A Review of Secondary Flow in Inertial Microfluidics

Abstract

Inertial microfluidic technology, which can manipulate the target particle entirely relying on the microchannel characteristic geometry and intrinsic hydrodynamic effect, has attracted great attention due to its fascinating advantages of high throughput, simplicity, high resolution and low cost. As a passive microfluidic technology, inertial microfluidics can precisely focus, separate, mix or trap target particles in a continuous and high-flow-speed manner without any extra external force field. Therefore, it is promising and has great potential for a wide range of industrial, biomedical and clinical applications. In the regime of inertial microfluidics, particle migration due to inertial effects forms multiple equilibrium positions in straight channels. However, this is not promising for particle detection and separation. Secondary flow, which is a relatively minor flow perpendicular to the primary flow, may reduce the number of equilibrium positions as well as modify the location of particles focusing within channel cross sections by applying an additional hydrodynamic drag. For secondary flow, the pattern and magnitude can be controlled by the well-designed channel structure, such as curvature or disturbance obstacle. The magnitude and form of generated secondary flow are greatly dependent on the disturbing microstructure. Therefore, many inventive and delicate applications of secondary flow in inertial microfluidics have been reported. In this review, we comprehensively summarize the usage of the secondary flow in inertial microfluidics.

Numerical Study of Multivortex Regulation in Curved Microchannels with Ultra-Low-Aspect-Ratio

“…The helical vortex is distinct from the Dean vortex in a curved channel and asymmetrically structured channel [ 14 , 18 ]. The formation of helical vortexes depends on several factors, including the ratio of the contraction area to the expansion area, shape and size of the micro-obstacle, the flow velocity, and the fluid inertia [ 13 , 16 ]. It is noteworthy that the fluid inertia is mainly dependent on Re .…”

“…However, the resulting reduction in the number of Dean vortexes negatively affected fluid manipulations. Accordingly, it is vital to yield the optimal number of micro-obstacles and flow rate when studying fluid applications [ 16 , 17 ]. These findings verify the value in using sequenced micro-obstacles in curved channels to modify the Dean vortex achieved and lay the theoretical basis for fluid mixing and particle manipulation [ 37 , 38 ].…”

“…Various channel designs that manipulate vortex actions to increase the interface area of fluid streams, depending on non-conventional applications of fluid inertia in microfluidic platforms, have been demonstrated [ 13 , 15 ]. The vortex usually appears in (i) straight channels with expansion–contraction arrays or disturbance obstacles [ 16 , 17 , 18 , 19 ], and (ii) curved channels with spiral or serpentine geometry [ 20 , 21 , 22 , 23 , 24 ]. Due to the fluid momentum mismatch between the near wall and the central area in these platforms, Dean vortex or Dean-like secondary vortex flows can be generated in the main channel section by a radial pressure gradient [ 13 ].…”

Inertial microfluidics: Recent advances

“…There are several published review articles on inertial microfluidics. For example, the reviews focus on structures [35–37], physics [38,39], computational methods [40], and secondary flow [41] in inertial microfluidics and the particle manipulations in non‐Newtonian fluid [42]. Different from these previous reviews, we focused on the bioparticles applications of inertial microfluidics.…”

Dean Vortex for Laminar Flows in Curved Pipes with Various Cross-Sections

“…Laminar fluid flow in curved pipes with a slowly varying radius of curvature and for various crosssections shapes is encountered in different areas such as heat transfer [1], chaotic mixing [2], separation of mixtures in pipes [3], or blood circulation in the arteries [4] among others. In inertial microfluidic technology, many applications use the properties of Dean's vortices to manipulate the motion of small particles at low cost based solely on the hydrodynamic properties of these flows, a detailed review of the use of secondary flows in microfluidics was recently presented by Zhao et al, [5]. For applications based on inertial focusing separation, cross-sections shapes are essentials for mastering the separation, which is generally carried out in spiral tubes, thus with small variations in the radius of curvature [6].…”