  Head-on collisions of two immiscible liquid droplets lead to a collision
complex, which may either remain stable in the form of a single compound drop,
or fragment into two main daughter droplets. This paper investigates the liquid
distribution developing in the two daughter droplets and which can be of three
types. Either two encapsulated droplets (single reflex separation) form, or a
single encapsulated drop plus a droplet made solely of the encapsulating
liquid, which can be found either on the impact side (reflexive separation) or
opposite to it (crossing separation). A large number of experimental and
simulation data covering collisions with partial and total wetting conditions
and with Weber and Reynolds numbers in the ranges of 2 - 720 and 66 - 1100,
respectively, is analyzed. The conditions leading to the three mentioned liquid
distributions are identified and described based on the decomposition of the
collision in two phases: (i) radial extension of the compound droplet into a
lamella and (ii) its relaxation into an elongated cylindrical droplet. In
accordance with these two phases, two dimensionless parameters, $\Lambda =
{\rho_i/\rho_o} {{We_i}^{-1/2}}$ and $N =
{\nu_o/\nu_i}~{\sigma_{o}/\sigma_{io}}$, are derived, which are built on the
collision parameters and liquid properties of the encapsulated inner droplet
(i) and the outer droplet (o) only. In agreement with the proposed
interpretation, the combination of these two parameters predicts the type of
liquid distribution. The predictions are found to be in very good agreement
with both experimental and numerical results.
