  Inelastic collisions occur among regolith particles, such as those in the
ejecta curtain from a crater, and may cause clustering or agglomeration of
particles and thus produce discrete patterns of ejecta deposits around a
crater. Previous studies have shown that clusters, and even agglomerates, are
formed via mutual, inelastic collisions of spherical particles due to adhering
forces between particles in granular streams. To investigate the condition of
agglomerate formation in granular streams, we conducted laboratory experiments
of granular streams using both spherical and irregular, non-spherical
particles. Measurements of particle adhesion in this study were performed using
a centrifugal separation method, in contrast to the previous study in which
atomic force microscopy (AFM) was used. This enabled simultaneous measurements
of multiple particles of various shapes for a statistical analysis of the
results. With similar relative velocities and adhesion values, irregular
particles were found to form agglomerates much more easily than spherical
particles. The axial ratio of the agglomerates of spherical particles and
irregular particles was similar and was in accordance with those observed in
previous laboratory studies, whereas the size of the agglomerates of irregular
particles was larger than the size of spherical particles. The degree of
agglomeration and the size of agglomerates can be used as an indicator of the
shape or adhesive force of the particles in granular stream. Our findings on
agglomeration in granular streams could provide new insights into the origin of
rays on airless bodies and grooves on Phobos.
