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*.