A Minireview on Inertial Microfluidics Fundamentals: Inertial Particle Focusing and Secondary Flow

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Review Article

A Minireview on Inertial Microfluidics Fundamentals: Inertial Particle Focusing and Secondary Flow Aram J. Chung

1,2,3,

*

Received: 30 January, 2019 / Accepted: 01 March, 2019 / Published online: 08 March, 2019 ⒸThe Korean BioChip Society and Springer 2019

Abstract In 1961, Segre and Silberberg first reported the tubular pinch effect and numerous theoretical studies were subsequently published to explain the inertial particle migration phenomenon. Presently, as fluid mechanics meets micro- and nanotechnology, theoretical studies on intrinsic particle migration and flow phenomena associated with inertia are being experimentally tested and validated. This collective study on the fluid-particle-structure phenomena in microchannels involving fluid inertia is called, “inertial microfluidics”. Beyond theoretical studies, now inertial microfluidics has been gaining much attention from various research fields ranging from biomedicine to industry. Despite the positive contributions, there is still a lack of clear understanding of intrinsic inertial effects in microchannels. Therefore, this minireview introduces the mechanisms and underlying physics in inertial microfluidic systems with specific focuses on inertial particle migration and secondary flow, and outlines the opportunities provided by inertial microfluidics, along with an outlook on the field. Keywords: Inertial microfluidics, Fluid inertia, Inertial particle migration, Secondary flow, Inertial microfluidic physics

1 School of Biomedical Engineering, Korea University, Seoul 02841, Republic of Korea 2 Department of Bioengineering, Korea University, Seoul 02841, Republic of Korea 3 Department of Bio-convergence Engineering, Korea University, Seoul 02841, Republic of Korea *Correspondence and requests for materials should be addressed to A.J. Chung ( [email protected])

Introduction In 1961, Segre and Silberberg first reported that randomly incoming millimeter-sized particles in a fluidic channel can laterally migrate towards the channel wall and form a tubular annulus positioned in a 1 cm diameter tube. This was termed as the tubular pinch effect1. This observation surprised the research community because until then it was believed that all suspended particles move towards a low shear region, which is the centerline of the channel. Subsequently, there were many theoretical investigations to physically explain this unexpected lateral migration behavior of particles, which originated from fluid inertia2-8. Since fluid mechanics met MEMS (microelectromechanical systems) technology, numerous researchers not only experimentally have validated the theoretical particle migration phenomenon in smaller dimensions (i.e., in the micrometer scale) of microfluidic channels, but also reported various applications demonstrating the power of fluid inertia that existed in the microchannels9. This collective study involving with fluid inertia in microchannels and their applications is now commonly referred to as “inertial microfluidics”. Inertial microfluidics is