Probing thermal waves on the free surface of various media: Surface Fluctuation Specular Reflection Spectroscopy

TL;DR

This paper introduces Surface Fluctuation Specular Reflection (SFSR) spectroscopy, an optical method to measure thermally excited surface waves, providing insights into material properties with high temporal resolution across various media.

Contribution

The paper presents a novel optical technique, SFSR spectroscopy, for probing thermally excited surface waves on diverse media, demonstrating its effectiveness and broad applicability.

Findings

Good agreement between experimental and theoretical spectra for liquids and solids

Applicable to a wide range of materials including complex fluids

Offers high temporal resolution for dynamical surface measurements

Abstract

Thermal motion gives rise to fluctuations in free surfaces; the propagation of the thermally excited waves on such surfaces depends on the mechanical properties of the medium. Their measurement can therefore provide information on those properties. We have developed an optical tool to probe the thermally excited waves on free surfaces: Surface Fluctuation Specular Reflection (SFSR) spectroscopy. It consists in measuring the fluctuations in the position of a laser beam, which is specularly reflected onto the free surface of a medium, and is therefore sensitive to the roughness of that surface. We show how the measured signal is related to the medium properties. We also present measurements performed on Newtonian liquids as well as on a viscoelastic solid; we show that, in all cases, there is a very good agreement between experimental and computed spectra. SFSR thus applies to a broad range of materials. It moreover offers a very good temporal resolution and should provide a useful tool for dynamical measurements on complex fluids.