(383i) The Surfactant Effect On the Dynamics of Drop Coalescence

Processes involving drop coalescence are commonly encountered in nature, as well as in spray , combustion and propulsion systems. A difficulty that arises in such systems is that due to the small time scale characteristics of the coalescence process in the conditions of interest, it is very difficult to capture and therefore control the details of the coalescence process.As a consequence of this difficulty, computational simulations may help to better understand the underlying physical mechanism behind drop coalescence.  Recent studies have mainly focused on the dynamics of the coalescence between two intact drops. Little is known about the effect of the insoluble surfactant on the dynamics of the coalescence despite the  widespread industrial usage.   Here we present direct numerical simulations that include the surfactant effect showing that the process of the coalescence is retarded with the existence of the surfactant on the interface that generates a significant surface tension gradient during the process at the bridge connecting the two drops .   A strong  Maranogoni stress is therefore induced , driving more fluid underneath the surface away from the neck region.  Since direct simulations of the free-surface system of governing equations, including fully coupled continuity, momentum and surfactant convection-diffusion equation, enabled a comprehensive analysis of the dynamics of the  coalescence  of two drops contaminated with insoluble surfactant , results from this study can have a significant impact on the more rational design of surfactant  additives for spray and propulsion systems.