Optomechanical micro-rheology of complex fluids at ultra-high frequency

TL;DR

This paper introduces a novel optomechanical micro-rheology technique for measuring the rheological properties of complex fluids at ultra-high frequencies (100 MHz - 1 GHz), revealing molecular relaxation processes and elastic responses.

Contribution

It develops an innovative, high-frequency optomechanical method with an analytical model to probe molecular dynamics in liquids in real-time.

Findings

Water remains Newtonian up to 1 GHz with pronounced compressibility effects.

1-decanol exhibits non-Newtonian, frequency-dependent viscosity with two relaxation times.

Elastic response at high frequencies allows estimation of single-molecule volume.

Abstract

We present an optomechanical method for locally measuring the rheological properties of complex fluids in the ultra-high frequency range (UHF). A mechanical disk of microscale volume is used as a small-amplitude oscillating probe that monitors the fluid at rest in thermal equilibrium, while the oscillation is detected by optomechanical transduction within a sub-millisecond measurement time, thanks to an optimized signal collection. An original analytical model for fluid-structure interactions is used to extract from these measurements the rheological properties of liquids over the frequency range 100 MHz - 1 GHz. This new micro-rheology method is calibrated by measurements on liquid water, in which we observe pronounced compressibility effects above 500 MHz, but which we show remains Newtonian all over the explored range. In contrast, measurements reveal that liquid 1-decanol exhibits a non-Newtonian behavior, with a frequency-dependent viscosity associated to several relaxation frequencies in the UHF. Our data agree well with an extended Maxwell model for the viscosity of this alcohol, involving two relaxation times of 797 and 151 picoseconds, which are respectively analyzed as supramolecular and intramolecular relaxation processes. A shear elastic response of the liquid appears at the highest frequencies, compatible with the entropic elasticity of molecules as they attempt to uncurl, and whose value enables estimating the volume of a single molecule of liquid. UHF optomechanical micro-rheology demonstrates here a direct mechanical access to the fast molecular dynamics at play in a liquid, in a quantitative manner and in a short time.