Characterization of the Capacitance of a Rotating Ring–Disk Electrode

The use of rotating ring–disk electrodes as generator-collector systems has so far been limited to the detection of Faradaic currents at the ring. As opposed to other generator-collector configurations, non-Faradaic detection has not yet been carried out with rotating ring–disk electrodes. In this study, a.c. perturbation based detection for measurement of the ring impedance is introduced. By using a conducting polymer-modified disk electrode in combination with a bare gold ring as a model, it is shown that the measured ring capacitance correlates with the polarization of the polymer film, most probably due to counter-ion exchange. A method of calculating the ring capacitance based on a small-signal sinusoid perturbation is described and the most important instrumental limitations are identified.

Rotating dust particles in the coma of comet 67P/Churyumov-Gerasimenko

Context. During September and October 2014, the OSIRIS cameras onboard the ESA Rosetta mission detected millions of single particles. Many of these dust particles appear as long tracks (due to both the dust proper motion and the spacecraft motion during the exposure time) with a clear brightness periodicity. Aims. We interpret the observed periodic features as a rotational and translational motion of aspherical dust grains. Methods. By counting the peaks of each track, we obtained statistics of a rotation frequency. We compared these results with the rotational frequency predicted by a model of aspherical dust grain dynamics in a model gas flow. By testing many possible sets of physical conditions and grain characteristics, we constrained the rotational properties of dust grains. Results. We analyzed on the motion of rotating aspherical dust grains with different cross sections in flow conditions corresponding to the coma of 67P/Churyumov-Gerasimenko qualitatively and quantitatively. Based on the OSIRIS observations, we constrain the possible physical parameters of the grains.