Dispersion and attenuation on the Brillouin sound waves of a lubricant: Di(2-ethylhexyl) sebacate under high pressures

Abstract

The Brillouin spectra of di(2-ethylhexyl) sebacate, which is a liquid lubricant known as DOS, were measured at up to 5 GPa at 25 °C and up to 2.5 GPa at 80 °C. At 25 °C, the Brillouin frequency linewidth (acoustic attenuation) has a large maximum at 0.1 MPa, and at 80 °C, it has a large broad maximum at 0.8 GPa. The Brillouin frequency shift (sound velocity) and linewidth obtained indicate that the large dispersion of the sound velocities of DOS occurs from 0.1 MPa at 25 °C and from 0.8 GPa at 80 °C. The origins of this attenuation and dispersion are discussed on the basis of the theory for a viscoelastic liquid. It is proposed that the large acoustic attenuation and dispersion of DOS are due to the production of higher-rank structures with nano-order domains in a polymeric liquid by pressurization. The results show that DOS is strongly viscoelastic above 0.8 GPa at 80 °C, but it is not viscous below 0.8 GPa at 80 °C, with the disappearance of the frequency dispersion. The result obtained is used to explain a limiting shear stress observed in a traction oil. Above a given sliding speed, the oil reaches the region of temperature and pressure in which its viscosity decreases with increasing shear rate and conveys a constant torque above some high shear rate. Then, the oil flows as a plastic solid at a limiting shear stress. These findings regarding the dynamical properties of DOS under high pressures are very useful for the production and analysis of lubricants and traction oils.