Metal scaffolds for acoustic trapping of polystyrene particles in water

The concept of ultrasonic 3D caging is based in wells designed to host three orthogonal half-wave acoustic resonances defined by its three dimensions generating a single-pressure-node in the center where particles collect to aggregate. A replacement of the planar shapes in these cavities by curved walls introduce changes in the pressure patterns, generating different effects on the particles exposed to the acoustic field. Here we present an experimental study of the acoustic behavior of aqueous suspensions of micron-sized polystyrene particles (Cv~1%) exposed to ultrasounds at a frequency f~1MHz in a metal scaffold made up of stainless steel wires crossed-linked orthogonally to form a mesh with a light of 1mm (something larger than λ/2). Once applied the acoustic field, the particles are rapidly attracted by the rods of the mesh distances much larger than their diameter (Φp = 6µm), where are trapped and remain adhered to the scaffold during the acoustic actuation and even later, providing stable aggregates. Hydrodynamic mechanisms associated to viscous disturbances and mutual radiation pressure induced by the metal rods could be responsible of this massive trapping effect on the polymeric particles according to previous theoretical and experimental studies carried by the authors in aerosols.