arXiv subjects

Pramod A Pullarkat

Publications and source records attributed to Pramod A Pullarkat.

3 recordsLinked to original sources

arXiv2020

Investigation of Soft and Living Matter using a Micro-Extensional Rheometer

Rheological properties of a material often require to be probed under extensional deformation. Examples include fibrous materials such as spider-silk, high-molecular weight polymer melts, and the contractile response of living cells. Such materials have strong molecular-level anisotropies which are either inherent or are induced by an imposed extension. However, unlike shear rheology, which is well-established, techniques to perform extensional rheology are currently under development and setups are often custom-designed for the problem under study. In this article, we present a versatile device that can be used to conduct extensional deformation studies of samples at microscopic scales with simultaneous imaging. We discuss the operational features of this device and present a number of applications.

cond-mat.soft↗

arXiv2019

Strain softening and stiffening responses of spider silk fibers probed using Micro-Extension Rheometer

Spider silk possesses unique mechanical properties like large extensibility, high tensile strength, super-contractility, etc. Understanding these mechanical responses require characterization of the rheological properties of silk beyond the simple force-extension relations which are widely reported. Here we study the linear and non-linear viscoelastic properties of dragline silk obtained from social spiders Stegodyphus sarasinorum using a Micro-Extension Rheometer that we have developed. Unlike continuous extension data, our technique allows for the probing of the viscoelastic response by applying small perturbations about sequentially increasing steady-state strain values. In addition, we extend our analysis to obtain the characteristic stress relaxation times and the frequency responses of the viscous and elastic moduli. Using these methods, we show that in a small strain regime (0-4%) dragline silk of social spiders shows strain-softening response followed by strain-stiffening response at higher strains (> 4%). The stress relaxation time, on the other hand, increases monotonically with increasing strain for the entire range. We also show that silk stiffens while ageing within the typical lifetime of a web. Our results demand the inclusion of the kinetics of domain unfolding and refolding in the existing models to account for the relaxation time behaviour.

cond-mat.soft↗

arXiv2006

Loss of Cell-Substrate Adhesion Leads to Periodic Shape Oscillations in Fibroblasts

We report the phenomenon of periodic shape oscillations occurring in 3T3 fibroblasts merely as a consequence of a loss of cell-substrate adhesion. The oscillatory behavior can last many hours at a constant frequency, and can be switched off and on using chemical agents. This robustness allows for the extraction of quantitative data using single cells. We demonstrate that the frequency of oscillations increases with increasing actomyosin contractility. Both the Myosin Light Chain Kinase as well as the Rho Kinase pathways are shown to operate during this process. Further, we reveal significant similarities between the oscillatory dynamics and the commonly observed phenomenon of blebbing. We show that both these processes ceases when extracellular calcium is depleted or when stretch activated calcium channels are blocked. This, along with the fact that these dynamical processes require actomyosin contactility points towards strong similarities in the respective mechanisms. Finally, we speculate on a possible mechanism for the observed dynamical phenomena.

physics.bio-ph↗