Quasi-two-dimensional coalescence of liquid in another liquid

Coalescence of liquid drops is a phenomenon familiar in everyday life. However, its dynamics has been actively explored in recent years. For example, in a confined space of Hele-Shaw cell, the coalescence of a highly viscous drop with a bath of the same liquid was studied, when the drop and bath were surrounded by a low viscosity liquid. It was found that the size of the neck bridging the drop and bath grows in two stages, both of which are governed by the viscosity of the drop[1].
Here, we performed the same experiment but with the viscosity contrast reversed (the drop viscosity is lower than the surrounding liquid) to seek different scaling regimes, by using a high-speed camera and a microscope lens. As a result, we discovered an initial regime in which the growing velocity of the neck is independent of the viscosity of the drop or the surrounding liquid. This is in contrast with a previous study concluding that the initial viscous dynamics is always governed by the viscosity of the drop[2].
[1] Yokota&Okumura, PNAS, 108 (2011) 6337.
[2] Paulsen, et al., Nat. Common., 5 (2014) 3182.