Focusing of Micrometer-Sized Metal Particles Enabled by Reduced Acoustic Streaming via Acoustic Forces in a Round Glass Capillary

Abstract

Two-dimensional (2D) 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 labor-intensive cleanroom production. Here, we present 2D metal microparticle 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 toward the capillary center. We experimentally investigate the particle line width for different particle materials and concentrations. We demonstrate the focus of copper particles approximately $1\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in diameter down to a line of width $60.8\ifmmode\pm\else\textpm\fi{}7.0\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and height $45.2\ifmmode\pm\else\textpm\fi{}9.3\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$, corresponding to a local concentration of 4.5% v/v, which is $90$ times higher than the concentration of the initial solution. Further, we achieve the focusing of 1-$\ensuremath{\mu}\mathrm{m}$ polystyrene particles, which is usually prevented due to acoustic streaming. Through numerical analysis, we reveal the mechanism enabling the manipulation of particles in the low-micrometer range. Due to a transition of the acoustic streaming patterns from two dominant vortices in the lower half to two dominant vortices in the upper half, the streaming velocity exhibits a local minimum while the overall acoustic energy density stays at a sufficiently high level for particle focusing, leading to a lower critical particle radius than in conventional rectangular microchannels. Finally, we use our method to eject copper particles through a tapered round capillary with an opening of $25\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in diameter, which would not be possible without particle focusing. Our setup can be utilized for various applications that otherwise might suffer from abrasion, clogging, and limited resolution.