  Two-dimensional metal particle focusing is an essential task for various
fabrication processes. While acoustofluidic devices can manipulate particles in
two dimensions, the production of these devices often demands a cleanroom
environment. Therefore, acoustically excited glass capillaries present a cheap
alternative to labour-intensive cleanroom production. Here, we present 2D metal
micro-particle focusing in a round glass capillary using bulk acoustic waves.
Excitation of the piezoelectric transducer at specific frequencies leads to
mode shapes in the round capillary, concentrating particles towards the
capillary centre. We experimentally investigate the particle linewidth for
different particle materials and concentrations. We demonstrate the focus of
copper particles with 1 $\mu$m in diameter down to a line of width 60.8 $\pm$
7.0 $\mu$m and height 45.2 $\pm$ 9.3 $\mu$m, corresponding to a local
concentration of 4.5 % v/v, which is 90 times higher than the concentration of
the initial solution. Through numerical analysis, we could obtain further
insights into the particle manipulation mechanism inside the capillary and
predict the particle trajectories. We found that a transition of the acoustic
streaming pattern enables us to manipulate particles close to the critical
particle radius. Finally, we used our method to eject copper particles through
a tapered round capillary with an opening of 25 $\mu$m in diameter, which would
not be possible without particle focusing. Our novel setup can be utilized for
various applications, that otherwise might suffer from abrasion, clogging and
limited resolution.
