Microfluidics is the science and technology of manipulating fluids at the microscale, where phenomena such as inertia, viscosity, and surface tension play important roles. One of the key concepts in microfluidics is Dean’s flow, which is the generation of two counter-rotating vortices in a curved channel due to the centrifugal force. Dean’s flow can be used for applications such as mixing and sorting of particles and cells in microchannels.
However, when the Dean number (De), which is a dimensionless parameter that measures the strength of the secondary flow, exceeds a critical value, Dean’s instability occurs. This means that the two vortices break up into multiple smaller ones, creating a complex flow pattern that can affect the performance of microfluidic devices.
A new study investigates Dean’s instability in high aspect ratio channels
A team of researchers from Hong Kong University of Science and Technology (HKUST) and Zhejiang University (ZJU) have recently published a paper in Scientific Reports, where they report the first experimental and numerical study of Dean’s instability in high aspect ratio channels on the deka-microns level for De > 162.
High aspect ratio channels are channels that have a large width-to-height ratio, which are commonly used in microfluidics to increase the throughput and reduce the pressure drop. However, the effects of channel geometry on Dean’s instability have not been well understood until now.
The researchers designed and fabricated four different channel geometries: straight, curved, serpentine, and tortuous. The tortuous channel is a new geometry that creates a rolled-up velocity profile, which enhances the secondary flow and allows easier Dean’s instability creation.
They then performed experiments and simulations to compare the flow patterns and mixing efficiencies among the four channel geometries at different Reynolds numbers (Re), which measure the ratio of inertial to viscous forces.
The tortuous channel outperforms the other geometries in terms of mixing efficiency
The results showed that the tortuous channel generated a higher De environment at the same Re compared to the other channels, and thus exhibited Dean’s instability at lower Re values. The tortuous channel also produced multiple vortexes that enhanced the mixing of fluids in the cross-section.
The researchers quantified the mixing efficiency by calculating the variance of dye concentration along the channel. They found that the tortuous channel had the highest mixing efficiency among all four geometries, followed by the serpentine, curved, and straight channels.
They also observed that increasing Re improved the mixing efficiency for all geometries except for the straight channel, which had no secondary flow.
The study offers more insights into the creation and applications of Dean’s instability
The study by Wong et al. offers more understanding of how channel geometry affects the creation and characteristics of Dean’s instability in high aspect ratio channels. It also demonstrates the potential for using multiple vortexes for applications such as mixing and cell sorting in microfluidics.
The researchers suggest that future work could explore other factors that influence Dean’s instability, such as fluid properties, channel dimensions, curvature radius, and bend angle. They also propose that more advanced fabrication techniques could be used to create more complex channel geometries that could further enhance Dean’s instability and its applications.