Three-Dimensional Trapping and Assembly of Small Particles with Synchronized Spherical Acoustical Vortices

Three-dimensional harmless contactless manipulation and assembly of\nmicro-objects and micro-organisms would open new horizons in microrobotics and\nmicrobiology, e.g. for microsystems assembly or tissue engineering. In our\nprevious work [Gong and Baudoin, Phys. Rev. Appl., 12: 024045 (2019)], we\ninvestigated theoretically the possibility to trap and assemble in two\ndimensions small particles compared to the wavelength with synchronized\nacoustical tweezers based on cylindrical acoustical vortices. However, since\nthese wavefields are progressive along their central axis, they can only push\nor pull (not trap) particles in this direction and hence are mainly limited to\n2D operations. In this paper, we extend our previous analysis and show\ntheoretically that particles can be trapped and assembled in three-dimensions\nwith synchronized spherical vortices. We show that the particles can be\napproached both laterally and axially and we determine the maximum assembly\nspeed by balancing the Stokes' drag force and the critical radiation force.\nThese theoretical results provide guidelines to design selective acoustical\ntweezers able to trap and assemble particles in three dimensions.\n