3. Results
3.1. Measurements results
3.1.1. Sound speed
The plots of measured longitudinal ultrasonic wave velocity c in the investigated edible oil (olive oil) as a function of pressure, at various temperatures, are shown in Fig. 2. The plot of sound speed (see Fig. 2) displays a piecewise monotonically continuous character of c as a function of pressure for various values of temperature. The increase in ultrasonic speed is due to the decrease in intermolecular free path length and adiabatic compressibility. Discontinuity in the plot indicates that the phase transition occurs. In Fig. 2, three different parts of each curve can be seen. At the beginning of measuring process (point a), the pressure was increased gradually up to the point b, (first part of each curve in Fig. 2). At the moment corresponding to point b, we stopped the compression, and the piston in the high-pressure chamber was fixed to enable the liquid-to-solid phase transition to occur undisturbed. The induction time is the time that elapses from the moment when the piston is stopped and the beginning of the phase transformation (marked by spontaneous pressure drop associated with transition). The observed spontaneous pressure drop indicates initiation of the liquid-to-solid phase transition. The pressure drop is due to a decrease in the liquid volume, and consequently the increase in the liquid density. These changes result from the phase transition that occurs spontaneously. Although the pressure drops, the speed of sound continues to increase (second part of each curve in Fig. 2). This indicates change in the physical properties of the liquid.
When the temperature during measurements was higher, the pressure, at which the liquid-to-solid phase transition began, increased. The stabilization of pressure and negligible changes in sound velocity indicated that the liquid-to-solid phase transition was completed (point c). At the termination of the phase transition process the pressure was increased again, in order to measure the sound velocity in the newhigh-pressure solid-like phase of olive oil (third part of each curve in Fig. 2). The further increase of velocity with increasing pressure was observed (up to point d).