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).