Probing local nonlinear viscoelastic properties in soft materials

TL;DR

This paper introduces an enhanced cavity expansion technique to measure local viscoelastic properties of soft materials, revealing rate-dependent behavior and addressing limitations of previous elastic-only methods.

Contribution

The authors develop a cavity expansion method that captures viscoelastic response at low to medium rates, improving local mechanical characterization of soft materials.

Findings

Significant rate-dependent viscoelastic response in PDMS rubber.

Conventional methods may overestimate elastic modulus due to high stretch rates.

The new protocol accurately distinguishes quasistatic and dynamic properties.

Abstract

Minimally invasive experimental methods that can measure local rate dependent mechanical properties are essential in understanding the behaviour of soft and biological materials in a wide range of applications. Needle based measurement techniques such as Cavitation Rheology and Volume Controlled Cavity Expansion (VCCE), allow for minimally invasive local mechanical testing, but have been limited to measuring the elastic material properties. Here, we propose several enhancements to the VCCE technique to adapt it for characterization of viscoelastic response at low to medium stretch rates ($10^{-2}$ - $1$ s${}^{-1}$). The proposed technique performs several cycles of expansion-relaxation at controlled stretch rates in a cavity expansion setting and then employs a large deformation viscoelastic model to capture the measured material response. Application of the technique to soft PDMS rubber reveals significant rate dependent material response with high precision and repeatability, while isolating equilibrated states that are used to directly infer the quasistatic elastic modulus. The technique is further established by demonstrating its ability to capture changes in the rate dependent material response of a tuneable PDMS system. The measured viscoelastic properties are used to explain earlier reports of rate insensitive material response by needle based methods: it is demonstrated that the conventional use of constant volumetric rate cavity expansion can induce high stretch rates that lead to viscoelastic stiffening and an illusion of rate insensitive material response. We thus conclude with a cautionary note on possible overestimation of the quasistatic elastic modulus in previous studies and suggest that the stretch rate controlled expansion protocol, proposed in this work, is essential for accurate estimation of both quasistatic and dynamic material parameters.