Effects of surfactant on lift coefficients of bubbles in linear shear flows

Effects of surfactant on the lateral motion of bubbles in linear shear flows are numerically investigated by using an interface tracking method. Ranges of the bubble Reynolds number Re, Eötvös number Eo, Morton number M and dimensionless shear rate Sr are 2 < Re < 70, 0.6 < Eo < 5, 10−6 < M < 10−4 and 0 ≤ Sr < 1, respectively. The main conclusions obtained are as follows: (1) the lift coefficient, CL, of a bubble decreases as the Langmuir number increases, (2) the surfactant decreases the critical values of Eo and Re, at which CL changes its sign, (3) the reduction in CL of bubbles of large Hatta numbers due to the presence of surfactant can be understood from the surface tension reduction, and (4) the Marangoni force affects CL of small Hatta number bubbles, and therefore, the surface immobilization effect of the Marangoni force should be taken into account to model CL of contaminated bubbles.