Centrifugal compression of soft particle packings - theory and experiment

TL;DR

This paper develops an exact theoretical method for calculating the compression height of elastic media under centrifugal force, validated by experiments on microgel beads, revealing their incompressibility and elastic properties.

Contribution

It introduces a general exact approach for centrifugal compression of soft particles with arbitrary stress-strain relations, including diverging stress cases, and applies it to experimental data on microgel beads.

Findings

Microgel beads are incompressible and deform without deswelling.

Young's modulus scales with cross-link density raised to approximately the 3.3 power.

Experimental results align with Hertzian elasticity at low accelerations.

Abstract

An exact method is developed for computing the height of an elastic medium subjected to centrifugal compression, for arbitrary constitutive relation between stress and strain. Example solutions are obtained for power-law media and for cases where the stress diverges at a critical strain -- for example as required by packings composed of deformable but incompressible particles. Experimental data are presented for the centrifugal compression of thermo-responsive N-isopropylacrylamide (NIPA) microgel beads in water. For small radial acceleration, the results are consistent with Hertzian elasticity, and are analyzed in terms of the Young elastic modulus of the bead material. For large radial acceleration, the sample compression asymptotes to a value corresponding to a space-filling particle volume fraction of unity. Therefore we conclude that the gel beads are incompressible, and deform without deswelling. In addition, we find that the Young elastic modulus of the particulate gel material scales with cross-link density raised to the power 3.3+-0.8, somewhat larger than the Flory expectation.