## 3 Results and Discussion
Apparent viscosity values -increase with time, but do not reach the values characteristic for the first shear stage, the greatest differences between \(\eta_{app}\) are visible in  
Figure 4: Blasius plot \(f_{D}=f(Re_{HBE})\) prepared on the base of rheological data obtained during flow experiment in capillary-pipe viscometer for xanthan gum solutions (0.25–1.00%) at the measurement temperature 21 °C. The values od Reynolds number and friction coefficients were calculated according to Equations (6) and (7). the range of the lowest shear rates [10]. For comparison, the same graph shows the results obtained using the method of measuring the static pressure drop while flowing through the capillary of solutions with a concentration of 0.25%, 0.5% and 0.75%. Changes in apparent viscosity reflect the rheological character of the system under study. The effect of xanthan gum concentration on the frictional properties of solutions is clearly visible [10; 14]. A comparison of \(\eta_{app}\) changes for a 0.5% solution tested with both methods shows the convergence of the results. The differences in the values of apparent viscosity are visible in the range of the lowest shear rate values. From the analysis of the graph (Figure 5) it can be seen that the steady state is reached faster for higher shear rates (lower values of apparent viscosity). This phenomenon is due to the fact that the increase in shear forces is associated with the supply of greater amounts of mechanical energy to the system that causes faster disintegration of the fluid structure. Figure 5 also shows the exemplary time series obtained with the same solution (0.50% xanthan gum) in a rotational rheometer. Initially, it is possible to observe discrepancies between the individual shear rates, but the increase in the value of \(\dot{\gamma}\) causes the differences to disappear. This phenomenon can be explained by the differences in the dissipation of mechanical energy by the tested systems. In the case of a capillary-pipe viscometer, the volume of the tested sample is over \(10^{4}\) cm\({}^{3}\), and in the case of a rotational rheometer, 75 cm\({}^{3}\). Such a significant difference in the volume of the tested samples translates into different thermal inertia and different capacities of mechanical energy accumulation in the fluid structure. After the fluid structure is destroyed (high shear rate values), the measured values become practically equal. A further analysis included a reduction of the obtained data to the \(\tau(\dot{\gamma})\), relationship, so the classical flow curves and the classical hysteresis loop test. These results are presented in Figure 6.  
Figure 5: Apparent viscosity change for different xanthan concentration across shear rate range from 5 s\({}^{-1}\) to 65 s\({}^{-1}\). In all cases, the results of measurements of the static pressure drop as a function of volumetric flow rate are presented after conversion to the values of the tangential stress \(\tau_{w}\) Equation (1) and shear rate \(\dot{\gamma}\) Equation (2). The model was fitted to the data thus obtained power Equation (4). The obtained rheological parameters are listed in Table 1, where additionally results obtained for the same preparations (solutions) of xanthan gum but with rotational rheometer, are presented. Analysis of the values of the H-B model parameters for the obtained for the capillary-pipe viscometer shows that as the concentration of xanthan gum in the test solution increased the value of the consistency coefficient (\(K\)) increases, which is as predicted, while the value of the rheological exponent (\(n\)) decreases. Parameter analysis for the \(up\) and \(down\) curves exhibit a decrease in the value of the \(K\) parameter for the \(down\) curve in relation to the \(up\) curve. This phenomenon can be explained by irreversible changes in structure caused by shear flow. The exponent the rheological \(n\) exhibit a similar tendency. The phenomenon of destroying the structure of xanthan gum solutions are best visible in the region of the lowest shear rates.