Fig. 1. An explanation of the impact parameter B , whereb is the distance from the centre of one drop to the relative
velocity vector placed to the centre of the other drop (m),v1 and v2 are the
respective speeds of smaller and bigger drops (m/s) andD1 and D2 are the
respective diameters of the drops (m).
Drop size ratio γ
\(\gamma=\frac{D_{2}}{D_{1}}\) (3)
\(\Delta=\frac{D_{1}}{D_{2}}\) (4)
Depending on these three parameters, the collision of two drops may have
five possible results \cite{Bre11,Nik09,Nob95,Jia03,Sar12,Kri15,Zha16,Est99,Ash17,Foc13}:
- slow coalescence,
- bounce,
- coalescence,
- reflexive separation,
- stretching separation.
The possible results of the collision of drops have been given in Fig.
2 \cite{Kim09}.
In case of reflexive and stretching separation, the satellite drops are
formed in addition to daughter drops. The mechanism of the formation of
satellite drops is described by the Plateau-Rayleigh instability \cite{Ray78,Wor08}. The
diameter d sat and numberN sat of satellite drops can be modelled using the
Munnannur-Reitz model \cite{Mun07}, whereby both of these depend on the Weber number.
Fig. 3 depicts the possibilities B = f (We ) as a
diagram \cite{Qia97}.
Situation B = 0 corresponds to the frontal impact of two drops.
The colliding drop size ratio in this diagram is γ = 1, which
corresponds to the situation in which the colliding drops have equal
diameters. If γ is increased to values 100 and more, then
cohesion forces increase the probability of coalescence of the drops. If
the ambient pressure p is increased, then the slow coalescence
area disappears as it becomes harder during collision to squeeze out the
gas (air) between the drops.