Millimetric core-shell drops via buoyancy assisted non-confined microfluidics

Core-shell drops having millimetric dimensions have attracted a wide range of applications. Such drops cannot be achieved in typical microfluidic devices due to buoyancy-driven phase separation difficulties at low flow rates. A buoyancy assisted approach based on two-phase flow (core and shell phase) with the outer phase as quiescent and non-confined, is presented for producing large core-shell drops. Buoyancy was found to be the dominant force for drop formation in the surfactant-free system, but considerably less effective in the presence of surfactants. The drop formation was mostly limited to the dripping regime when the surfactant concentrations were low. A simple force-balance model is developed for the prediction of the core-shell and the core drop sizes, which are validated against the data obtained in a surfactant-free system. The suggested method gives highly monodisperse (coefficient of variation smaller than 3%) core-shell drops (radius R similar to 800-3000 mu m) with a wide range of absolute (t similar to 30-1000 mu m) and relative (t/R similar to 0.03-0.80) shell thickness. (C) 2015 Elsevier Ltd. All rights reserved.