Ultrasonic near-field based acoustic tweezers for the extraction and manipulation of hydrocarbon droplets

Radiation pressure from acoustic and electromagnetic fields can generate forces sufficient to trap and manipulate objects. In most cases, the objects are pre-existing, but it is also possible for the forces to essentially create the target objects. Recently, we reported on the ability of high power ultrasound to extract and controllably manipulate droplets from the organic solvent CCl4 using a near-field type of acoustic tweezers [Lirette et al., Phys. Rev. Appl. 12, 061001 (2019)]. The extraction used a fraxicon lens which produced trapping zones in the near-field. With the addition of extraction to trapping and manipulation, the process can be considered a form of contact-free pipetting. In the present work, we demonstrate the capability of this system to co-axially extract two droplets of SAE30ND motor oil (between 70%–80% liquid hydrocarbon) at a water interface against a positive radiation pressure. In the experiments with oil, several differences in the process have been observed relative to the CCl4 study: a second near-field trapping zone is revealed; the surface deformation is small and opposite to the direction of extraction; the extraction and trapping forces are sufficient to overcome both interfacial tension and buoyancy; and the target liquid has distinct physical properties, such as density, viscosity, and acoustic impedance. Non-contact and label-free extraction of oil droplets remotely in an aqueous environment could have significant biological and environmental applications. Finding that the process works with two distinct liquids demonstrates its more general applicability and broadens its potential uses.