# Irreversible hardening of a colloidal gel under shear: the smart response of natural rubber latex gels

## TL;DR

This study reveals that natural rubber latex gels undergo irreversible strain hardening under shear, with local structural rearrangements contributing to increased elastic modulus, demonstrating a unique smart response of these colloidal gels.

## Contribution

It uncovers the phenomenon of irreversible strain hardening in natural rubber latex gels and links it to homogeneous local structural rearrangements.

## Key findings

- Gels exhibit irreversible increase in elastic modulus after large strains.
- Hardening occurs over a wide deformation range, maintaining elastic properties.
- Ultrasound imaging shows homogeneous local rearrangements during hardening.

## Abstract

Natural rubber is obtained by processing natural rubber latex, a liquid colloidal suspension that rapidly gels after exudation from the tree. We prepared such gels by acidification, in a large range of particle volume fractions, and investigated their rheological properties. We show that natural rubber latex gels exhibit a unique behavior of irreversible strain hardening: when subjected to a large enough strain, the elastic modulus increases irreversibly. Hardening proceeds over a large range of deformations in such a way that the material maintains an elastic modulus close to, or slightly higher than the imposed shear stress. Local displacements inside the gel are investigated by ultrasound imaging coupled to oscillatory rheometry, together with a Fourier decomposition of the oscillatory response of the material during hardening. Our observations suggest that hardening is associated with irreversible local rearrangements of the fractal structure, which occur homogeneously throughout the sample.