2.6. Rheological properties of the emulsions
The mechanical properties of the emulsions were evaluated determining frequency (f ) sweeps and time-dependent recovery measurements using a Discovery HR-3 Hybrid Rheometer (TA Instruments, New Castle, DE, USA) equipped with a stainless-steel parallel plates’ geometry (40 mm diameter) with the surface of the upper plate sand-blasted. The temperature of the sample (25°C) was controlled using a Peltier system located on the base of the equipment. Initially, a sample of the emulsion (25°C) was placed on the base of the geometry preset at 25°C to then setting the upper plate to a gap of 1000 µm. Applying a constant strain (γ) we determined the f sweeps of the emulsion measuring the storage (G’) and loss (G”) modulus. The γ applied varied between 0.06% and 0.10% as a function of the emulsion, but always was within the linear viscoelastic region (LVR) of the material previously established from the γ sweeps using a f of 1 Hz. Additionally, to establish the plastic region of the emulsions we did an onset point analysis of the G’ values obtained from the γ sweeps determined using af of 1 Hz. With the elastic and plastic regions established we determined the time-dependent recovery behavior of the emulsions, initially applying during 60 s an γ within the elastic region (i.e., within the LVR) of the corresponding sample while measuring G’ (G’0). Afterwards, we applied during 30 s an γ within the plastic region (i.e., an γ above the G’-G” cross point, γ*) followed by the measurement of G’ while applying during 300 s an γ within the LVR. From a master curve of the time-dependent recovery profile of two emulsions (n = 2), we calculated the instantaneous (R10 s) and extended (R300 s) elasticity recovery as:
R10 s = (G’10 s)100/G’0 s Equation 1
R300 s = (G’300 s)100/G’0 s Equation 2
where G’10 s and G’300 s corresponds to the G’ mean values determined after 10 s and 300 s, respectively, of concluding the application of γ*.