OIL-IN-WATER NANOEMULSIONS 341 network structure created between dispersed phase droplets. The practical implication of this behavior is that emulsions show high stability and longer storage life if they exhibit domination of G′ response over G″ (23). In point of emulsifi cation methods, emulsions prepared using ultrasonic cavitation exhib- ited better viscoelastic responses when compared to those prepared by rotor–stator ho- mogenizer. The fact that the G′ response of emulsions prepared by ultrasonic cavitation was higher than the response of emulsions prepared by rotor–stator homogenizer is evi- dence that the degree of elastic property of emulsions prepared by ultrasonic cavitation is higher. This in turn suggests that emulsions prepared by ultrasonic cavitation are more stable and possess longer shelf life compared to emulsions prepared by rotor–stator ho- mogenizer. As G″ response of emulsions prepared by ultrasonic cavitation was higher compared to emulsions prepared by rotor–stator homogenizer at all measured frequency domains, it is evident that the emulsions prepared by ultrasonic cavitation systems fl ow and spread more easily compared to corresponding emulsions prepared by rotor–stator homogenizer system. Plot of phase angle δ as a function of frequency ω is given in Figures 7 and 8. In Figure 7, the phase angle is less than 20° across the entire frequency range, indicat- ing that the system tends to show an elastic response to shear. On the other hand in Figure 8, the phase angle at low frequency shows that the response of the system is elastic. As the frequency increases, the response of the system becomes more viscous. Such behavior is a common feature in viscoelastic systems. Figure 5. A frequency sweep of an emulsion system that shows an elastic response to shear. The diagram shows the storage (G′ ) and loss (G″ ) moduli for an emulsion system prepared by rotor–stator homogenizer. Figure 6. A frequency sweep of an emulsion system that shows an elastic response to shear. The diagram shows the storage (G′ ) and loss (G″ ) moduli for an emulsion system prepared by ultrasonic cavitation.

JOURNAL OF COSMETIC SCIENCE 342 RHEOLOGICAL CHARACTERIZATION 4: TIME-DEPENDENT VISCOMETRY Time-dependent viscometry was used to explore the consequences of structural changes during fl oc development. Emulsions were initially sheared at high stress (10 Pa) to breakup any existing fl ocs. After 1 min, this stress was removed and a constant low shear stress (0.1 Pa) was applied for 300 s where viscosity measurements taken every 1 s with increasing temperature in the range from 25°C to 50°C. Figures 9 and 10 show the ap- parent viscosity versus time plot at elevated temperature for the stabilized emulsion pre- pared by rotor–stator emulsifi cation and ultrasonic cavitation, respectively. After the applied stress was removed, an immediate increase in apparent viscosity was observed, which could be attributed to the onset of fl occulation. The apparent viscosity was found to reach a maximum of 109.69 Pa·s and 70.79 Pa·s at 25°C, after which it was followed by a steady decrease to 108.50 Pa·s and 70.09 Pa·s over the remaining time (300 s) for emulsions prepared by rotor–stator and ultrasonic cavitation, respectively. It can be seen that there was no change in emulsion viscosity over the time at elevated tempera- tures, as would be expected for an emulsion not displaying time-dependent fl occulation. CONCLUSION The ultrasonic cavitation as compared to rotor–stator is a viable method for producing nanoemulsions of palm oil esters in water with mean particle sizes down to 62.99 nm and Figure 8. A phase angle graph shows how the phase angle varies with frequency for emulsion system pre- pared by ultrasonic cavitation. Figure 7. A phase angle graph shows how the phase angle varies with frequency for emulsion system pre- pared by rotor–stator homogenizer.