Three-dimensional hydrodynamic focusing in two-layer polydimethylsiloxane (PDMS) microchannels

Abstract

In this work, we designed and fabricated a three-dimensional hydrodynamic focusing microfluidic device. The device comprises a two-layer PDMS microchannel structure. There are four inlet ports and one outlet port. The fluids are all injected by syringe pumps. A sample flow stream was first vertically constrained into a narrow stream, and then horizontally focused on one small core region from a cross-section perspective, which is useful for cell/particle counting. We showed the numerical and experimental images of the focused stream shape from a cross-section perspective; experimental images were captured using a confocal fluorescence microscope. We also investigated the effect of channel aspect ratio on the vertical focusing effect using CFD simulations. The results showed that the sample flow can be focused successfully in the lower aspect ratio of the main channel (slightly greater than 0.5) in our design. Furthermore, the effect of the Reynolds number on the vertical focusing effect was also investigated. The numerical results showed that the rectangular-like shape of the focused stream from the cross-section perspective was deformed as the Reynolds number was high due to stronger secondary flows produced in the vertical focusing unit. This phenomenon was also demonstrated experimentally. The device only works well at low Reynolds numbers (approximately less than 5). The device can be integrated into an on-chip flow cytometer.

Review Article: Recent advancements in optofluidic flow cytometer

“…Microfluidic systems in this realm offer the benefit of highly predictable behavior that does not depend on sample size desirable qualities for a microfluidic sample positioning system. Numerous structures [17][18][19][20][21][22][23][24][25] have been designed for hydrodynamic flow focusing under this approximation and will be discussed in Sec. II A.…”

“…19,24 Groisman and other groups fabricated multilayered, threedimensional microfluidic devices to achieve confinement in the vertical direction. 17,21,23,25 However, these techniques have not been widely adopted in practice due to increased complication in fabrication process or the need to increase overall fluid flow rates ͑and thus overall particle velocity͒.…”

Two simple and rugged designs for creating microfluidic sheath flow

“…Clearly, the stripe configuration for producing sheath flow has an advantage in that it requires only one inlet for sheath flow, greatly reducing the number of pumps compared to other microfluidic systems that completely ensheath a core fluid. 26,[28][29][30][31][32] Once the number of stripes is established for a given channel aspect ratio, the sheathing will remain essentially in the center of the channel unless the flow rate is significantly changed. Although the chevron design requires one more pump than the stripe design, it does offer a degree of flexibility.…”

“…26,[28][29][30][31][32] Typically, two additional input channels focus the stream vertically as well as horizontally. From the standpoint of cytometry, this is a far better situation, because the sample is now completely isolated from the channel surface, and the position of the particles to be analyzed is fixed.…”

Particle focusing by 3D inertial microfluidics

“…Unfortunately, due to the intrinsic 2D nature of the standard fabrication techniques, this achievement is difficult to obtain. So far, different methods have been proposed to obviate this limitation, but they usually envisage the use of external fields (such as acoustic 6 , electrical 7 or optical fields 8 ) or the presence of multi-lateral sheaths flows [9][10][11][12] . In recent years, inertial microfluidics has been successfully proposed to effectively manipulate particle positions in microfluidic channels [13][14][15] , by simply exploiting the channel geometry and the flow rate.…”