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

Abstract

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 when 1＜ η ＜6. Furthermore, the influences of viscosity ratio and droplet size on the film drainage time ( T dr ) 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 dr ~ ( l d ) 0.25±0.04 and T dr ~ ( η ) −0.1±0.02 . For droplet coalescence of same viscosity, the relation of critical capillary number with two-phase viscosity ratio and dimensionless droplet size is Ca c = 0.48 λ 0.26 l −2.64 , 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 c = 0.11 η −0.07 l s −2.23 l d 0.16 .