The manipulation of particles in various media is critically important for research and applications in biotechnology, chemistry, and environmental science. Microfluidics has been widely employed to mix, sort, focus, and separate particles suspended in different fluids. Among the numerous microfluidic approaches, elasto-inertial microfluidics—a passive particle manipulation method that combines elasticity and inertia in the flow—has gathered significant attention due to its high precision in focusing and separating particles. This technique enables the focusing of randomly distributed particles in non-Newtonian fluids without the need for external fields, and facilitates their separation based on size. Elasto-inertial microfluidics has previously been used to separate circulating tumor cells (CTCs) or bacteria from blood samples. Despite the high interest in this field, particle behavior in elasto-inertial flow has not been fully explored, as it relies on the complex interplay between particles and the forces acting upon them. Previous studies have primarily focused on numerical approaches or have been limited to low flow rates and throughput. Additionally, many studies utilized multiple inlets in the microfluidic design to better control fluid dynamics, leading to further dilution of the sample and decreased throughput.

In this work, we aimed to conduct an extensive study to elucidate particle behavior in elasto-inertial flow by combining experimental and numerical approaches. Subsequently, we applied our findings to an innovative microchannel design to achieve high-throughput and high-resolution particle separation, beneficial for future applications in biotechnology, medicine, and environmental science.

Particle focusing in elasto-inertial flow depends on several parameters such as particle size, microchannel dimensions, fluid rheology, and flow rate. Furthermore, microchannel design (straight, spiral, serpentine, etc.) determines the final equilibrium position of particles in the channel. To develop practical devices in the future using elasto-inertial microfluidics, it was crucial to expand our understanding of particle behavior in elasto-inertial flow by investigating each parameter affecting particle focusing and migration. To achieve this, we designed a single-inlet straight microfluidic channel featuring a focusing section and a migration section. We fabricated microchannels with a high aspect ratio, making the channel height greater than its width. This high aspect ratio straight microchannel induces single-stream particle focusing at the channel center, allowing us to better analyze particle behavior in both unfocused and focused states. We conducted particle focusing experiments using three different particle sizes, two different aspect ratios, and four different viscoelastic concentrations over a wide range of flow rates (1–250 µL/min) to obtain a comprehensive dataset. Our extensive investigation demonstrated that particle focusing does not depend on a single parameter, and can be achieved at both low and high flow rates by appropriately tuning the parameters. This finding is particularly important for applications requiring high flow rates and throughput, which were previously considered challenging in elasto-inertial flows.

Following particle focusing, we extensively studied their migration to achieve size-based separation, the ultimate goal for biomedical and environmental applications. Here, the specifics of the microchannel design are critical. Our unique high aspect ratio single-inlet straight microfluidic channel employs a two-step process: prefocusing of particles in the focusing section followed by size-based separation in the migration section. The initial focusing section aligns particles at the channel center when parameters are correctly tuned. At the end of the focusing section, we split our microfluidic channel into two parallel straight channels, one slightly more resistant than the other. This configuration pushes prefocused particles near the channel wall in the lower resistance channel. Since elasto-inertial flow causes particles to find equilibrium positions at the channel center, all particles migrate from the channel wall toward the center. We leverage this migration for particle separation, which is highly size-dependent. Additionally, by employing a single inlet in our microchannel, we not only increase the overall throughput of the system for particle separation but also achieve simpler and more precise control over the system.

Our experiments showed that migration is faster near the channel wall and slows as particles approach the center. Larger particles reach the center first and maintain their position, while smaller particles continue migrating until they eventually reach the center. To support our experimental findings on particle migration, we performed numerical studies that confirmed the size-dependence of migration and the decreasing migration velocity as particles approach the center. We demonstrated that this differential migration enables high-resolution particle separation (5 µm, 7 µm, and 10 µm). Based on these observations, the length of the migration section can be adjusted to separate different-sized particles effectively. It is important to emphasize that this strategy works well due to the initial prefocusing of particles at the channel center. Additionally, we showed that increasing the channel height while maintaining the same width leads to higher throughput, an essential factor for practical applications.

In this work, we comprehensively demonstrated how randomly distributed particles can be focused in a microfluidic channel by tuning parameters such as microchannel geometry, fluid properties, and particle size. Subsequently, we illustrated how to separate prefocused particles of different sizes by utilizing differential migration in our two-step microfluidic channel. Overall, this study sheds light on the complex behavior of particles in elasto-inertial flow and highlights the potential of high aspect ratio single-inlet microchannels for achieving high-throughput and high-resolution particle separation, which is valuable for biomedical and environmental applications.

Our findings have significant implications for the future of particle manipulation using elasto-inertial microfluidics. By providing a deeper understanding of the factors influencing particle behavior in elasto-inertial flows, our research paves the way for the development of more efficient and precise microfluidic devices. These devices could be employed in a wide range of applications, from medical diagnostics to environmental monitoring. The ability to achieve high-throughput and high-resolution separation of particles opens up new possibilities for advancements in various fields, ultimately contributing to improved health outcomes and a better understanding of environmental processes.