Microbubbles for optofluidics: controlled defects in bubble crystals

We use liquid-gas microfluidics as a low cost, tunable microstructuring tool, with envisioned applications in optics. In order to obtain relevant geometries for optics, beyond simple self-assembled crystals, we propose an original approach excluding bubbles from chosen zones thanks to tiny pillars. To assess the strength of exclusion mechanism, we predict the behavior of a single flattened bubble in front of a 15 cylindrical thin pillar located in a rectangular microchannel. The model compares the hydrodynamic force Ffluid that pushes the bubble and the force Fs, due to surface tension, resulting from the surface augmentation when the bubble rises over the pillar. The resulting predictions have been confirmed by experimental results which showed that the bubble goes above the pillar if Fs < Ffluid, and goes around it in other 20 cases. Consistently with this model, bubble crystals with controlled lacuna defects of one, two or a line of bubbles have been successfully produced. Using a photosensitive polymer as a carrier liquid, static bubble crystals have also been realized.