Single-beam acoustical tweezers

Dexterous contactless manipulation of microscopic matter in three dimensions is possible since the development of optical tweezers. This scheme using a single laser beam can handle particles with forces in the piconewton range with subnanometer resolution. Although various acoustical traps, mainly using standing wave fields, have been proposed over the last 50 years, several difficulties have prevented the demonstration of acoustical tweezers using a single beam of ultrasound. Deriving the analytical expression for the acoustic radiation force applied on an elastic sphere, the feasibility and the experimental demonstration of the trapping of small elastic particles with a single ultrasonic beam in three dimensions was recently achieved. These acoustical tweezers upscale by several orders of magnitude the range of forces and particle sizes available in the realm of contactless manipulation.