Tunable subwavelength ultrasound focusing in mesoscale spherical lenses using liquid mixtures

In this work, we present a configurable spherical lens for underwater focusing applications, which consists on a hollow ABS container filled with a liquid mixture. Two miscible liquids with different sound speeds are required to implement this novel configurable lens. We show that by adjusting the mixing ratio between the volumes of both liquids, the sound speed of the liquid mixture can be accurately selected. This results in a modification of the acoustic jet properties and a continuous tuning on the lens focal length. This procedure can be fully automatized providing a dynamic control mechanism that can shift the lens focal length to any desired value inside a continuous range in both directions. Depending on the acoustic properties of the selected liquids, subwavelength resolution or even beyond the diffraction limit resolution can be achieved. We provide experimental measurements for ethanol-water mixtures achieving subwavelength resolution for a certain focal length ranging between 34.6 and 42.8 mm.