Marangoni effects cause non-monotonic damping of droplet oscillations*

Experiments have often shown that the oscillations of droplets coated with surfactant are more damped than those with pure droplets, though the frequency is largely unchanged. This is attributed to Marangoni effects: the non-uniform surfactant distribution causes gradients of surface tension and induced flows that oppose the oscillation. In this talk we will show that the decay rate is a non-monotonic function of surfactant strength: a peak in the decay rate is observed at relatively low surfactant strengths followed by a further decrease as the surfactant strength increases (though still more damped than the zero-surfactant case for sufficiently large Laplace numbers). We discuss the physical origin of this surprising phenomenon and the issues that this non-monotonicity causes when trying to experimentally determine surfactant strength from observations of oscillation.