Transient Profiles and Development of Laminar Flow
Flow in microchannels is exceptional because of the very low tendency to build turbulence and eddies. Even at quite high flow velocities the velocity field stays stratified – in technical language: the flow stays laminar. The velocity distribution inside the channel usually follows the Poiseuille profile in cylindrical channels and a more complicated though well-known profile composed of hyperbolic and trigonometric functions in rectangular channels.
However this is not always the case. It is worth to note that the flow profile shortly after inlets and changes of channel geometries may follow other profiles, which reach into the new channel geometry for some distance. However, these transient structures die out and only the well-known profiles survive on the long-term. If working close to the inlet or a perturbation inside the channel it has to be taken into account that the shear strength and the flow velocity in the channel centre may not be the expected ones. It is wise to measure such quantities only at some distance from inlets and perturbations where such transients have died out.