## 3 Results and Discussion
### Various Flow Rate Patterns of Pure Liquid Controlled with Syringe Pump  
Before testing the blood with the present method, glycerin solutions (glycerin = 10%, 20%, and 30%) were used as the test fluid for validating the present method. 1\(\times\) PBS was selected as the reference fluid for clearly visualizing the interface between both fluids in the coflowing channel. Here, a syringe pump was applied to control the flow rate of the test fluid in two different patterns, including a constant flow rate and a sinusoidal flow rate with different periods. First, while the flow rate of each fluid increased stepwise from 0.2 mL/h to 1.6 mL/h at intervals of 0.2 mL/h, both fluids were supplied into the microfluidic device at the same flow rate (\(Q_{r}\) = \(Q_{t}\)). The variations in pressure and shear stress were obtained by increasing the flow rate of the test fluid. Figure 3A showed the temporal variation of \(\beta\) as well as the flow rate (\(Q_{r}\) or \(Q_{t}\)) with respect to glycerin. The results showed that \(\beta\) tended to increase at higher concentrations of the glycerin solution. Above 0.4 mL/h, it remained unchanged with respect to \(Q_{t}\). Using Equation (3), the viscosity of each concentration of the glycerin solution was obtained with respect to the flow rate (\(Q_{t}\)). Figure 3B showed the variations in the viscosity of the glycerin solution with respect to \(Q_{t}\). At lower flow rates (i.e., \(Q_{t}\) \(\leq\) 0.4 mL/h), the viscosity of the glycerin solution exhibited fluctuations with respect to \(Q_{t}\) because the syringe pump induced unstable fluid flows at sufficiently low pressure (i.e., low flow rate or low fluidic viscosity) [58; 59]. However, at higher flow rates (i.e., \(Q_{t}\) \(>\) 0.4 mL/h), the viscosity remained constant with respect to \(Q_{t}\). The results indicated that the glycerin solution behaved as a Newtonian fluid. The viscosity of the test fluid was represented as the mean \(\pm\) standard deviation from several viscosities obtained for each fluid rate. The corresponding viscosity of each glycerin solution was obtained as (a) \(\mu_{t}\) = 1.408 \(\pm\) 0.033 cP for \(C_{glycerin}\) = 10%, (b) \(\mu_{t}\) = 1.885 \(\pm\) 0.08 cP for \(C_{glycerin}\) = 20%, and (c) \(\mu_{t}\) = 2.274 \(\pm\) 0.059 cP for \(C_{glycerin}\) = 30%. When compared with the reference viscosity reported in a previous study [53], linear regression analysis yielded a higher value of correlation (\(R^{2}\) = 0.94). In addition, the present method underestimated the values slightly. Based on the quantitative comparison, the viscosity of the test fluid obtained with the present method was used to evaluate the shear stress of the test fluid stream using Equation (5). As shown in Figure 3C, variations in \(P_{r}\) and \(P_{t}\) were obtained with respect to \(Q_{t}\) and \(C_{glycerin}\). Variations in ND were also summarized with respect to \(Q_{t}\) as well as glycerin. From the results, the pressure of both streams (\(P_{r}\), \(P_{t}\)) increased linearly with respect to \(Q_{t}\). Above \(Q_{t}\) = 0.4 mL/h, the ND was less than 5%. Both streams had approximately the same pressure. \(P_{t}\) required two vital factors (i.e., flow rate, and viscosity) in advance. As \(P_{r}\) could be evaluated by measuring \(\beta\), the pressure of the test fluid stream could be quantified by measuring \(P_{r}\) indirectly. Figure 3D showed the variations in \(\tau_{r}\) and \(\tau_{t}\) with respect to \(Q_{t}\) and \(C_{glycerin}\). Variations in ND were also obtained with respect to \(Q_{t}\) and glycerin. Similar to the variations in pressure (\(P_{r}\), \(P_{t}\)), both the shear stresses (\(\tau_{r}\), \(\tau_{t}\)) increased linearly with respect to \(Q_{t}\). The ND was less than 5% above \(Q_{t}\) = 0.4 mL/h. Thus, the shear stress of the test fluid stream (\(\tau_{t}\)) could be obtained indirectly by measuring \(\tau_{r}\). Second, 1\(\times\) PBS was supplied to the microfluidic device at a constant flow rate of 1 mL/h (\(Q_{r}\) = 1 mL/h).