  This paper investigates a clustering instability of a freely falling granular
jet composed of 100 micron glass spheres. The granular flow out of a circular
nozzle starts out spatially uniform and then, further downstream, breaks up
into well defined clusters. The role of air is investigated in this phenomenon
by changing the ambient air pressure down to 1/5000th atm. An optical method is
used that measures inhomogeneities in the flow in order to quantify the growth
of the clusters. Clustering is observed down to the lowest pressure and the
presence of air leads to larger drops but does not initiate the drop formation.
The analysis shows that the drop size is set by fluctuations on the order of
the size of the particles at the nozzle.
