Observation of selective optical manipulation of particles in acoustic levitation

Acoustic Radiation Force is commonly used to create stable large-scale aggregates of particles in levitation (so-called "acoustic levitation") in a micro-cavity. The authors show in the following work that this well-known and well-controlled aggregation process can be reversed without contact or external flow if the aggregated particles are enlightened with the proper optical wavelength. This coupled optics and acoustics effect has been observed with various kinds of particles and different optic wavelengths, showing high reproducibility. The phenomenon is studied using fluorescent micro-metric polystyrene particles without flow, and the effects of acoustic energy and illumination power have been quantitatively assessed. It is then exploited to separate a mix of particles with identical mechanical properties based on their different optic absorption. If the phenomenon is not well understood, some possible mechanisms are proposed and discussed that could be responsible for the rapid ejection of the objects in levitation from the illuminated area. Since it is a tag free phenomenon that does not need high energies to happen and since it works with biological objects such as algae, red blood cells, and bacteria, it may open the way to a broad range of applications.