CµFD: Simulation of Microfluidic Systems

We discuss the applicability of standard CFD techniques in µ-fluidics, highlight challenges and introduce methods which allow for corresponding extensions. The focus is especially on problems of major practical importance, namely liquid mixing in micro mixers and free-surface micro flows. With respect to the former two approaches are presented allowing eliminating the problems due to numerical discretisation errors for specific cases of miscible liquids. In the case of emulsion formation of immiscible liquids the RayleighPlateau decay is identified to be the driving mechanism for droplet formation under certain process conditions. Furthermore we investigate capillary filling of a narrow slit, where a special focus is put on the effects induced by a dynamic variation of the contact angle. The test case shows that on practically manageable grids the dynamic behaviour of the contact angle cannot be sufficiently reproduced. A result of the studies performed is the possibility to incorporate the correct contact-angle dynamics even on comparatively coarse grids by introducing a macroscopic slip range at the 3-phase contact line. By virtue of such an “artificial slip method”, correct results for free-surface micro flows can be obtained on grids with a moderate number of computational cells, without the need to resolve the contact line very accurately. Finally, a micro fluidic application involving both mixing of miscible liquids and free-surface flows is exemplarily outlined as one of the future challenges.