Microscopic Viscoelasticity: Shear Moduli of Soft Materials Determined from Thermal Fluctuations

TL;DR

This paper introduces a high-resolution microrheology technique using thermal fluctuations of embedded particles to measure local viscoelastic properties of soft materials across a broad frequency range.

Contribution

It presents a passive interferometric method for determining shear moduli from thermal fluctuations, extending accessible frequency ranges for microrheology.

Findings

F-actin solutions exhibit omega^(3/4) shear modulus scaling.

PAAm gels show omega^(1/2) shear modulus scaling.

Method enables detailed viscoelastic characterization of soft materials.

Abstract

We describe a high-resolution, high-bandwidth technique for determining the local viscoelasticity of soft materials such as polymer gels. Loss and storage shear moduli are determined from the power spectra of thermal fluctuations of embedded micron-sized probe particles, observed with an interferometric microscope. This provides a passive, small-amplitude measurement of rheological properties over a much broader frequency range than previously accessible to microrheology. We study both F-actin biopolymer solutions and polyacrylamide (PAAm) gels, as model semiflexible and flexible systems, respectively. We observe high-frequency omega^(3/4) scaling of the shear modulus in F-actin solutions, in contrast to omega^(1/2) scaling for PAAm.