Pressure dependence of the viscoelastic properties of castor oil

Abstract

Relatively few results are available concerning the effect of hydrostatic pressure on the viscoelastic properties of liquids. For pure liquids data obtained in the relaxation region by variation of pressure can be fitted by the same equation for the complex compliance which was found by Barlow, Erginsav and Lamb to describe the behaviour of supercooled liquids at atmospheric pressure: J^*(j_ω)=J_∞+1//_jωη+ 2(J_∞/jωη)^½. (1), J_∞ is the limiting compliance at high effective frequencies.

At atmospheric pressure the behaviour of castor oil differs markedly from that described by eqn (1) and, for this reason, a series of measurements has been made on this liquid at frequencies of 10 and 30 MHz in the temperature range –30 to +30°C and for pressures up to 300 MN/m² at intervals of 25 MN/m². Values of density, ρ, viscosity, η, and the resistive component, R_L, of the shear mechanical impedance have been determined. Results show that G_∞(= 1/J_∞) increases linearly with pressure as for pure liquids and the viscoelastic behaviour can be represented by the modified equation, found previously for mixtures of liquids: J^*(jω)=J_∞[1 +1//_jωτm]+2KJ∞//_(jωτm)^½,(2) where τ_m is the Maxwell relaxation time (=ηJ_∞).

The value of K for castor oil is 2.6 and results at various temperatures and pressures are fitted to this equation within experimental error.