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Chris Kleijn (TUD) Dynamics of droplet breakup in a T- junction The application of bubbles and droplets in microfluidics requires precise control and manipulation of droplet sizes. One of the basic techniques to manipulate droplet sizes is the passive breakup of droplets at a T-junction: a single droplet is split into two daughter droplets by extensional flow [1]. In this paper, we present a computational fluid dynamics (CFD) study on the dynamical behaviour of the breakup of droplets in a three-dimensional T-junction microchannel. A finite volume based opensource CFD package, OpenFOAM [2], was used to perform the numerical simulations. The fluid interface was modeled using the Volume of Fluid (VOF) method with interface sharpening techniques. We quantitatively describe the breakup process and the mechanism of droplet pinch-off. Similar to the formation of droplets at T-junctions [3], the interface of the droplet first moves at a nearly constant speed, after which it rapidly collapses prior to breakup. As we will show, droplet dynamics in two- and three-dimensional simulations is similar at the first stage but significantly different prior to breakup. Two-dimensional simulations do not capture the rapid collapse such that the breakup time is significantly overestimated. By contrast, the rapid collapse is accurately captured in threedimensional simulation and its onset agrees well with our theoretical model. The results presented in this paper are useful for the design of droplet-based laboratories-on-a-chip and stress the need to perform simulations in three dimensions to capture the interfacial dynamics of droplet in microfluidic devices. References: [1] D. R. Link, S. L. Anna, D. A. Weitz & H. A. Stone (2004). Geometrically mediated breakup of drops in microfluidic devices. Phys. Rev. Let. 92, 054503.& [2] H. G. Weller, G. Tabor, H. Jasak and C. Fureby (1998). A tensorial approach to computational continuum mechanics using object-oriented techniques. Com. in Phys. 12, 6, 620-631.& [3] V. van Steijn, C. R. Kleijn & M. T. Kreutzer (2009). Flows around confined bubbles and their importance in triggering pinch-off. Phys. Rev. Let. 103, 214501.