Published online by Cambridge University Press: 06 May 2024
A lattice Boltzmann method is used to explore the effect of surfactants on the unequal volume breakup of a droplet in a T-junction microchannel, and the asymmetry due to fabrication defects in real-life microchannels is modelled as the pressure difference between the two branch outlets ($\Delta {P^\ast }$). We first study the effect of the surfactants on the droplet dynamics at different dimensionless initial droplet lengths (
$l_0^\ast $) and capillary numbers (Ca) under symmetric boundary conditions (
$\Delta {P^\ast } = 0$). The results indicate that the presence of surfactants promotes droplet deformation and breakup at small and moderate
$l_0^\ast $ values, while the surfactant effect is weakened at large
$l_0^\ast $ values. When the branch channels are completely blocked by the droplet, a linear relationship is observed between the dimensionless droplet length (
$l_d^\ast $) and dimensionless time (
${t^\ast }$), and two formulas are proposed for predicting the evolution of
$l_d^\ast $ with
${t^\ast }$ for the two systems. We then investigate the effect of the surfactants on the droplet breakup at different values of
$\Delta {P^\ast }$ and bulk surfactant concentrations (
${\psi _b}$) under asymmetric boundary conditions (
$\Delta {P^\ast } \ne 0$). It is observed that, as
$\Delta {P^\ast }$ increases, the volume ratio of the generated droplets (
${V_1}/{V_2}$) decreases to 0 in both systems, while the rate of decrease is higher in the clean system, i.e. the presence of surfactants could cause a decreased pressure difference between the droplet tips. As
${\psi _b}$ increases,
${V_1}/{V_2}$ first increases rapidly, then remains almost constant and finally decreases slightly. We thus establish a phase diagram that describes the
${V_1}/{V_2}$ variation with
$\Delta {P^\ast }$ and
${\psi _b}$.