Coalescence dynamics of two droplets of different viscosities in T-junction microchannel with a funnel-typed expansion chamber

The coalescence behavior of two droplets with different viscosities in the funnel-typed expansion chamber in T-junction microchannel was investigated experimentally and compared with droplet coalescence of the same viscosity. Four types of coalescence regimes were observed: contact non-coalescence,squeeze non-coalescence, two-droplet coalescence and pinch-off coalescence. For droplet coalescence of different viscosities, the operating range of non-coalescence becomes narrowed compared to the droplet coalescence of same viscosity, and it shrinks with increasing viscosity ratio η of two droplets,indicating that the difference in the viscosity of two droplets is conducive to coalescence, especially when1<η<6. Furthermore, the influences of viscosity ratio and droplet size on the film drainage time(T) and critical capillary number(Ca)c) were studied systematically. It was found that the film drainage time declined with the increase of average droplet size, which abided by power-law relation with the size difference and viscosity ratio of the two droplets: T～(l)and T～(η). For droplet coalescence of same viscosity, the relation of critical capillary number with two-phase viscosity ratio and dimensionless droplet size is Ca= 0.48 λl, while for droplet coalescence of different viscosities,the scaling of critical capillary number with dimensionless average droplet size, dimensionless droplet size difference and viscosity ratio of two droplets is Ca= 0.11ηll.