Pressure is one basic parameter involved in microfluidic systems. In this study, we developed an easy capillary-based method for measuring fluid pressure at one or multiple locations in a microchannel. The principal component is a commonly used capillary (inner diameter of 400 μm and 95 mm in length), with one end sealed and calibrated scales on it. By reading the height (h) of an air-liquid interface, the pressure can be measured directly from a table, which is calculated using the ideal gas law. Many factors that affect the relationship between the trapped air volume and applied pressure (papplied) have been investigated in detail, including the surface tension, liquid gravity, air solubility in water, temperature variation, and capillary diameters. Based on the evaluation of the experimental and simulation results of the pressure, combined with theoretical analysis, a resolution of about 1 kPa within a full-scale range of 101.6–178 kPa was obtained. A pressure drop (Δp) as low as 0.25 kPa was obtained in an operating range from 0.5 kPa to 12 kPa. Compared with other novel, microstructure-based methods, this method does not require microfabrication and additional equipment. Finally, we use this method to reasonably analyze the nonlinearity of the flow–pressure drop relationship caused by channel deformation. In the future, this one-end-sealed capillary could be used for pressure measurement as easily as a clinical thermometer in various microfluidic applications.

The sorting of target particles from heterogeneous samples is challenging yet crucial for cell biology research and clinical diagnosis. Among various microfluidic methods, the use of cavity-based laminar vortex combined with inertial focusing is a powerful label-free passive technique for the selective sorting of large rare cells with high purity and concentration from billions of blood cells. However, this technology faces the challenge of improving the cavity holding capacity of trapped particles. This paper describes a round cavity-based vortex sorting method and presents a novel judgment criterion. The proposed round cavity achieves a holding capacity of entrapped target particles that is 2.2–7.8 times higher than that of rectangular cavities. By comparing the particle recirculating orbits and the simulated vortex morphology in round and rectangular cavities, a mechanism whereby particles/cells are held within the cavities is investigated. It is found that the area ratios (S = Ap/Ac) of the particle orbit area (Ap) to the cavity area (Ac) are 0.56 and 0.95 for the rectangular and round cavities, respectively. The results show that the round cavity provides more efficient space for recirculating particles and has better sorting performance. This round cavity-based vortex sorting method will be useful for clinical applications.

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Particle orbital motion in a hydrodynamic vortex confined in a microcavity is a relatively new issue of fluid mechanics. In this study, we use a high-speed microscopy system to visualize the phenomenon of particle two-orbital motion within a laminar microvortex. Specifically, a finite-size particle recirculates along a small inner orbit and a large outer orbit alternately and periodically. The influences of the inlet Reynolds number (Re = 110–270), particle diameter (d = 20 and 30 μm), and microcavity size on the particle orbiting behaviors are investigated. The vortical flow field, orbital morphology, and particle velocity variations are characterized quantitatively to elucidate the mechanisms of particle recirculation along the dual orbits. The particle orbital motion results from the combined effects of hydrodynamic forces, particle slingshot effect, and particle–wall interactions in a complex way. The findings of this study could deepen the understanding of the particle orbital motion in a microvortex.