A Valve-Less Capillary System: A Novel Approach for Passive Flow Control on Chip Ahmed Taher 1,2* , Benjamin Jones 1 , Paolo Fiorini 1 and Liesbet Lagae 1,2 1 Life Science Technologies Department, IMEC v.z.w., Leuven, Belgium and 2 Department of Physics, KU Leuven, Leuven, Belgium ABSTRACT A major challenge in passive microfluidics is providing a reliable means of flow control. In this paper, we propose a novel concept for controlling the liquid flow in a capillary microfluidic system based on capillary pressure difference in a branched network of microchannels. A capillary system was designed to mix two fluids while avoiding the introduction of air bubbles using this valve-less concept. The system does not use either active or passive capillary trigger valves unlike typical mixing sections for capillary-driven systems. The design concept was verified by experimental characterization of the fabricated devices. A simple analytical criteria for whether or not the devices function correctly was developed and a comparison with experimental results yielded excellent agreement. KEYWORDS: Capillary microfluidics, Valve-less, Passive valves, lab-on-a-chip INTRODUCTION Most of point of care devices use microfluidic systems to achieve portable rapid analysis of small sample volumes for wide range of biological applications. Capillary based microfluidic devices are perfectly suitable for such applications as they do not need external power for fluids manipulation. However, a major challenge in passive microfluidics is providing a means of flow control. Typically valves, such as capillary trigger valves [1-2], are used. However, planar micromachined capillary trigger valves have proven to be unreliable [1]. Safavieh and Juncker [3] reported a two-level valve design in conjunction with a hydrophobic cover to improve reliability. The drawback to this design is the increase in complexity of the manufacturing process. Here we propose a novel concept for controlling the liquid flow in a capillary microfluidic system without using valves. The design concept was verified by experimental characterization of the fabricated devices. DESIGN CONCEPT The basic concept of the valve-less capillary system relies on a branched network of microchannels of varying width, as shown in figure 1. Fluid will flow preferentially in branch 1 as long as the condition of equation 1 is satisfied. A sudden widening of branch 1 causes the flow to stop in this branch and the flow is diverted into branch 2 (again, as long as equation 1 is satisfied). Figure 1: A schematic drawing of capillary flow in a branched microchannel. The fluid moves in a channel of fluidic resistance Rup (a). When the flow arrives to the branching point (b), in general it is di- vided between the two branches but if the condition of Eq. 1 is satisfied it flows only in the first branch. If there is an expansion in the first branch (c), the flow stops and starts to flow in the second branch if the condition of Eq. 1 can be applied between Pc2 and Pc3. 1770 978-0-9798064-8-3/μTAS 2015/$20©15CBMS-0001 19 th International Conference on Miniaturized Systems for Chemistry and Life Sciences October 25-29, 2015, Gyeongju, KOREA