Figure 4a,b represent the changes in viscosity as a function of shear rate for ASP and SP microemulsions. The concentration of ASP varies from 20 to 80% WC, while the concentration of SP ranges from 20 to 60% WC. The temperature and shear rate were at 60 °C and 1 to 70 s
−1, respectively. In
Figure 4a, the log/log plot shows the effect of ASP concentrations on the functional relationship of viscosity and shear rate at 60% oil concentration. The viscosity of all microemulsions decreases as the shear rate increases, showing the typical behaviour of the pseudoplastic fluid. Increasing the ASP concentration resulted in more adsorption of the thickening agent through the oil-water interface, thus producing an apparent higher microemulsion viscosity. The marked rheology of ASP microemulsion comes from the HPAM polymer’s large macromolecular weight. The entanglement of macromolecule chains increases at a high concentration of ASP, which then causes an increase in viscosity of the microemulsion [
10]. The shear-thinning behaviour phenomenon is related to the orientation of the macromolecule along the streamline of the flow [
15]. At low shear rate, the entanglement of macromolecules causes the formation of aggregates, resulting in high viscosity fluid. As the shear rate was applied to the fluid, it destroyed the aggregates, and the dispersing molecules were arranged along the flow direction, declining the flow of the fluid resistance and subsequently decreasing the apparent viscosity [
16].
Figure 4b indicates the viscous behaviour of SP as a function of shear rate. The viscosity of SP emulsions was in the range of 0 to 0.8 mPa.s, which is slightly lower than the viscosity of the ASP emulsion with a range of 0.2 to 1.4 mPa.s. The presence of alkaline in the system leads to an increase in the microemulsion viscosity. The high viscosity produced by the addition of alkali is due to a rise in pH value. As the pH value increases, the degree of hydrolysis increases and thrusts more negative charges on the polymer chain; thus, the polymer molecule expands and produces a higher viscosity [
17]. The Power Law Equation (1) illustrates the relationship between viscosity and shear rate [
18]:
where, K,
, and n are the viscosity, consistency index, shear rate, and power-law index, respectively. The power-law index provides information about the effects of shear on the system. The microemulsion shows shear-thinning behaviour when the value of n is below one, and the lower the value of n, more shear-thinning behaviour of the emulsion is produced.
Table 4 indicates the rheological parameters obtained after fitting the data obtained in
Figure 4a,b to the Power Law Equation (1). The plot from the figure and the data from the table indicate that consistency index K increases as the concentration of ASP and SP increases, and the values obtained for n by model fitting are less than 1, which appears on the shear-thinning system.
The shear properties of the microemulsion can also be characterized by the Herschel-Bulkeley Equation (2) [
19]:
where
τ is the shear stress (Pa),
is shear rate, n is flow index,
is the yield stress, and K is the consistency index.
Figure 5a,b indicate the plot of shear stress as a function of shear rate for 20, 40, 50, 60, 80% WC ASP and 20, 40, 50, 60% WC SP microemulsions, respectively. Based on
Figure 6, it was observed that the microemulsions of ASP and SP in different water cuts (WC) show dual flow behaviour at different shear rates. The microemulsion demonstrated Newtonian fluid behaviour (at constant viscosity) at a low shear rate (below 5 s
−1), and the transition of Newtonian to non-Newtonian fluid behaviour occurs at 5 s
−1 shear rate. Beyond the 5 s
−1 shear rate, the microemulsions are found to exhibit non-Newtonian behaviour (the apparent viscosity decreases as stress increases). The shear rate value for the transition from Newtonian to non-Newtonian behaviour is called critical shear rate. The non-Newtonian behaviour microemulsion can be characterized by the value of K, n, and
from the Herschel-Bulkeley Equation (2), where the microemulsion is classified as shear-thinning fluid with yield stress when
= 0, 0 < n < 1, and K > 0 [
19].
Table 5 indicates the Herchel-Bulkeley parameters obtained on fitting the data shown in
Figure A1a,b (
Appendix A). The parameters obtained show that the microemulsions behave as a shear-thinning fluid. For ASP, 80% of WC microemulsions display the lowest yield stress value, while it is 40% WC for SP microemulsions. Yield stress is defined as the minimum shear stress required to initiate flow of the fluid [
20]. The microemulsions start to behave as a non-Newtonian fluid when the applied stress is higher than the yield stress. Separation tends to occur for a meagre yield stress value [
19].