Role of Marangoni stress during breakup of surfactant-covered liquid threads: Reduced rates of thinning and microthread cascades

Adsorption onto and lowering of surface tension sigma of fluid interfaces by surfactants is exploited in drop formation (e.g., inkjet printing) where a thinning liquid thread (radius h) connects an about-to-form drop to the liquid that remains hanging from the nozzle when the former falls from it. Surfactants can affect thread pinch-off in two ways: first, by lowering s, they lower capillary pressure (sigma/h), and second, as surfactant concentration along the interface can be nonuniform, they cause the interface to be subjected to a surface tension gradient or Marangoni stress. Recent studies show that the location where the thread breaks is devoid of surfactant, and others assert that the influence of Marangoni stress on pinch-off is negligible. We demonstrate by simulations and experiments that surfactants play a major role in drop formation and that Marangoni stresses acting near but not at the pinch point give rise to reduced rates of thread thinning and formation of multiple microthreads that distinguish pinch-off of surfactant-covered threads from surfactant-free ones. Thinning at finite Reynolds and Peclet numbers, Re and Pe, is shown to exhibit intermediate scaling regimes that have heretofore only been observed during pinch-off of threads undergoing creeping flow (Re = 0) while convection of surfactant is weak compared to its diffusion (Pe < 1).