THURSDAY, Feb. 15, 12 p.m. 11 FLOOR OF THE GAMOW TOWER
Alberto Fernandez-Nieves
Division of Engineering and Applied Sciences Harvard University
Drops and jets in co-flowing liquids
We study the dripping-to-jetting transition of a liquid that is injected into a second, immiscible, coaxially flowing liquid; a slightly more complicated problem than dripping and jetting in a conventional kitchen faucet. We show that these transitions are controlled by two non-dimensional numbers: The capillary number of the outer liquid (Caout) and the Weber number of the inner liquid (Wein). These numbers establish the relative importance of viscous and inertial forces with respect to surface tension forces, respectively. When jetting is forced by Caout, the diameter of the jet narrows in the downstream direction. For these jets, the drop diameter and jet length scaling are well predicted assuming that the Rayleigh-Plateau instability is convected downstream. However, when jetting is forced by Wein, the diameter of the jet widens in the downstream direction. In this case, the resultant drop size can not be predicted assuming that the Rayleigh-Plateau instability drives the break-up of the jet. Instead, we believe that these jets break-up due to an absolute instability: Perturbations grow in time at a fixed spatial location irrespective of external noise. Overall, our results highlight the importance of the instability nature for producing stable monodisperse emulsions using microfluidic-based devices.