Fundamental Mechanisms in Inertial Microfluidics

Inertial microfluidics leverages the finite inertia of a fluid flowing through microscale channels—typically at Reynolds numbers between 1 and 100—to exert predictable lateral forces on suspended particles or cells. As fluid streams negotiate curved or straight channel geometries, two main inertial forces arise: the shear-gradient lift, which pushes particles from regions of high to low shear, and the wall-induced lift, which repels them from channel boundaries. The interplay of these forces drives particles toward precise, size-dependent equilibrium positions within the cross-section of the channel, enabling continuous, high-throughput focusing, sorting, or separation without external fields. Because it operates passively—relying solely on channel design and flow rate—inertial microfluidics offers label-free, gentle handling of biological samples, making it a powerful tool for applications ranging from blood plasma separation to rare cell isolation.

Particle Pairs Formation 

Isolated particle behaviour is fairly well understood, with general rule of thumb for device designers larger and softer particles migrating to equilibrium positions closer to the channel centre. However, when particles are part of a suspension, particle-particle ineractions can occur leading to particle pair and train formation, with constant and predictable axial spacing between the particles. These pairs and trains can be beneficial or detrimental to device performance depending on the application and are therefore very important. However, we do not have a full understanding of the mechanisms invloved or the contributing parameters.

The effect of particle softness

We have shown that particle softness has a significant effect on the formation of particle pairs. 

The effect of different particle sizes

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The effect of different particle softnesses

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