(432i) Formation of Secondary Dean Vortices in Low-Aspect Ratio Curved Microchannels

Flows in curved confinements are ubiquitous in nature and industrial applications. The centrifugal force in flow through a curved channel initiates a hydrodynamic instability that results in the development of a pair of counter-rotating vortices, the so-called Dean flow. When this secondary flow toward the concave wall becomes too strong to be dissipated by viscous effects, an additional pair of vortices emerges near the outer wall. While the onset of Dean flow has been widely reported in the context of various applications such as particle sorting and fluid mixing, the formation of the additional pair of vortices is not well characterized. Combining numerical and scaling analysis, we identify the critical condition for the onset of secondary vortices in rectangular channels of different aspect ratio 𝛾, and show that it depends on 𝛾^1/2Dn, where Dn is the Dean number. We also examine the development length for the formation of the additional vortices and show that it can be characterized in terms of the effects of convective transport and viscous dissipation. The development of secondary Dean vortices can have a profound effect on particle capture and separation.