Investigation of drop coalescence using tomographic PIV*

High-speed tomographic PIV was used to obtain evolving volumetric velocity fields of the coalescence of single drops and two side-by-side drops through liquid/liquid interfaces. Reynolds numbers (Re=$\rho _{s}$U$_{\sigma }$D/$\mu _{s})$ based on surface tension velocity (U$_{\sigma }$=D/t$_{\sigma })$ and surrounding ambient fluid were 8-10, and the viscosity ratio between the fluid drop and surrounding fluid was 0.14. The coalescence process investigated is driven by gravity and thus the initial drops are non-spherical and the interface is deformed by the drops. Previously, Mohamed-Kassim {\&} Longmire (2004) showed that under these conditions, the film rupture typically occurs off-axis, and therefore the flow is three-dimensional. For a single drop, volumetric velocity vector fields are used to characterize the asymmetric film rupture occurring for 0$<$t/t$_{\sigma }<$0.1 and the subsequent symmetric development of the velocity and vorticity fields for 0.1$<$t/t$_{\sigma }<$1.6. It is shown that even though the film rupture occurs off-axis, the capillary waves and the collapse of the drop into a vortex ring are relatively axisymmetric. For two side-by-side drops, the first drop to coalesce ruptures off-axis on the side closest to the second drop. The volumetric velocity and vorticity fields indicate an asymmetric collapse of the drop for 0.1$<$t/t$_{\sigma }<$1.6 due to the deformation of the interface by the second drop while the capillary waves are axisymmetric.