Proposed viscosity measurement method using fluidic flow switching in microfluidic channels based on hydrodynamic balancing with a label-free operation. (A) Schematic diagram of the proposed method using a simple microfluidic device which consists of two inlets and two outlets, two side channels for guiding two fluids, and one junction channel connecting the two parallel side channels for detecting flow direction depending on the flow-rate ratio ( Q ref / Q test ). (a) When the viscosity of a test fluid is larger than that of reference fluid ( μ test > μ ref ), the test fluid moves toward the right direction in the junction channel due to higher pressure at the left junction rather than the right junction ( P L > P R ). (b) The reference fluid reversely moves toward the left direction in the junction channel, at a specific flow-rate ratio ( Q ref S / Q test ). The viscosity of the test fluid to that of the reference fluid can be identified by monitoring the specific flow-rate ratio, at which fluidic flow-switching phenomenon occurs in the junction channel. (B) The microfluidic system which is composed of two syringe pumps for delivering the sample and reference fluids, two flow stabilizers for regulated fluidic flows, and the microfluidic device for fluid viscosity identification. The flow switching in the junction channel was monitored using an optical microscopy with a CCD camera. (C) Viscosity measurements of (a) silicone oil (test) in relation to DIW (ref), and (b) blood (test) in relation to 1 × PBS solution (ref). The test fluid reversely flows from the right direction ( Q ref = Q test ) to the left direction in the junction channel, at a specific switching flow rate ( Q ref S / Q test ).
Discrete circuit model for the proposed microfluidic system. The governing parameters include the flow rates ( Q test , Q ref ) for the test and reference fluids, the fluidic resistances ( R L , R R , R R-test , R R-ref , RJA ) for the two side channels and the junction channel, and the fluidic pressures ( P L , P R ) at the junction points ( L, R ) under a negligible compliance effect in the microfluidic device.
Performance evaluation results of the proposed method for various design parameters, including the fluidic resistance ratio ( R J L / R L ), the length of the junction channel ( L J ), and the width ratio between the side channel and the junction channel ( W / W J L ). (A) Microscopic images showing fluidic flow direction in the junction channel depending on flow-rate ratio ((a) Q ref / Q test = 1, (b) Q ref / Q test = 3, (c) Q ref / Q test = 4, and (d) Q ref / Q test = 4.08) for the 40% Glycerin solution as the test fluid and DIW as the reference fluid. The fluidic flow of the Glycerin was reversely moved in the junction channel from right direction (R) to left direction (L), at a specific flow rate ratio of Q ref S / Q test = 4.08). (B) Variation of viscosity ratios ( μ test / μ ref ) identified using the proposed method with respect to the fluidic resistance ratios ( R J L / R L ) between the junction channel and the side channel. The normalized differences (NDs) compare the results obtained by the proposed method and the conventional method. (C) Variations of viscosity ratios identified by the proposed method with respect to various lengths ( L J ) of the junction channel ranging from 250 μm to 1800 μm. (D) Variations of viscosity ratios measured by the proposed method with respect to different width ratios ( W / W J L ) between the side channel and the junction channel.
Typical microscopic images captured by the optical microscope for illustrating the performance of the proposed device (W = 50 μm) for various fluids, including Glycerin, plasma, and oil, respectively. In this experiment, DIW was applied as the reference fluid. (A) Microscopic images captured for measuring viscosity of different concentrations of Glycerin ((a) CGlycerin = 10%, (b) CGlycerin = 20%, (c) CGlycerin = 30%, and (d) CGlycerin = 40%). (B) Microscopic snapshots captured for identifying fluid viscosity for two different concentrations of plasma ((a) Cplasma = 50%, and (b) Cplasma = 100%). (C) Viscosity variations of test fluids ((a) Glycerin (40%) vs. DIW, (b) Plasma vs. DIW, and (c) Silicon oil vs. DIW) depending on heat treatment (200 °C for 12 h) of the microfluidic device. (D) Microscopic images showing change of flow direction of the silicone oil in the junction channel depending on flow-rate ratio ((a) Q ref / Q test = 1, (b) Q ref / Q test = 2, (c) Q ref / Q test = 3, (d) Q ref / Q test = 4.4, and (e) Q ref S / Q test = 4.57).
Variation of blood viscosities determined by the proposed method, with respect to several factors such as hematocrit at inlet, chemically hardened RBCs by GA (glutaraldehyde), and widths of side channels. (A) The ratio of blood viscosity to that of the reference fluid (1 × PBS) with varying hematocrit of blood sample at inlet. The images captured by an optical microscopy were monitored to identify the fluidic flow-switching in the junction channel for various flow-rate ratios ((a) Q ref / Q test = 1, (b) Q ref / Q test = 1.9, (c) Q ref / Q test = 2.25, and (d) Q ref S / Q test = 2.37) for blood sample of 50% hematocrit. (B) Variation of blood viscosity measured by the proposed method with respect to different concentrations of GA ranging from 0% to 1%. (C) Viscosity measurements of blood samples (Hct = 40%) and the Glycerin (40%) with respect to channel widths (50 μm to 3000 μm) of the both side channels. Microscopic images at the right side represent the fluidic flow-switching in the junction channel for (a) Glycerin ( Q ref S / Q test = 4.34) and (b) blood ( Q ref S / Q test = 3.45). (D) Variation of blood viscosities identified by the proposed device (W = 2000 μm) using two different blood samples ((a) RBCs in 1 × PBS suspension (Hct = 40%) and (b) RBCs in plasma suspension (Hct = 40%)) with respect to shear rates.
Quantitative comparison of the viscosity ratios determined by the proposed method (W = 50 μm, R JL / R L = 0.1) and by the conventional methods for several pure liquids and different phases (single phase and oil–water phase).
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