Local elastic perturbation of colloidal suspensions near the colloidal glass transition

TL;DR

This study uses magnetic probes and confocal microscopy to locally measure the viscoelastic properties of dense colloidal suspensions near the glass transition, confirming continuum models at microscopic scales.

Contribution

It introduces a method for local elastic perturbation and measurement in colloids, linking microscopic responses to macroscopic rheology near the glass transition.

Findings

Probe particles exhibit a 1/r decay in oscillation amplitude, indicating a homogeneous viscoelastic response.

Measured storage and loss moduli align qualitatively with traditional rheology results.

Continuum descriptions remain valid at length scales comparable to particle size.

Abstract

Isolated microscopic magnetic particles are used to induce local perturbations in dense colloidal suspensions by rotating an external magnet. Confocal microscopy enables tracking of both the magnetic probe particle and adjacent colloidal particles. A probe particle moves with a circular trajectory. Knowing the external force and measuring the amplitude and phase of the probe motion allows us to infer the storage and loss moduli of colloidal suspensions at various volume fractions. These measurements are in qualitative agreement with previous results from conventional rheology. To further analyze the system's response, the oscillatory amplitude of colloidal particles is evaluated as a function of distance from the probe, revealing a 1/r decay in amplitude, consistent with a homogeneous viscoelastic material. These observations confirm that continuum descriptions of the colloidal samples are effective down to length scales comparable to the particle diameter.