Drop coalescence in planar extensional flow and gravity

A film drainage model, which allowed for the effect of the ratio of the viscosities of the drop and liquid film on the interfacial mobility, is found to represent reasonably well the published experimental data on the coalescence times of drops in two different flow fields at a constant force. The data of Yang et al. [2001. The coalescence of two equal-sized drops in a two-dimensional linear flow. Physics of Fluids 13, 1087-1106] on the head-on collision of two equisized drops at the center of the inflow axis under planar extensional hyberbolic flow in a four-roll mill, and the data of several authors on the coalescence time of a single drop with its bulk homophase at a deformable interface under gravity are considered. The scaled coalescence time increased linearly with the product of the viscosity ratio and the applied external to interfacial tension forces ratio over three orders of magnitude. Higher applied external forces resulted in larger film areas while more viscous drops reduced the interfacial mobility thereby increasing the coalescence times.