Keywords: Lab-on-a-CD, centrifugal pressure, capillary valve, surface tension, two-phase microfluidics, CFD.

Flow sequencing is probably the most important task performed by Lab-on-a-CD devices since it is responsible for the movement of the liquid between its various components [1]. An incorrect conception and distribution of these components may cause an undesired mixing between two liquids compromising the accomplishment of the required tasks. To solve this problem Lab-on-a-CD uses capillary valves [2].

Capillary valves are pressure barriers created by a sudden expansion in the microchannel cross section and rely on the capillary force to stop the flow. Hence, the capillary force keeps the flow blocked until a change in the liquid pressure occurs [3]. Taking into account the capillary valves main purpose, the criterion employed in this study considered the valve to be overcome only when the liquid volume fraction inside the downstream microchannel (located after the valve) was different from zero. Based on this criterion the influence of the valve on liquid flow pattern and, consequently, on the imposed pressure value was studied recurring to a commercial computational fluid dynamics (CFD) package (Gambit®2.2.30 and FLUENT®6.2.16).

The numerical results obtained revealed several interesting aspects. First, it was verified the major importance that surface tension phenomenon has in the liquid flow progression, acting simultaneously as a resistance (due to the liquid-gas interfacial surface tension) and as a driving flow force (due to the hydrophilic wetting capillary phenomenon in the walls). Second, it was demonstrated how these two ambiguous characteristics influence the valve filling process. This was performed studying the same flow with and without the presence of the surface tension. Finally, the influence of a valve geometrical parameter (the height, in the case studied) over the value of the imposed pressure required for the liquid to overcome the valve was investigated. This last study revealed a linear increase between these parameters.

1

M. Grumann, T. Brenner, C. Beer, R. Zengerle, J. Ducrée, "Visualization of flow patterning in high-speed centrifugal microfluidics", Review of Scientific Instruments 76 - American Institute of Physics, 025101, 1-6, 2005. doi:10.1063/1.1834703

2

J. Zeng, K.B. Greiner, M. Deshpande, J.R. Gilbert, "Fluidic capacitance model of capillary-driven stop valves", ASME, 1-7, 2000.

3

J.V. Zoval, M.J. Madou, "Centrifuge-Based Fluidic Platforms", Proceedings of the IEEE, 92(1), 140-153, 2004. doi:10.1109/JPROC.2003.820541

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