Vertical oscillations of droplets in small droplet clusters

Small clusters of condensed microdroplets can levitate above a heated water surface. Vertical oscillations of small clusters (one, two, and three droplets) in the frequency range below 90 Hz are studied and compared with recently reported horizontal oscillations. Two reproducible peak frequencies are at 41.6 ± 0.4 Hz and at 49.5 ± 0.2 Hz. These frequencies are much higher than those for horizontal oscillations (between 1.61 Hz and 5.96 Hz); however, corresponding amplitudes are much smaller, which suggests equal energy distributions between the degrees of freedom. The values of effective stiffness constants are obtained from the data on equilibrium height of levitation of droplets of different sizes, and possible sources of oscillations are discussed. The oscillations are caused by aerodynamic forces, and these are same forces which are responsible for the stability of the droplet cluster. [Display omitted]