SLIMPICODA · Synthesis of Liquid-filled Micro-particles as Icy Cosmic Dust Analogues

Synthesis of Liquid-filled Micro-particles as Icy Cosmic Dust Analogues

The in situ analysis of cosmic dust can provide a large amount of information not only about the cosmic dust itself, but also of its sources. Examples of sources of cosmic dust encountered in the solar system are asteroids, comets, molecular clouds, supernovae, Wolf-Rayet stars, atmosphereless objects. The physical characteristics of cosmic dust may be investigated using a variety of techniques in situ (i.e. not including the analysis of cosmic dust samples collected from terrestrial locations) or after the exposure of capture instruments or spacecraft surfaces to the dust flux and subsequent return to Earth. For example, compositional analysis may be undertaken directly on the particles themselves, using a time of flight impact ionisation mass spectrometer, or at a later date on crater residues or particles captured in aerogel. Cosmic dust is usually encountered in space at a considerable relative velocity, usually from 1-2 km/s up to speeds in excess of 150 km/s (generally known as hypervelocities). To calibrate spacecraft-based instrumentation, or investigate the effects of high levels of physical shock on the particles and the surfaces they impact, laboratory calibration and simulation is required. Typically cosmic dust may include one or more of the following major components: minerals, metals and organics, and it is therefore necessary to create analogue particles with compositions which reflect the diversity encountered in space. The production of mineral and refractory organic particles is reasonably well documented for use in hypervelocity impact research (albeit still not without its own challenges), however the production and acceleration of mineral-organic volatile-rich micron-scale particles for this purpose had not yet occurred and forms the primary objective for this project. A secondary objective was to apply thin metallic coatings to the particles, enabling them to attain suitable charge to electrostatically accelerate. The initial period of the project was spent undertaking arrival logistics. This period was followed by comprehensive training on the departmental Hitachi S3400-N Scanning Electron Microscope, gold sputter coater and Horiba LabRAM-HR Raman Spectrometer. There then followed a period during which several iterations of the liquid-filled particle synthesis were attempted. The flexible modular work-package approach outlined in the proposal envisaged repeated attempts at the production, characterisation and testing of liquid-filled particles. Each work-package therefore has the same structure: synthesis, sealing, characterisation, testing, coating and use. During the project several methods of synthesis, sealing, characterisation and testing were tried. Characterisation was undertaken using the Hitachi S3400-N SEM or Hitachi 4700 FEG-SEM with Bruker X-Flash detector (for which further training was provided) prior to sealing. Sealing was attempted using mercaptopropyl trimethoxysilane or polyvinylpyrrolidone and subsequent deposition of silica, from Sodium Metasilicate, or metal. Particle testing at Kent, if deemed appropriate, was performed using a small test vacuum chamber. Particles suspected of containing liquid were accelerated onto aerogel using the Light Gas Gun at Kent (for which training was provided). As an analogue for water-rich cosmic dust grains, samples of hydrated minerals (serpentine and antigorite) have been coated with platinum to use in an electrostatic accelerator in Boulder, as part of a collaboration with existing project collaborators in Germany and colleagues in the USA. During this process training on a Thermo-Fisher Surfer BET analyser, planetary ball mill, as well as a He-pycnometer was provided. Efforts have also been made to develop new metallic coating methods, to enable the particles to charge and accelerate in an electrostatic accelerator. A small 20 kV test electrostatic accelerator has been built, based on the technical designs for those in Germany and the USA. This has enabled particles designed for use in electrostatic accelerators to be tested after metal coating (with either Pt or Sn). Description of the main results achieved so far •After coating with a thin layer of polypyrrole by our Sheffield collaborators, the grains formed by condensation around phenyltrimethoxysilane droplets were tested (again in Sheffield) using thermogravimetric analysis, to determine the polypyrrole loading. During this process unusual mass-loss behaviour was observed, which is in agreement with a fraction of the particles containing unreacted, liquid, phenyltrimethoxysilane. The particles have been successfully accelerated by a colleague, at the dust accelerator in Heidelberg and low velocity impact-ionisation mass spectra are awaiting analysis. These spectra, if not too noisy, may indicate whether liquid organics were encapsulated in the grains. Further particles have been produced and tested in the light gas gun, fired onto aerogel. •A test bench electrostatic accelerator, with detection electronics and control PC has been built, enabling metal-coated particles to be tested. •Silica-shelled, presumably initially water-filled, particles were produced by the hexadecane/water/surfactant method, appearing as deflated “balloons” in SEM images. Repeated attempts at creating a thicker shell have been made, with the latest method involving treatment with polyvinylpyrrolidone and further silica deposition. •A new, facile method for applying thin electroless coatings of Sn to mineral grains has been developed. The expected final results are to produce liquid-filled microparticles for use in hypervelocity impact experiments. They will enable the simulation of hypervelocity impacts of ice, or volatile, rich cosmic dust grains found in space, in the laboratory, and as such will assist in the European participation of future space missions. The societal impact of such missions is well documented, as they not only provide vital and unique information about the environment around us, but also enthuse the public and the next generation of young scientists.

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