Effects of insoluble surfactants on the nonlinear deformation and breakup of stretching liquid bridges

During the emission of single drops and the atomization of a liquid from a nozzle, threads of liquid are stretched and broken. A convenient setup for studying in a controlled manner the dynamics of liquid threads is the so-ca lled liquid bridge, which is created by holding captive a volume of liquid between two solid disks and pulling apart the two disks at a constant veloc ity. Although the stability of static bridges and the dynamics of stretchin g bridges of pure liquids have been extensively studied, even a rudimentary understanding of the dynamics of the stretching and breakup of bridges of surfactant-laden liquids is lacking. In this work, the dynamics of a bridge of a Newtonian liquid containing an insoluble surfactant are analyzed by s olving numerically a one-dimensional set of equations that results from a s lender-jet approximation of the Navier-Stokes system that governs fluid flo w and the convection-diffusion equation that governs surfactant transport. The computational technique is based on the method-of-lines, and uses finit e elements for discretization in space and finite differences for discretiz ation in time. The computational results reveal that the presence of an ins oluble surfactant can drastically alter the physics of bridge deformation a nd breakup compared to the situation in which the bridge is surfactant free . They also make clear how the distribution of surfactant along the free su rface varies with stretching velocity, bridge geometry, and bulk and surfac e properties of the liquid bridge. Gradients in surfactant concentration al ong the interface give rise to Marangoni stresses which can either retard o r accelerate the breakup of the liquid bridge. For example, a high-viscosit y bridge being stretched at a low velocity is stabilized by the presence of a surfactant of low surface diffusivity (high Peclet number) because of th e favorable influence of Marangoni stresses on delaying the rupture of the bridge. This effect, however, can be lessened or even negated by increasing the stretching velocity. Large increases in the stretching velocity result in interesting changes in their own right regardless of whether surfactant s are present or not. Namely, it is shown that whereas bridges being stretc hed at low velocities rupture near the bottom disk, those being stretched a t high velocities rupture near the top disk. (C) 1999 American Institute of Physics. [S1070-6631(99)02705-1].