Influence of Surfactants on the Dynamics of Fluid-Liquid Interfaces

Abstract

In many two-phase fluid-liquid systems at least one phase contains surface active agents (surfactants for short) which are adsorbed preferentially at the interface Γ(t) due to minimization of free surface energy. Important examples are emulsification processes and bubbles rising in a bubble column through water containing a surfactant - unmeant as a contamination or by determined addition in order to increase the efficiency of the column. The adsorption of a surfactant at a fluid-liquid interface causes a decrease of the surface tension, depending on the area specific concentration cΓ of the adsorbed surfactant, i.e. σ=f(cΓ)(1) with a decreasing function f. The adsorbed surfactant is distributed on the interface due to convective and diffusive interfacial fluxes. The resulting spatial inhomogeneity leads to surface gradients of the surface tension, ∇Γσ(cΓ), which effect the hydrodynamics via the interfacial momentum jump condition [pI−S]nΓ=σκnΓ+∇Γσ(cΓ).(2) These additional so-called Marangoni stresses often result in a pronounced change of the dynamical behavior.