Extracting the dynamic correlation length of actin networks from microrheology experiments

TL;DR

This paper presents a method to extract the dynamic correlation length of actin networks from microrheology experiments, revealing its dependence on filament length and challenging previous assumptions about its relation to mesh size.

Contribution

It introduces a detailed analysis scheme to determine the dynamic correlation length from microrheology data and explores its relation to filament length and network structure.

Findings

Dynamic correlation length can be extracted from microrheology measurements.

Reducing filament length below the crossover distance increases the dynamic correlation length.

The dynamic correlation length is not necessarily equal to the mesh size of the network.

Abstract

The mechanical properties of polymer gels based on cytoskeleton proteins (e.g. actin) have been studied extensively due to their significant role in biological cell motility and in maintaining the cell's structural integrity. Microrheology is the natural method of choice for such studies due to its economy in sample volume, its wide frequency range, and its spatial sensitivity. In microrheology, the thermal motion of tracer particles embedded in a complex fluid is used to extract the fluid's viscoelastic properties. Comparing the motion of a single particle to the correlated motion of particle pairs, it is possible to extract viscoelastic properties at different length scales. In a recent study, a crossover between intermediate and bulk response of complex fluids was discovered in microrheology measurements of reconstituted actin networks. This crossover length was related to structural and mechanical properties of the networks, such as their mesh size and dynamic correlation length. Here we capitalize on this result giving a detailed description of our analysis scheme, and demonstrating how this relation can be used to extract the dynamic correlation length of a polymer network. We further study the relation between the dynamic correlation length and the structure of the network, by introducing a new length scale, the average filament length, without altering the network's mesh size. Contrary to the prevailing assumption, that the dynamic correlation length is equivalent to the mesh size of the network, we find that the dynamic correlation length increases once the filament length is reduced below the crossover distance.