Lagrangian flow structures in 3D AC electro-osmotic microflows

Flow forcing by AC electro-osmosis (ACEO) is a promising technique for actuation and manipulation of microflows. Utilisation to date mainly concerns pumping and mixing. However, emerging micro-fluidics applications demand further functionalities. The present study explores first ways by which to systematically realise this in three-dimensional (3D) microflows using ACEO. This exploits the fact that continuity "organises" Lagrangian fluid trajectories into coherent structures that geometrically determine the transport properties. 3D Lagrangian flow structures typically comprise families of concentric (closed) streamtubes, acting both as transport barriers and transport conduits, embedded in chaotic regions. Representative case studies demonstrate that ACEO, possibly in combination with other forcing mechanisms, has the potential to tailor these features into multi-functional Lagrangian flow structures for various transport purposes. This may pave the way to "labs-within-a-channel" that offer the wide functionality of labs-on-a-chip yet within one microflow instead of within an integrated system