Formation of bubbles and foams in gelatine solutions within a vertical glass tube

Dispersions of air in gelatine solutions were experimentally investigated in a 2.75 mm circular vertical tube coupled to a microfluidic device. Air and gelatine solutions (1%, 3% and 5% w/w) were mixed in the microfluidic device to produce steady flows of microbubbles by controlling liquid and gas volumetric flow rates. A two-phase flow map and transition lines between flow regimes were examined. In the bubbling regime (e.g., low air fraction) it was possible to produce mono and polydispersed micron-size air bubbles of controllable sizes. In the foam regime (e.g., high air fraction) the foam structure mainly depended on the liquid flow rate. Moreover, uniform foams moved as rigid bodies along the capillary tube, lubricated at the wall by the gelatine solution.