3D visualization of convection patterns in lab-on-chip with open microfluidic outlet

Open-outlet microfluidics is getting more and more attention, thanks to the generation of capillarity-driven flows which simplify the connection with the macro-world. It is known that convection flows are generated at the interface with air, i.e., the meniscus. Several works have investigated evaporation-induced convection, but its effect on particle position control in open-outlet biodevices is still not characterized. In this paper, we present the results of 3D measurement of particle traces near the meniscus in an open-outlet vertical 400 μm micro-channel filled with a water-based saline solution. Using a standard optical microscope and a system of mirrors, we observe the 3D position of individual micro-beads floating in the solution, in a way akin to particle image velocimetry technique. A single vortex is generated at the meniscus and occupies the whole region under observation at a distance of 1.5–2.7 mm from the meniscus. The generation of the convection pattern and the vortex rotational speed are described. The convection patterns disappear when evaporation is inhibited, while both the vortex generation and the rotational speed are faster for highly saline solutions. These results are relevant to the design of biochips which require control of the particle position in a fluid since they emphasize that in open-outlet microfluidic systems not only the gravitational fall but also the convection drag must be counteracted.