Surfactant Effects on Highly Nonequilibrium Surfaces:

Surfactants and Drop Detachment

Kathleen J. Stebe

Department of Chemical & Biomolecular Engineering

Joint appointments:

Mechanical Engineering, Materials Science & Engineering, Biomedical Engineering

Johns Hopkins University

3400 North Charles Street

Baltimore, MD 21218

 

When a buoyant viscous drop is injected into a viscous fluid, it evolves to form a distended shape that detaches via the rapid formation and pinching of a neck.  The effects of surfactants in altering this process are studied numerically.  In the absence of surfactants, surface contraction is most pronounced in the vicinity of the neck.  Thus, when surfactants are present, they accumulate there and alter the ensuing dynamics by reducing the surface tension that drives the contraction.  The surface tension is described by a nonlinear surface equation of state that accounts for the maximum packing of surfactant in a monolayer.  When surfactant adsorption-desorption is very slow, interfaces dilute significantly during drop expansion, and drops form necks which are only slightly perturbed in their dynamics from the surfactant-free case.  When adsorption-desorption dynamics are comparable to the rate of expansion, drops break at the primary neck at low surfactant coverage, at both the primary and secondary necks at moderate coverages, only at the secondary neck at higher coverages, or fail to neck at elevated coverages.  When surfactant adsorption-desorption kinetics are rapid, the surface remains in equilibrium with the surrounding solution, and drops break like surfactant-free drops with a uniform surface tension.   These arguments are used to construct a phase diagram of drop neck shapes as a function of surfactant coverage.  A map of neck/no-neck thresholds is also constructed as a function of surfactant coverage and sorption dynamics, suggesting that drop detachment can be used as a means of characterizing surfactant dynamics.