Short-time inertial response of viscoelastic fluids measured with Brownian motion and with active probes

TL;DR

This study investigates short-time stress propagation in viscoelastic fluids using optical trapping and particle tracking, revealing vortex-like flow patterns that differ between simple liquids and viscoelastic solutions.

Contribution

It provides direct experimental observation of stress propagation dynamics in viscoelastic fluids at short times using innovative optical and measurement techniques.

Findings

Vortex-like flow patterns are detected in stress propagation.

Stress propagation in simple liquids is diffusive.

In viscoelastic solutions, stress spreads super-diffusively, influenced by shear modulus.

Abstract

We have directly observed short-time stress propagation in viscoelastic fluids using two optically trapped particles and a fast interferometric particle-tracking technique. We have done this both by recording correlations in the thermal motion of the particles and by measuring the response of one particle to the actively oscillated second particle. Both methods detect the vortex-like flow patterns associated with stress propagation in fluids. This inertial vortex flow propagates diffusively for simple liquids, while for viscoelastic solutions the pattern spreads super-diffusively, dependent on the shear modulus of the medium.