Cycloid motions of grains in a dust plasma

Hypocycloid and epicycloid motions of irregular grains （pine pollen） are observed for the first time in a dust plasma in a two-dimensional （2D） horizontal plane. These cycloid motions can be regarded as a combination of a primary circle and a secondary circle. An inverse Magnus force originating from the spin of the irregular grain gives rise to the primary circle. Radial confinement resulting from the electrostatic force and the ion drag force, together with inverse Magnus force, plays an important role in the formation of the secondary circle. In addition, the cyclotron radius is seen to change periodically during the cycloid motion. Force analysis and comparison experiments have shown that the cycloid motions are distinctive features of an irregular grain immersed in a plasma.

Morphology analysis of tracks in the aerogels impacted by hypervelocity irregular particles

As an attractive collector medium for hypervelocity particles,combined with outstanding physical properties and suitable compositional characteristics,SiO2 aerogel has been deployed on outer space missions and laser shock-loaded collection experiments.In this paper,impact experiments were conducted to understand the penetration process of irregular grains,irregular Al_(2)O_(3) grains with two different sizes and speeds(~110μm@7 km/s,~251μm@2.3 km/s)at various density silica aerogels.By classifying the shapes of projectile residues and tracks,the morphology of tracks was analyzed.It was observed that there were several kinds of typical tracks in the penetration of irregular grains,accompanied by residues with the shapes of near-sphere,polyhedron,streamlined body wedge,and rotator.The rotational behavior was demonstrated by the final status of one flake projectile as direct evidence.In addition,there was no obvious relationship between the track length and experimental parameters,which may be caused by the uncertain interaction between aerogels and irregular particles.In addition,it confirmed the existence of fragmentation,melting situation by observing the shape of the impact entrance hole.At the same time,optical coherence tomography was used to observe the detail of tracks clearly,which provided a method to characterize the tracks nondestructively.