In lab-on-chip devices, on which complete (bio-)chemical analysis laboratories are miniaturized and integrated, it is essential to manipulate fluids in sub-millimetre channels and sub-microlitre chambers. A special challenge in these small micro-fluidic systems is to create good mixing flows, since it is almost impossible to generate turbulence. We propose an active micro-fluidic mixing concept inspired by nature, namely by micro-organisms that swim through a liquid by oscillating microscopic hairs, cilia, that cover their surface. We have fabricated artificial cilia consisting of electro-statically actuated polymer structures, and have integrated these in a micro-fluidic channel. Flow visualization experiments show that the cilia can generate substantial fluid velocities, up to 0.6 mm s⁻¹. In addition, very efficient mixing is obtained using specially designed geometrical cilia configurations in a micro-channel. Since the artificial cilia can be actively controlled using electrical signals, they have exciting applications in micro-fluidic devices.

A perspective is given on fictitious domain methods for deformable bodies that exert large motions induced by unsteady flow. In these methods an Eulerian and Lagrangian formulation are employed for the fluid and solid, respectively, and both bodies are coupled using a Lagrange multiplier. The use of a multiplier allows the solid not to be an integral part of the fluid mesh. Three variations of the fictitious domain method that have been published before, are compared to an ALE method in two numerical experiments and in conclusion the advantages, disadvantages and differences for the different approaches are discussed.