The oscillating drop technique revisited

The oscillating drop technique-in combination with some kind of levitation-is widely used today for the measurement of the surface tension and viscosity of liquids. It is based on Rayleigh's theory for the surface tension driven oscillations of a spherical, force-free, liquid drop. Unfortunately, this ideal case is never realized in practice. Whereas the influence of the levitation field and finite viscosity on the oscillation frequencies has been discussed by several authors, more practical questions related to the observation of these oscillations have not been addressed before and are the topic of this contribution. In particular, the influence of translational or rotational sample movement, and additional effects due to signal processing are discussed. It is hoped that this paper will help to avoid the many pitfalls inherent in the oscillating drop technique and will thus lead to unambiguous and reliable results.