High-Frequency Shear Viscosity of Low-Viscosity Liquids

Abstract

A thickness shear quartz resonator technique is described to measure the shear viscosity of low-viscosity liquids in the frequency range from 6 MHz to 130 MHz. Examples of shear-viscosity spectra in that frequency range are presented to show that various molecular processes are accompanied by shear-viscosity relaxation. Among these processes are conformational variations of alkyl chains, with relaxation times τ η of about 0.3 ns for n-pentadecane and n-hexadecane at 25 • C. These variations can be well represented in terms of a torsional oscillator model. Also featured briefly are shear-viscosity relaxations associated with fluctuations of hydrogen-bonded clusters in alcohols, for which τ η values between 0.3 ns (n-hexanol) and 1.5 ns (n-dodecanol) have been found at 25 • C. In addition, the special suitability of high-frequency shearviscosity spectroscopy to the study of critically demixing mixtures is demonstrated by some illustrative examples. Due to slowing, critical fluctuations do not contribute to the shear viscosity at sufficiently high frequencies of measurements so that the non-critical background viscosity η bg of critical systems can be directly determined from highfrequency shear-viscosity spectroscopy. Relaxations in η bg appear also in the shearviscosity spectra with, for example, τ η ≈ 2 ns for the critical triethylamine-water binary mixture at temperatures between 10 • C and 18 • C. Such relaxations noticeably influence the relaxation rate of order parameter fluctuations. They may be also the U. Kaatze (B) 2089 reason for the need of a special mesoscopic viscosity when mutual diffusion coefficients of critical polymer solutions are discussed in terms of mode-coupling theory.

Structural Relaxation and Viscoelasticity of a Higher Alcohol with Mesoscopic Structure

“…As seen in Figure , the normalized spectra appear to be bimodal, with the reduced relaxation frequencies of 2πη 0 ν ≈ 20 and 400 MPa. The bimodal viscoelastic relaxation of higher alcohols was suggested by the combination of the transverse and longitudinal ultrasonic spectroscopies, which was later confirmed by MD simulation . The presence of the bimodal relaxation has also been confirmed in a deeply supercooled region using conventional rheometers. − …”

Collective Mesoscale Dynamics of Liquid 1-Dodecanol Studied by Neutron Spin-Echo Spectroscopy with Isotopic Substitution and Molecular Dynamics Simulation

“…Viscoelastic spectra of liquid 1-alcohols are known to exhibit bimodal relaxation. − Our recent molecular dynamics (MD) simulation study demonstrated, from the comparison between the viscoelastic spectra and the intermediate scattering function (ISF), that the slower relaxation mode can be assigned to the coupling with the mesoscopic dynamics, whereas the faster mode can be ascribed to microscopic structural relaxation . The picture obtained by the MD simulation was later confirmed experimentally on 3,7-dimethyl-1-octanol using γ-ray quasi­elastic scattering spectroscopy .…”

Coupling between Structural and Dielectric Relaxations of Methanol and Ethanol Studied by Molecular Dynamics Simulation

“…Viscoelastic relaxation is another target for the comparison with the dielectric relaxation spectrum. The frequency-dependent shear viscosity of n -alcohols can be measured at the ambient condition by means of transverse ultrasonic technique. , Because many monoalcohols are easily supercooled, a conventional mechanical rheometer can also be applied to the deeply supercooled state. Jakobsen and co-workers measured the dielectric and viscoelastic spectra of supercooled 2-ethyl-1-hexanol and 2-butanol at various temperatures near the glass transition and demonstrated that the time scale of the viscoelastic relaxation is close to that of the high-frequency dielectric process …”