Experimental investigation of the collision and optical properties of (sub)µm-sized ice and ice-coated dust particles

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We propose to perform low-velocity impact experiments of (sub)μm-sized water ice, ice-coated silica, and pure silica particles with ice, silica, and ice/dust targets in a wide range of temperatures and under high-vacuum conditions. These experiments are intended to simulate inter-particle collisions in the densest and coldest interstellar clouds, for which grain growth by coagulation has been postulated and observed. Above all, we are interested in the sticking threshold for water-ice particles as a function of grain size, which is widely used in the literature but for which empirical data are lacking. On top of that, we also intend to measure the charge transfer in these collisions to assess whether triboelectric charging is a possible source for free charge carriers in dense interstellar clouds in which the classical sources of ionization, i.e. ultraviolet radiation and cosmic rays, are not effective. Our study will be complemented by infrared measurements of loose particle ensembles with which we will gain insight into the optical properties of coagulated spherical ice and ice-coated dust particles. A deeper understanding of the combined processes of coagulation, fragmentation, triboelectric charging, and optical properties of small particle agglomerates is required to calibrate models of the dust-mass evolution, the chemical evolution, and the radiative energy transport in molecular clouds.