Surfactant effects on drop formation in co-flowing fluid streams

We conduct a numerical study to investigate the effect of surfactants on the drop formation process in a co-flowing system using a hybrid volume-of-fluid (VOF) method combined with a front-tracking scheme. The drop and bulk phases are treated as incompressible Newtonian fluids, and the surfactants are modeled using a Langmuir adsorption framework. We consider the effect of soluble surfactants in the adsorption–desorption limit on the drop formation process. A drop in the co-flowing geometry typically breaks up at the primary neck, close to the primary drop, in the absence of surfactants. When surfactants are present, they accumulate in the neck region resulting in Marangoni stresses that slow down the neck thinning rate. This results in longer breakup times with larger drop volumes. At high surfactant coverages, the primary neck formation slows down enough and breakup occurs at the secondary neck, close to the remnant drop. Increasing the outer co-flowing flow weakens the retarding effect of the high surfactant coverage leading to breakup again at the primary neck. The adsorption–desorption kinetics also affects the neck breakup position, and the primary drop volume and breakup time depend non-linearly on the Biot number. For any given Biot number, a critical fractional coverage exists beyond which the drop fails to neck. The presence of a confining wall may lower the value of the critical equilibrium fractional coverage required for the drop to enter the no-necking regime.

Neck formation modes are affected by the equilibrium fractional coverage, x and the flow rate ratio, Q2/Q1. Figure optionsDownload as PowerPoint slideHighlights
► Numerical study of drop formation in a co-flowing geometry with soluble surfactants.
► Surfactants accumulate in the neck region slowing down the primary neck formation.
► Outer co-flowing flow washes away surfactants from neck weakening Marangoni stresses.
► Fractional coverage, flow rate ratio, and Biot number affects necking location.
► Necking can be completely inhibited by moving the confining walls inwards.