Figure 6. (a) Flow curve (Log plot of shear stress and shear rate) of 5
mg/mL BSA solution sheared in the range of 1 to 1000
s-1, and (b) Corresponding changes in dissipation
energy with time at the respective temperatures. The experiment was
performed using MCR75 rheometer with parallel plate geometry. The
dissipation energy is calculated from the respective experimental data
as \(\mathbf{\tau\times\gamma}\).
The energy involved in the shearing process, the dissipation energy, was
calculated as the product of shear stress and shear rate or the square
of the shear rate and the viscosity
(\(\tau\times\gamma\ or\ \gamma^{2}\times\eta\))
(Barnes et al. 1989;
Hill et al. 2006). The corresponding
generations of dissipation energy with time during the shearing process
at the respective temperatures are shown in Figure 6 (b). The
dissipation energy was found to increase with time or shear rate.
Similar to flow curves, there was only one slope at 55 °C and two
different slopes at higher temperatures. In fact, transitions between
these two slopes were observed at about 1500 and 2500 kJ/mol at 65 and
60 °C, respectively (Figure 6 (b)). This indicated that the higher
thermal energy at 65°C compensated to achieve the aggregation at lower
dissipation energy.
Although the magnitude of the dissipation energy was higher than the
energy required to break intramolecular hydrogen bonds, yet the
aggregation was not observed at the lower temperature 55 °C. This
indicated the loss/transfer of dissipation energy to the neighbouring
water molecules and surrounding and hence the first step (unfolding) of
the aggregation process could not initiate. However at higher
temperatures 60 and 65 °C, where unfolding was achieved by the thermal
energy, dissipation energy has more impact
(Dobson et al. 2017) and was found to
accelerate the second step (fibrillation) of the aggregation process and
the rates of aggregation were enhanced by 1.5 folds (Figure 3).