We report the use of focused acoustic beams to eject discrete droplets of controlled diameter and velocity from a free-liquid surface. No nozzles are involved. Droplet formation has been experimentally demonstrated over the frequency range of 5–300 MHz, with corresponding droplet diameters from 300 to 5 μm. The physics of droplet formation is essentially unchanged over this frequency range. For acoustic focusing elements having similar geometries, droplet diameter has been found to scale inversely with the acoustic frequency. A simple model is used to obtain analytical expressions for the key parameters of droplet formation and their scaling with acoustic frequency. Also reported is a more detailed theory which includes the linear propagation of the focused acoustic wave, the coupling of the acoustic fields to the initial surface velocity potential, and the subsequent dynamics of droplet formation. This latter phase is modeled numerically as an incompressible, irrotational process using a boundary integral vortex method. For simulations at 5 MHz, this numerical model is very successful in predicting the key features of droplet formation.

Liquid handling instruments for life science applications based on droplet formation with focused acoustic energy or acoustic droplet ejection (ADE) were introduced commercially more than a decade ago. While the idea of “moving liquids with sound” was known in the 20th century, the development of precise methods for acoustic dispensing to aliquot life science materials in the laboratory began in earnest in the 21st century with the adaptation of the controlled “drop on demand” acoustic transfer of droplets from high-density microplates for high-throughput screening (HTS) applications. Robust ADE implementations for life science applications achieve excellent accuracy and precision by using acoustics first to sense the liquid characteristics relevant for its transfer, and then to actuate transfer of the liquid with customized application of sound energy to the given well and well fluid in the microplate. This article provides an overview of the physics behind ADE and its central role in both acoustical and rheological aspects of robust implementation of ADE in the life science laboratory and its broad range of ejectable materials.

An actuation method for atomic force microscope (AFM) cantilevers in fluids is reported. The radiation pressure generated by a focused acoustic transducer at radio frequency (rf) (100–300 MHz) exerts a localized force of controlled amplitude at a desired location on the AFM cantilever. This force can be used to measure the spring constant and other dynamic properties of the cantilever. Furthermore, by amplitude modulating the rf signal input to the acoustic transducer, the cantilever is actuated in the dc–MHz frequency range. This provides a broadband actuation and characterization method for AFM cantilevers with arbitrary geometry. The technique is demonstrated on AFM cantilevers with spring constants in the 0.01–10 N/m range using a micromachined acoustic transducer/Fresnel lens structure operating at 179 MHz in water.

An acoustic microscope using water as the coupling medium has been operated at frequencies up to 4.4 GHz. The signal-to-noise ratio is sufficient for imaging and the acoustic images demonstrate that the resolution is better than 2000 Å when the instrument is operated in the nonlinear regime.

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