Shapes and oscillations of raindrops with reduced surface tensions: Measurements at the Mainz vertical wind tunnel

Important characteristics of raindrops pertinent to fields in atmospheric sciences such as weather radar or pollution scavenging are drop shape, oscillation frequency and amplitude, as well as the internal circulation. Atmospheric raindrops are never pure water drops but contain additional components like aerosol particles and dissolved species. Surface active substances, when present, reduce the surface tension of raindrops and, thus, increase the drop deformation which in turn affects breakup and coalescence, pollutant scavenging, and, finally, the formation of precipitation. Experiments were performed at the Mainz vertical wind tunnel with raindrops freely suspended at their terminal velocities in a vertical air stream. Drops with various reduced surface tensions and sizes between 3.5 and 7.5 mm equivalent diameter were investigated. The most relevant microphysical parameters of the drops, i.e. terminal velocity, shape, mean axis ratio, oscillation modes, frequencies and amplitudes, and the internal circulation were measured and compared to those of pure water drops. The experimental data were also compared to the results of a theoretical drop shape model which indicated that the model is applicable to atmospheric drops with reduced surface tensions. From a novel frequency analyzing method it was found that the oscillation modes cover not only the (2,0) and (2,1) but also the (2,2) modes. Depending on the existence of a surfactant film on the drop surface, drops with the same surface tension can show presence or absence of internal circulation.

Research highlights► We studied shapes, terminal speeds and oscillation of raindrops with reduced surface tension. ► A novel frequency analyzing method for the identification of the (2,2) mode was introduced. ► Drops with reduced surface tensions oscillate in (2,0), (2,1) and also in the (2,2) modes. ► Absence of internal circulation depends on the existence of a surfactant film on the drop surface.