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 = 0.48λ^0.26l^-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 = 0.11η^-0.07l_s^-2.23l_d^0.16.

Key words: Microchannel, Droplet, Coalescence, Viscosity ratio, Critical capillary number

摘要： 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 = 0.48λ^0.26l^-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 = 0.11η^-0.07l_s^-2.23l_d^0.16.

关键词: Microchannel, Droplet, Coalescence, Viscosity ratio, Critical capillary number