arXiv subjects

Yogesh M Joshi

Publications and source records attributed to Yogesh M Joshi.

25 records · Page 2Linked to original sources

arXiv2012

Interface Induced Anisotropy and nematic glass/gel state in Jammed Aqueous Laponite Suspensions

Aqueous suspensions of Laponite, a system composed of disk-like nanoparticles, are found to develop optical birefringence over several days, well after the suspensions solidified due to jamming. The optical anisotropy is particularly enhanced near the air Laponite suspension interface over length-scales of several millimetres, which is beyond five orders of magnitude larger than the particle length scale, suggestive of large scale ordering influenced by the interface. The orientational order increases with time and is always greater for higher concentration of salt, higher concentration of Laponite and higher temperature of the suspension. While weakly birefringent, Laponite suspensions covered by paraffin oil do not show any enhancement in the optical anisotropy near the interface compared to that in the bulk. We suggest that the expedited structure formation near the air interface propagating progressively inside the sample is responsible for the observed behaviour. We discuss the observed nematic ordering in the context of glasslike and gellike microstructure associated with aqueous Laponite suspensions.

cond-mat.soft↗

arXiv2012

Hyper-aging Dynamics of Nano-clay Suspension

Aqueous suspension of nanoclay Laponite undergoes structural evolution as a function of time, which enhances its elasticity and relaxation time. In this work we employ effective time approach to investigate long term relaxation dynamics by carrying out creep experiments. Typically we observe that the monotonic evolution of elastic modulus shifts to lower aging times while maxima in viscous modulus gets progressively broader for experiments carried out on a later date since preparation (idle time) of nanoclay suspension. Application of effective time theory produces superposition of all the creep curves irrespective of their initial state. The resulting dependence of relaxation time on aging time shows very strong hyper aging dynamics at small idle times, which progressively weakens to demonstrate linear dependence in the limit of very large idle times. Remarkably this behavior of nanoclay suspension is akin to that observed for polymeric glasses. Consideration of aging as a first order process suggests that continued hyper-aging dynamics causes cessation of aging. The dependence of relaxation time on aging time, therefore, must attenuate eventually producing linear or weaker dependence on time in order to approach progressively low energy state in the limit of very large times as observed experimentally. We also develop a simple scaling model based on a concept of aging of an energy well, which qualitatively captures various experimental observations very well leading to profound insight into the hyper-aging dynamics of nano-clay suspensions.

cond-mat.soft↗

arXiv2012

Delayed Yielding in Creep, Time - Stress Superposition and Effective Time Theory for a Soft Glass

In this work we investigate creep flow of aqueous suspension of Laponite, a model soft glassy material, at different aging times and stresses. We observe that this system shows time - aging time - stress superposition over a range of aging times and stresses when real time scale is transformed into effective time scale. Existence of superposition in an effective time domain facilitates prediction of long and very short time rheological behavior. Analysis of the observed behavior from effective time approach suggests that superposition is possible only when the shape of a relaxation time spectrum is preserved at various aging times and stresses. We also observe that creep curves at low aging times and greater stresses demonstrate delayed but sudden yielding. The critical time, at which material yields, increases with increase in aging time and decrease in stress. We argue that local rejuvenation of part of the glassy material causes variation in the rate of evolution of relaxation modes. The resulting interplay between aging and rejuvenating modes lead to delayed yielding as observed experimentally.

cond-mat.soft↗

arXiv2011

Prediction of long and short time rheological behavior in soft glassy materials

We present an effective time approach to predict long and short time rheological behavior of soft glassy materials from experiments carried out over practical time scales. Effective time approach takes advantage of relaxation time dependence on aging time that allows time-aging time superposition even when aging occurs over the experimental timescales. Interestingly experiments on variety of soft materials demonstrate that the effective time approach successfully predicts superposition for diverse aging regimes ranging from sub-aging to hyper-aging behaviors. This approach can also be used to predict behavior of any response function in molecular as well as spin glasses.

cond-mat.soft↗

arXiv2010

Effect of temperature on aging and time-temperature superposition in nonergodic Laponite suspensions

We have studied the effect of temperature on aging dynamics of laponite suspensions by carrying out the rheological oscillatory and creep experiments. We observed that at higher temperatures the mechanism responsible for aging became faster thereby shifting the evolution of elastic modulus to lower ages. Significantly, in the creep experiments, all the aging time and the temperature dependent strain data superposed to form a master curve. Possibility of such superposition suggests that the rheological behavior depends on the temperature and the aging time only through the relaxation processes and both the variables do not affect the distribution but only the average value of relaxation times. In addition, this procedure allows us to predict long time rheological behavior by carrying out short time tests at high temperatures and small ages.

cond-mat.soft↗

arXiv2008

Ageing under stress and mechanical fragility of soft solids of laponite

In this work, we investigate the ageing behavior of soft glassy solids of aqueous suspension of laponite under shear flow. We observe that when an imposed time is normalized by a dominating relaxation time of the system, the rheological response at different ages shows superposition. Analysis of this behavior suggests that the structural evolution with age under a deformation field, as represented by the dependence of dominant relaxation mode on age, becomes weaker as the system becomes progressively less homogeneous due to enhanced attractive interactions caused by addition of salt. Creep-recovery behavior at same elastic modulus shows more viscous dissipation for a system having more salt, demonstrating increase in the mechanical fragility. This study shows that an increase in the concentration of salt, which enhances attractive interactions and causes greater inhomogeneity, leads to a state wherein the particles are held together by weaker interactions. This work leads to important insights into how microstructure affects the ageing dynamics. We discuss the observed behavior in the context of ageing in colloidal glasses and gels of aqueous suspension of laponite.

cond-mat.soft↗

arXiv2008

Rheological Behavior of Aqueous Suspensions of Laponite: New Insights into the Ageing Phenomena

In this paper, ageing behavior of suspensions of laponite with varying salt concentration is investigated using rheological tools. It is observed that the ageing is accompanied by an increase in the complex viscosity. The succeeding creep experiments performed at various ages showed damped oscillations in the strain. The characteristic time-scale of the damped oscillations, retardation time, showed a prominent decrease with the age of the system. However, this dependence weakens with an increase in the salt concentration, which is known to change microstructure of the system from glass-like to gel-like. We postulate that a decrease in the retardation time can be represented as a decrease in the viscosity (friction) of the dissipative environment surrounding the arrested entities that opposes elastic deformation of the system. We believe that ageing in colloidal glass leads to a greater ordering that enhances relative spacing between the constituents thereby reducing the frictional resistance. However, since a gel state is inherently different in structure (fractal network) than that of a glass (disordered), ageing in the same does not induce ordering. Consequently, we observe inverse dependence of retardation time on age becoming weaker with an increase in the salt concentration. We analyze these results from a perspective of ageing dynamics of both glass state and gel state of laponite suspensions.

cond-mat.soft↗