Acoustic Metal Particle Focusing in a Round Glass Capillary

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.