Simulations of surfactant effects on the dynamics of coalescing drops and bubbles

Abstract

We present simulations of coalescence in the presence of surfactant. We consider a fluid-fluid interface where we track surfactant concentration. Our model is applicable to a soap bubble merging with a suspended soap film and to a surfactant covered liquid drop merging with a reservoir. In both cases, we determine the regime in which coalescence is only partial. Along with viscous effects, represented by the Ohnesorge number, the elasticity of the surface tension relative to the surfactant concentration is seen to play a key role and exhibits a surprising nonmonotonic influence, for which we present a physical mechanism. The effects of gravity are also simulated, along with effects of differing initial conditions, as well as those of uneven initial surfactant concentration, as are likely to arise in physical applications. We describe how the presence of surfactants can influence a coalescence cascade.

An experimental study on the drop/interface partial coalescence with surfactants

“…[23][24][25][26] Blanchette, Messio, and Bush 12 studied the influence of surface tension gradients on the partial coalescence of a droplet with a liquid/air interface and estimated that the surfactants would concentrate at the neck at the early stages of coalescence. The results agreed with recent numerical studies 2,27 which showed that surfactants accumulate at the edges as the neck grows after the film rupture, increasing significantly their concentration locally. Martin and Blanchette 2 found numerically that pinch-off and partial coalescence are inhibited in the presence of surfactants.…”

“…This low-pressure area has also been reported in the numerical studies by Ding et al 35 and Martin and Blanchette. 2 The reduced pressure drives the edges of the neck to contact with each other and causes the pinch-off at t = 1.37t σ .…”

Partial coalescence of soap bubbles

“…3,4, 7 The ratio is always in the range 0.35 < R D /R < 0.6, in good agreement with the simulations of Martin and Blanchette. 11 The distribution of R D /R in Figure 5(b) indicates a peak centered at 0.48 < R D /R < 0.50.…”

“…4 The presence of surfactants in the drop or bath can either inhibit or favor the phenomenon. 10,11 While the partial coalescence of drops has been widely investigated, 12,13 the analogous phenomenon for bubbles has only been considered by Martin and Blanchette, 11 whose numerical simulations indicate the possibility of partial coalescence. In this letter, we present the results of the first experimental investigation of the merger of a soap bubble with a soap film.…”

“…Moreover, the weakly damped residual waves on the film generated by prior events interfere with the process and may displace the bubble from the film. Martin and Blanchette 11 predict that up to 10 steps should be possible in such a cascade, but they assume that the film is quiescent and do not consider the possibility of trapping. Another scenario not considered by Martin and Blanchette 11 but evident in our experiments is an intermediate situation between trapping and partial coalescences, in which a delayed partial coalescence occurs.…”