Optical study of shear and longitudinal acoustic waves and complex relaxation dynamics of glass forming liquids

Abstract

The spectroscopic technique Impulsive Stimulated Scattering (ISS) was refined and used to study the complex structural relaxation dynamics of glass forming liquids, allowing both empirical modeling and testing of the predictions of the mode-coupling theory (MCT). Longitudinal and shear acoustic waves throughout much of the MHz frequency range, time-dependent thermal expansion on nanosecond and microsecond scales, and slower thermal diffusion were all monitored in real time. The data were used to construct complex longitudinal modulus spectra spanning from, 30 kHZ to 3 GHz, and complex shear modulus spectra from - 10 MHz to 1 GHz. In the liquid tetramethyl tetraphenyl trisiloxane, experiments which verified timetemperature superposition of its relaxation dynamics permitted construction of a master plot of scaled relaxation spectra in the entire temperature range studied. MCT predictions of power-law frequency dependencies of the high and low frequency wings of the loss modulus yielded a high-frequency exponent parameter in good agreement with the width of the non-exponential relaxation kinetics. The low-frequency exponent did not agree with the predicted value. In triphenyl phosphite, measurements of the measured shear relaxation spectrum over two decades in frequency revealed that it does not match the previously measured longitudinal spectrum, suggesting that different underlying degrees of freedom contribute to shear and compressional relaxation. Measurement of shear wave propagation as a function of temperature lent credence to the dominance of the temperature dependence of the transport by the instantaneous shear modulus. These measurements also call into question other relationships drawn between glass mechanical behavior and the supercooled liquid fragility. In work conducted collaboratively, the ISS technique was employed in singles hot measurements of liquid benzene under conditions of shock loading. The results indicate that benzene remains in a liquid state for at least 200 ns after the shock's arrival. ISS was also used to characterize both the thermal transport and mechanical properties of nanofluids.

(cont.) Finally, results of ISS acoustic measurements of thin films and their relationship with the study of glass forming liquids are briefly discussed.