  Hypothesis: Droplet coalescence is a common phenomenon and plays an important
role in many applications. When two liquid droplets are brought into contact, a
liquid bridge forms and expands quickly. Different from miscible droplets, an
extra immiscible interface exists throughout the coalescence of immiscible
droplets and is expected to affect the evolution of the liquid bridge, which
has not been investigated. We hypothesized that the liquid bridge of immiscible
droplets exhibits a different growth dynamics. Experiments: We experimentally
study the coalescence dynamics of immiscible droplets. The evolution of the
liquid bridge is measured and compared with miscible droplets. We also propose
a theoretical model to analyze the effects of immiscibility. Findings: We find
that immiscibility plays different roles in the viscous-dominated and
inertia-dominated regimes. In the initial viscous-dominated regime, the
coalescence of immiscible droplets follows the linear evolution of the bridge
radius as that of miscible droplets. However, in the later inertia-dominated
regime, the coalescence of immiscible droplets is slower than that of miscible
droplets due to the water-oil interface. By developing a theoretical model
based on the force balance, we show that this slower motion is due to the
immiscible interface and the extra interfacial tension. In addition, a modified
Ohnesorge number is proposed to characterize the transition from the
viscous-dominated regime to the inertia-dominated regime.
