In microchannels with typical dimensions from 10 μm to few hundreds μm, the flow is dominated by viscous forces, leading often to laminar flow conditions. At the entrance or in bends and curves, where the flow is accelerated or changes its direction, inertial forces generate transverse flow velocities. Due to continuity, compensating transverse velocity components generate vortex pairs, such as Dean flow in circular bends. The flow is still laminar, steady, and shows no statistically distributed fluctuations typical for turbulent flow. This deviation from straight laminar conditions, often in larger channels (100 μm to few mm) or for higher flow rates, is called transitional flow. That embraces the first occurrence of pulsating vortices, period doubling of vortex pairs, flow bifurcation, and regularly fluctuating wake flow or vortex shedding. With increased flow velocity, this process leads to chaotic flow phenomena being first evidence of turbulence. This paper describes the transitional flow characteristics in single channel elements such as bends and T-junction as well as around fins and posts in channels. These elements are used to augment the transport characteristics in microchannels for enhanced heat and mass transfer and for performing chemical reactions in microreactors. The profound understanding of the flow characteristics is fundamental for the understanding of transport phenomena. Additionally, this knowledge can be used to design successful microstructured devices for various applications by knowing how to generate and control vortices in microchannels. Concepts from chaotic advection are presented here to describe vortex flow and related transport characteristics. Though recent advances has shed new light on transport phenomena in complex channel structures, many issues are still unknown and huge potential is hidden in optimized channel devices.

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