Probing thermal waves on the free surface of various media: Surface fluctuation specular reflection spectroscopy

Abstract

Thermal motion gives rise to fluctuations in free surfaces; measurement of the thermally excited waves on such surfaces provides information on the mechanical properties of the medium. 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 that is specularly reflected onto the free surface of a medium. The position of the reflected beam is sensitive to the roughness of the probed surface; the thermal waves are detected by subtracting the light intensities collected on the two quadrants of a photodiode, on which the beam is centered. 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.

Surface fluctuations of liquids confined on flat and patterned solid substrates

“…Surface fluctuations, and its fluctuating position can be related to the fluctuating local slope of the liquid surface [16]. Such a measurement mixes the contributions from several surface modes, unlike a light scattering experiment in which a single spatial mode is selected.…”

“…Since, in each direction, we deal with signals that are normalized by the total intensity of the considered two quadrants, those signals can be compared even though the total intensity may slightly differ. We have shown that, taking into account the contributions from all the spatial modes of the surface, the PSD S(ω) can be approximated by [16] …”

“…The oil used with the patterned substrate is a specially designed oil (Cargille) whose refractive index matches that of the epoxy resin (n = 1.52). Its surface tension has been measured as γ = 38 mN/m from surface fluctuation measurements of a large depth sample [16].…”

“…Here we measure surface fluctuations of low-viscosity liquids with a technique based on the detection of local fluctuations in the surface slope of liquid surfaces [16]. Variations as small as 10 −6 rad can be measured, which allows for the measurement of fluctuation modes with wavelength in the range 10-100 μm.…”

Spectral properties of thermal fluctuations on simple liquid surfaces below shot-noise levels

“…In this work, we detect reflected light from surfaces to measure their inclinations [5][6][7] and apply this to thermal surface fluctuations of simple liquids. The measurements are somewhat complementary to the spectral measurements performed at specific wavelengths.…”