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

## TL;DR

This study characterizes the viscoelastic properties of spider silk using a novel Micro-Extension Rheometer, revealing strain-dependent softening and stiffening behaviors, and insights into relaxation times and aging effects.

## Contribution

It introduces a new rheological technique to analyze non-linear viscoelastic responses of spider silk, providing detailed insights into strain effects and aging.

## Key findings

- Spider silk exhibits strain-softening at low strains and strain-stiffening at higher strains.
- Stress relaxation time increases monotonically with strain.
- Silk stiffens with web aging, indicating changes over the web's lifetime.

## Abstract

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.