Fig. 4. Trapping of silica gel microspheres with ultrasound at specified points in 3D.

(A) Schematic of the experimental setup (2.25 MHz) using two holograms and a removable particle container. After initial resuspension by shaking, the particles (white dots) sediment under gravity g until they pass a trapping site (red cross). The trap locations represent vertices of platonic solids: (B) octahedron and (C) icosahedron. Scale bar, 10 mm.