The condensational growth of water droplets in technical and natural systems varies with environmental conditions such as droplet size, droplet composition, air humidity, temperature, and turbulence. This contribution addresses the influence of external electrical fields on the condensation process. Although electrical fields exist in the atmosphere, for example in thunderstorm clouds, and although it is generally accepted that electrical fields exert an influence on the condensation process, no quantitative description of this influence at ambient temperatures exists. We present laboratory experiments and a theoretical model to further develop understanding of the influence of electric fields on condensation at various temperatures. The levitated droplet cluster technology is applied to study the kinetics of droplet growth in an external electric field at temperatures between 50 °C and 70 °C. The theoretical model is designed to mirror the experimental conditions as precisely as possible. Experimental and kinetic model results are in qualitative agreement in that the relative contribution of the electrically induced contribution to total condensation reduces with increasing temperature. Vice versa, the electrically induced condensation significance is expected to further increase with decreasing temperatures below 50 °C. We expect that electro-condensation will be the dominating process at room temperatures and even more at temperatures near 0 °C, at electric field strengths typical for clouds. Further studies are needed to extend the experimental and theoretical temperature range to conditions typical for clouds in the atmosphere.

The forces acting on levitating water microdroplets in the structure of a droplet cluster (A.A. Fedorets, 2004) have been analyzed. It is found that microdroplets in the cluster should undergo...

This paper aims to study the path oscillations of single, spherical water droplets levitated in a wind tunnel to better comprehend the mechanical motion of small raindrops. The observations were carried out in the Mainz vertical wind tunnel. The discrete, fast Fourier transform was used to determine the oscillatory frequencies of the droplet paths, and the Hilbert transform was applied to analyze their instantaneous frequency stability. Both transversal and streamwise components of the path oscillations are described with typical frequencies of several tens of Hertz. The studied oscillations elongate the paths, reduce the terminal velocity of the smallest droplets, and make the rain droplet fall in a non-uniform motion even after reaching terminal velocity. The terminal velocity can be considered as physically having been reached if our proposed practical criterion is satisfied. From a fluid mechanics perspective, the paper fills an experimental gap in the studies of the paths oscillations of single, liquid spheres of microscopic sizes at low Bond numbers <1 and relatively low Reynolds numbers ~10².

The forces acting on freely levitating water microdroplets in the structure found 15 years ago and called the droplet cluster (A. A. Fedorets, 2004) are considered. It is shown that lowfrequency...