Computational Microfluidics for Miniaturized Bio-Diagnostics Devices using the Multiphysics code “TransAT” C. Narayanan, D. Lakehal * *ASCOMP GmbH, Zurich, Switzerland, lakehal@ascomp.ch ABSTRACT In this paper we have presented new developments achieved on the computational microfluidics front using the dedicated code TransAT. In particular, we have shown how it behaves for the prediction of the Marangoni effects within the Interface Tracking Concept, which can be used in controlling the dynamics of micro-droplets in bio-chips. We have also presented first results of a sub-grid scale ultra-thin film model, capable to mimic the wettability of inner surfaces allowing for a realistic representation of what might be expected in arterial microbubble delivery in gas embolotherapy. Keywords: Microfluidics, Thin fim, Marangoni, effects 1 INTRODUCTION Microfluidic devices are now used for such diverse applications as DNA microarrays, drug screening, sensors, and in clinical and forensic analysis. Typical microfluidics flows feature free-surface motion evolving (sometimes) in porous media or as falling films, spreading and dewetting of (complex) liquids on solid or liquid substrates, chemical reaction of binary mixtures, micro-bubbles and beads control and manipulation, phase change or transition. The control of such micro-flow systems is central to future technological advances in emerging technologies, like biological reactors, microreactors, biochannel arrays, and labs-on-chip. It is expected that robust, accurate and fast response computational microfluidics solutions will play a key role in the development in this new business segment. In practical microfluidics applications the flow involves phenomena acting at different time and length scales. At each level of the scale cascade, the physics of the flow is amenable to numerical prediction by scale-specific strategies. In this paper will present our recent computation results obtained with our CFD code TransAT, in which interfacial flows are treated using the Level Set method. As bio-chips may comprise various components, a new fully automatized version has been developed for microfluidics applications, using IST (Immersed Surfaces Technology) to map complex geometries into a rectangular Cartesian grid. Since IST forces the grid to remain Cartesian and equidistant, high-order schemes (up to 3rd order for flux convection and 3 rd order WENO schemes for free surface flows) can maintain their high degree of accuracy. Further, to better resolve boundary-layer regions, near wall flow areas are treated by another new feature, namely the BMR (Block- based Mesh Refinement), in which sub-scale refined blocks are placed around each component. The combination IST/BMR can save up to 70% grid cells in 3D. In this paper we discuss simulation examples treated with this approach (see details in [1]). We will particularly focus on the role played by the Marangoni effects in controlling the dynamics of micro-droplets in bio-chips, and the way ultra- thin film can be predicted using a sub-grid scale model. 2 PREDICITING INTERFACIAL MICROFLUIDCS FLOWS Interfacial flows refer to multi-phase flow problems that involve two or more immiscible fluids separated by sharp interfaces which evolve in time. Typically, when the fluid on one side of the interface is a gas that exerts shear (tangential) stress upon the interface, the latter is referred to as a free surface. Interface tracking methods (ITM) are schemes capable to locate the interface, not by following the interface in a Lagrangian sense (e.g., by following marker points on the interface), but by capturing the interface by keeping track, in an Eulerian sense (the grid is fixed), of the evolution of an appropriate field such as a level-set function or a volume-fraction field. Examples and classifications are provided in [2]. Application of ITM’s to microfluidics flows requires particular attention to the way surface forces and triple-line dynamics are handled. 2.1 Mathematical Formulation ITMs are based on solving a single-fluid set of conservation equations with variable material properties and surface forces. The coupled fluid and heat transfer equations in incompressible flow conditions take the form . 0 ∇ = u (1) ( ) ( ) ( ) . . t s g w pI F F F ρ ρ μ ∂ +∇ + =∇ ∇ + + + u uu u (2) ( ) ( ) ( ) . . "' t CpT CpT T Q ρ ρ λ ∂ +∇ =∇ ∇ + u (3) where u is the velocity vector, ρ is the density, p is the pressure, I is the identity matrix, and μ is the dynamic viscosity. The source terms in (2) represent body forces (F g ), surface tension (F s ) defined by NSTI-Nanotech 2008, www.nsti.org, ISBN 978-1-4200-8505-1 Vol. 3 429