Inertial focusing of a dilute suspension in pipe flow

What is it about?

How can you direct particles flowing in a pipe where you want? This question has many practical implications and motivates the development of inertial microfluidics, where we rely on inertia to manipulate particles. This adventure started sixty years ago when researchers observed particles flowing in a pipe migrate laterally, concentrating in a thin annular region. They attributed this effect to fluid inertia: the particle's lateral equilibrium position results from the competition between the centrifugal force in the center of the pipe and a centripetal one at the walls. Researchers have recently observed the exotic phenomenon of a secondary annulus appearance. However, no one understood the reason for this puzzling behavior.

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Photo by Shubham Dhage on Unsplash

Photo by Shubham Dhage on Unsplash

Why is it important?

Understanding the physical mechanism behind the migration of particles flowing in a simple or complex channel is essential to develop devices to separate particles based on their properties. For example, we can use fluid inertia to separate particles based on their size or shape or isolate pathological cells such as cancer cells or malaria-infected red blood cells.