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}:
  1. slow coalescence,
  2. bounce,
  3. coalescence,
  4. reflexive separation,
  5. 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.