Dual roles of liquid viscosity in influencing bouncing and coalescence of two identical droplets

Droplet collision experiments and VOF simulations were conducted for a wide range of Ohnesorge numbers (Oh) with the particular focus on clarifying the role of liquid viscosity in influencing droplet bouncing and coalescence. The prominent experimental finding is that increasing Oh from 0.009 to 0.621 tends to enhance and then suppress droplet bouncing. The VOF simulations revealed the underlying physics of gas film drainage, which is modulated by liquid viscosity. In the range of Oh=0.009∼0.067, the gas film drainage time is dominantly affected by the drainage velocity in the early stage of droplet collision when the droplet deformation is small. Increasing Oh attenuates the drainage velocity due to the reduced impact kinetic energy by viscous dissipation. The drainage velocity attenuation prolongs the drainage time and subsequently enhances droplet bouncing. In the range of Oh=0.067∼0.621, the gas film drainage time is dominantly affected by the film length in the late stage of droplet collision when the droplet deformation is large. Increasing Oh suppresses gas film spreading and therefore shortens the film length for drainage, consequently suppressing droplet bouncing.