The lateral migration of microspheres across streamlines induced by elasticity and inertia in a square microchannel flow of viscoelastic fluids is investigated using a holographic microscopy technique. We experimentally demonstrate the exact particle positions driven by the elasticity of fluid in the channel cross-section. The effects of the blockage ratio, flow rate, and shear-thinning property of the viscoelastic fluids on particle migration are evaluated. In particular, the focusing patterns of microspheres in three-dimensional volume are analyzed under different conditions, namely, dominant inertia, dominant elasticity, and the combined effects of inertia and elasticity.

Red blood cells (RBCs) are important to dictate hemorheological properties of blood. The shear-thinning effect of blood is mainly attributed to the characteristics of the RBCs. Variations in hemorheological properties alter flow resistance and wall shear stress in blood vessels. Therefore, detailed understanding of the relationship between the hemorheological and hemodynamic properties is of great importance. In this study, blood viscosity and blood flow were simultaneously measured in the same microfluidic device by monitoring the flow-switching phenomenon. To investigate blood flows according to hemorheological variations, the flow rate of blood samples (RBCs suspended in autologous plasma, dextran-treated plasma, and in phosphate buffered saline solution) was precisely controlled with a syringe pump. Velocity profiles of blood flows were measured by using a micro-particle image velocimetry technique. The shape of velocity profiles was quantified by using a curve-fitting equation. It is found that the shape of the velocity profiles is highly correlated with blood viscosity. To demonstrate the relationship under ex vivo conditions, biophysical properties and velocity profiles were measured in an extracorporeal rat bypass loop. Experimental results show that increased blood viscosity seems to induce blunt velocity profile with high velocity component at the wall of the microchannel. Simultaneous measurement of blood viscosity and velocity profile would be useful for understanding the effects of hemorheological features on the hemodynamic characteristics in capillary blood vessels.

Fluctuations in flow rate invariably occur in microfluidic devices. This fluidic instability results in a deteriorating performance and the suspension of their unique functions occasionally. In this study, a fluidic-LPF (low pass filter), which is composed of an ACU (air compliance unit) and a FCSP (fluidic channel with high fluidic resistance for sufficient preload), has been proposed for providing the stabilization of hydrodynamic flow in microfluidic devices. To investigate the characteristics of various fluidic networks including our fluidic-LPF, we used a parametric identification method to estimate the time constants via a transient response that was based on a discrete parameter model. In addition, we propose the use of a pulsation index (PI) to quantify the fluctuations in flow rate. We verified the formula for PI derived herein by varying individually both the periods and the air compliance volumes in the ACU, both theoretically and experimentally. We found that the PI depended strongly on either the time constants or the periods of the flow rates at the inlet. Additionally, the normalized differences between the experimental results and the theoretical estimations were less than 6%, which shows that the proposed formula for PI can provide an accurate quantification of the fluctuations in flow, and estimate the parametric effects. Finally, we have successfully demonstrated that our fluidic-LPF can regulate fluctuations in the flow at extremely low flow rates (∼ 10 μL h⁻¹) and can also control severe fluidic fluctuations (PI = 0.67) with excessively long periods (100 s) via a microfluidic viscometer. We therefore believe that the stabilization of hydrodynamic flow using a fluidic-LPF could be used easily and extensively with a range of microfluidic platforms that require constant flow rates.

The accurate viscosity measurement of complex fluids is essential for characterizing fluidic behaviors in blood vessels and in microfluidic channels of lab-on-a-chip devices. A microfluidic platform that accurately identifies biophysical properties of blood can be used as a promising tool for the early detections of cardiovascular and microcirculation diseases. In this study, a flow-switching phenomenon depending on hydrodynamic balancing in a microfluidic channel was adopted to conduct viscosity measurement of complex fluids with label-free operation. A microfluidic device for demonstrating this proposed method was designed to have two inlets for supplying the test and reference fluids, two side channels in parallel, and a junction channel connected to the midpoint of the two side channels. According to this proposed method, viscosities of various fluids with different phases (aqueous, oil, and blood) in relation to that of reference fluid were accurately determined by measuring the switching flow-rate ratio between the test and reference fluids, when a reverse flow of the test or reference fluid occurs in the junction channel. An analytical viscosity formula was derived to measure the viscosity of a test fluid in relation to that of the corresponding reference fluid using a discrete circuit model for the microfluidic device. The experimental analysis for evaluating the effects of various parameters on the performance of the proposed method revealed that the fluidic resistance ratio (RJL/RL, fluidic resistance in the junction channel (RJL) to fluidic resistance in the side channel (RL)) strongly affects the measurement accuracy. The microfluidic device with smaller RJL/RL values is helpful to measure accurately the viscosity of the test fluid. The proposed method accurately measured the viscosities of various fluids, including single-phase (Glycerin and plasma) and oil-water phase (oil vs. deionized water) fluids, compared with conventional methods. The proposed method was also successfully applied to measure viscosities of blood with varying hematocrits, chemically fixed RBCS, and channel sizes. Based on these experimental results, the proposed method can be effectively used to measure the viscosities of various fluids easily, without any fluorescent labeling and tedious calibration procedures.