Near-contact motion of surfactant-covered spherical drops: Ionic surfactant

A lubrication analysis is presented for near-contact axisymmetric motion of spherical drops covered with an insoluble nondiffusing surfactant, The sur factant equation of state is arbitrary; detailed results are presented for ionic surfactants. The qualitative behavior of the system is determined by the dimensionless force parameter (F) over cap, the external force normaliz ed by the maximum resistance force generated by Marangoni stresses. For (F) over cap > 1 drops coalesce on a time scale commensurate with the coalesce nce time tau(0) for drops with clean interfaces. For (F) over cap < I, the system evolves on the time scale tau(0) until Marangoni stresses approximat ely balance the external force; thereafter a slow evolution occurs on the S tokes time scale. In the long-time regime a self-similar surfactant concent ration profile is attained that scales with the extent of the near-contact region. The gap width decreases exponentially with time but slower than for rigid particles because of surfactant backflow. For (F) over cap < I, drop coalescence does not occur without van der Waals attraction. Quantitative results depend only moderately on the surfactant equation of state. (C) 199 9 Academic Press.