Until recently, it was widely accepted that an ambient flow affects the near-contact motion of nearly-spherical drops only through the pushing force generated by hydrodynamic stresses acting on the drop interfaces; the ambient flow was believed to have no direct influence on the film drainage dynamics, even for drops with tangentially-mobile interfaces. According to this view, the near-contact motion of drops is the same whether pushed together by an ambient flow or by an equivalent set of body forces. Recently, however, the validity of this picture has been questioned.

We showed that an ambient flow can qualitatively affect film drainage and thus the near-contact motion of drops with tangentially-mobile interfaces, even under small-deformation conditions; noncoalescencing stationary states were found for drops in compressional flows and exponential film drainage was observed for drops in extensional flow (Nemer et. al., Phys. Rev. Lett., vol. 92, art. 114501, 2004). By contrast, Baldessari and Leal (Colloid Interface Sci., vol. 289, p. 262, 2005) present an argumentsupporting the traditional view, asserting that an ambient flow has no direct influence on film drainage under small-deformation conditions. In this presentation, we will explain why the ambient flow does, in fact, affect film drainage qualitatively at long times. We describe a new thin-film simulation that correctly incorporates the effects of the outer ambient flow. Our results support the predictions of Nemer et. al. (2004) for drops in compressional flows but also reveal sustained oscillations about the predicted stationary state for flow strengths below a critical value. We also present an analysis that explains exponential film drainage between drops in extensional flow.