuvenated state under application of deformation field. Particularly model proposes distinction between various kinds of rheological effects for different combinations of parameters. Interestingly, when relaxation time evolves stronger than linear, the model predicts various features observed in soft glassy materials such as thixotropic and constant yield stress, thixotropic shear banding, and presence of residual stress and strain.

cond-mat.soft↗

arXiv2014

Linear Viscoelasticity of Soft Glassy Materials

Owing to lack of time translational invariance, aging soft glassy materials do not obey fundamental principles of linear viscoelasticity. We show that by transforming the linear viscoelastic framework from the real time domain to the effective time domain, wherein the material clock is readjusted to account for evolution of relaxation time, the soft glassy materials obey effective time translational invariance. Consequently, we demonstrate successful validation of principles of linear viscoelasticity (Boltzmann superposition principle and convolution relation for creep compliance and stress relaxation modulus) for different types of soft glassy materials in the effective time domain.

cond-mat.soft↗

arXiv2013

Investigation of the dynamical slowing down process in soft glassy colloidal suspensions: comparisons with supercooled liquids

The primary and secondary relaxation timescales of aging colloidal suspensions of Laponite are estimated from intensity autocorrelation functions obtained in dynamic light scattering (DLS) experiments. The dynamical slowing down of these relaxation processes are compared with observations in fragile supercooled liquids by establishing a one-to-one mapping between the waiting time since filtration of a Laponite suspension and the inverse of the temperature of a supercooled liquid that is rapidly quenched towards its glass transition temperature. New timescales, such as the Vogel time and the Kauzmann time, are extracted to describe the phenomenon of dynamical arrest in Laponite suspensions. In results that are strongly reminiscent of those extracted from supercooled liquids approaching their glass transitions, it is demonstrated that the Vogel time calculated for each Laponite concentration is approximately equal to the Kauzmann time, and that a strong coupling exists between the primary and secondary relaxation processes of aging Laponite suspensions. Furthermore, the experimental data presented here clearly demonstrates the self-similar nature of the aging dynamics of Laponite suspensions within a range of sample concentrations.

cond-mat.soft↗

arXiv2013

Tailoring Relaxation Time Spectrum in Soft Glassy Materials

Physical properties of out of equilibrium soft materials depend on time as well as deformation history. In this work we propose to transform this major shortcoming into gain by applying controlled deformation field to tailor the rheological properties. We take advantage of the fact that deformation field of a certain magnitude can prevent particles in an aging soft glassy material from occupying energy wells up to a certain depth, thereby populating only the deeper wells. We employ two soft glassy materials with dissimilar microstructures and demonstrate that increase in strength of deformation field while aging leads to narrowing of spectrum of relaxation times. We believe that, in principle, this philosophy can be universally applied to different kinds of glassy materials by changing nature and strength of impetus.

cond-mat.soft↗

arXiv2012

Shear Mediated Elongational Flow and Yielding in Soft Glassy Materials

In this work, we study the deformation behavior of thin films of various soft glassy materials that are simultaneously subjected to two creep flow fields, rotational shear flow by applying torque and elongational flow by applying normal force. The generic behavior under the combined fields is investigated in different soft glassy materials with diverse microstructure such as: hair gel, emulsion paint, shaving foam and clay suspension. Increase in strength of one stress component while keeping the other constant, not only leads to an expected enhanced deformation in its own direction, but also greater strain in the other direction. Herschel Bulkley model is observed to explain this behavior qualitatively. Elongational flow induced in the materials eventually causes failure in the same. Interestingly time to failure is observed to be strongly dependent not just on normal force but also on the applied rotational shear stress. We believe that the presence of a three dimensional jammed structure, in which overall unjamming can be induced by applying stress having sufficient magnitude irrespective of the direction leads to the observed behavior. In addition, we observe self-similarity in the elongational as well as rotational strain time curves corresponding to various combinations of both the fields. This observation suggests a mere shift in the time-scales involved keeping the path followed in the process unchanged. A phase diagram is also constructed for various soft glassy materials by determining different combinations of orthogonal stresses beyond which materials yield. Estimated yield stress in the limit of flow dominated by applied tensile force on the top plate demonstrates scatter, which might be originating from fingering instability. Except this deviation, yielding is observed when the invariant of stress tensor exceeds yield stress, validating the Von Mises criterion.

cond-mat.soft↗

arXiv2012

Delayed solidification of soft glasses: New experiments, and a theoretical challenge

When subjected to large amplitude oscillatory shear stress, aqueous Laponite suspensions show an abrupt solidification transition after a long delay time tc. We measure the dependence of tc on stress amplitude, frequency, and on the age-dependent initial loss modulus. At first sight our observations appear quantitatively consistent with a simple soft-glassy rheology (SGR)-type model, in which barrier crossings by mesoscopic elements are purely strain-induced. For a given strain amplitude γ0 each element can be classified as fluid or solid according to whether its local yield strain exceeds γ0. Each cycle, the barrier heights E of yielded elements are reassigned according to a fixed prior distribution ρ(E): this fixes the per-cycle probability R(γ0) of a fluid elements becoming solid. As the fraction of solid elements builds up, γ0 falls (at constant stress amplitude), so R(γ0) increases. This positive feedback accounts for the sudden solidification after a long delay. The model thus appears to directly link macroscopic rheology with mesoscopic barrier height statistics: within its precepts, our data point towards a power law for ρ(E) rather than the exponential form usually assumed in SGR. However, despite this apparent success, closer investigation shows that the assumptions of the model cannot be reconciled with the extremely large strain amplitudes arising in our experiments. The quantitative explanation of delayed solidification in Laponite therefore remains an open theoretical challenge.

cond-mat.soft↗

arXiv2012

Self-Similarity in Electrorheological Behavior

In this work we study creep flow behavior of suspension of Polyaniline (PANI) particles in silicone oil under application of electric field. Suspension of PANI in silicone oil, a model electrorheological fluid, shows enhancement in elastic modulus and yield stress with increase in the magnitude of electric field. Under creep flow field, application of greater magnitude of electric field reduces strain induced in the material while application of greater magnitude of shear stress at any electric field enhances strain induced in the material. Remarkably, time evolution of strain in PANI suspension at different stresses, electric field strengths and concentrations show superposition after appropriate shifting of the creep curves on time and strain axes. Observed electric field - shear stress - creep time - concentration superposition demonstrates self-similarity in electrorheological behavior. We analyze the experimental data using Bingham and Klingenberg - Zukoski model and observe that the latter predicts the experimental behavior very well. We conclude by discussing remarkable similarities between observed rheological behavior of ER fluids and rheological behavior of aging soft glassy materials.

cond-mat.soft↗

arXiv2012

Time Temperature Superposition in Soft Glassy Materials

Soft glassy materials are out of thermodynamic equilibrium and show time dependent slowing down of the relaxation dynamics. Under such situation these materials follow Boltzmann superposition principle only in the effective time domain, wherein time dependent relaxation processes are scaled by a constant relaxation time. In this work we extend effective time framework to successfully demonstrate time - temperature superposition of creep and stress relaxation data of a model soft glassy system comprised of clay suspension. Such superposition is possible when average relaxation time of the material changes with time and temperature without affecting shape of the spectrum. We show that variation in relaxation time as a function of temperature facilitates prediction of long and short time rheological behavior through time - temperature superposition from the experiments carried out over experimentally accessible timescales.

cond-mat.soft↗

arXiv2010

Time-aging time-stress superposition in soft glass under tensile deformation field

We have studied the tensile deformation behaviour of thin films of aging aqueous suspension of Laponite, a model soft glassy material, when subjected to a creep flow field generated by a constant engineering normal stress. Aqueous suspension of Laponite demonstrates aging behaviour wherein it undergoes time dependent enhancement of its elastic modulus as well as its characteristic relaxation time. However, under application of the normal stress, the rate of aging decreases and in the limit of high stress, the aging stops with the suspension now undergoing a plastic deformation. Overall, it is observed that the aging that occurs over short creep times at small normal stresses is same as the aging that occurs over long creep times at large normal stresses. This observation allows us to suggest an aging time - process time - normal stress superposition principle, which can predict rheological behaviour at longer times by carrying out short time tests.

cond-mat.soft↗