Equilibrium Morphological Phase Diagram of Drops in Hydrophilic Cylindrical Channels

The equilibrium morphology of a drop in a horizontal tube can provide useful information for two-phase flow in microfluidics devices in which the capillary force dominates. A drop-in-tube system is analogous to a drop-on-fiber one and two conformations are obtained, adhered drop and liquid slug, by the approaches of experiments and surface evolver (SE) simulations. The adhered drop conformation tends to exist at small volume, whereas the liquid slug conformation is favored at larger volume. Around the transition volume between the two conformations, both morphologies can coexist. The experimental results are consistent with those of simulation outcomes. The morphological phase diagram of the drop-in-tube system is constructed via SE simulations by varying the drop volume and contact angle. Three regimes can be identified through the upper and lower boundary curves: adhered drop only, liquid slug only, and coexistence. Compared to the case with negligible gravity, the adhered drop is more favored than the liquid slug in the presence of gravity. As a result, the coexistence regime expands substantially.