  An advanced Volume of Fluid (VOF) method is presented that enables performant
three-dimensional Direct Numerical Simulations (DNS) of the interaction of two
immiscible fluids in a gaseous environment with large topology changes, e.g.,
binary droplet collisions. One of the challenges associated with the
introduction of a third immiscible phase into the VOF method is the
reconstruction of the phase boundaries near the triple line in arbitrary
arrangements. For this purpose, an efficient method based on a Piecewise Linear
Interface Calculation (PLIC) is shown. Moreover, the surface force modeling
with the robust Continuous Surface Stress (CSS) model was enhanced to treat
such three-phase situations with large topology changes and thin films. A
consistent scaling of the fluid properties at the interfaces ensures energy
conservation. The implementation of these methods in the multi-phase flow
solver Free Surface 3D (FS3D) allowed a successful validation. A qualitative
comparison of the morphology in binary collisions of immiscible droplets as
well as a quantitative comparison regarding the threshold velocities that
distinguish different collision regimes shows excellent agreement with
experimental results. These simulations enable the evaluation of experimentally
inaccessible data like the contributions of the kinetic, surface and
dissipative energy of both immiscible liquids during the collision process.
Furthermore, the comparison with binary collisions of the same liquids
highlights similarities and differences between the collisions. Both can
support the modeling of the immiscible liquid interaction in the future.
