The coalescence of an impinging droplet colliding with a sessile droplet
at an angle(θi) is investigated by numerical simulation. The range of θi
is 0° - 60° and the surface wettability are set as hydrophilic or
hydrophobic, and both of them can affect the droplet mergence behavior.
By using a modified mixing function, the dimensionless total mixing time
τm can be calculated. The results show that there is no clear effect of
θi on τm on a hydrophobic surface, while τm increases as θi increases on
the hydrophilic surface. With the Weber number(We) ranging from 5.65 to
22.7 and the Ohnesorge number(Oh) ranging from 0.136 to 0.214, we find
τm hardly changes with We and Oh. By dividing the mergence and mixing
process in a convection and a diffusion stage, we find that the
diffusion is much larger than the convection time.