Dynamic vertical positioning of liquid droplets in transducer-reflector sound field arrangement

• Applicants: Professor Dr. Dieter Bothe; Professor Dr. Mario Kupnik
• Subject Area: Measurement Systems ; Mathematics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 542327521

This research project addresses the acoustic levitation of liquid droplets using ultrasound, motivated by the fact that in preliminary work we have discovered a new method that enables vertical positioning of liquid droplets in and also against the direction of gravity. This is even possible with the simplest form of an acoustic levitator using standing waves – the so-called transducer-reflector arrangement. It means that a simple commercially available ultrasonic transducer positioned at a defined distance from a flat or curved surface is sufficient to move liquid droplets from one position to another. It happens in a completely contactless and therefore sterile manner, even over obstacles in between. In this transducer-reflector arrangement, we can gradually lift the object to be levitated from the reflector surface to more than just the first (state-of-the-art) sound pressure node by modulating the ultrasonic transducer's electrical signal accordingly. The levitator does not move during this raising or lowering process. In the meantime, we master this reproducibly for polystyrene spheres, but for liquid droplets it is much more difficult. Reproducibility can only be achieved by numerical modeling of the dynamics occurring, which requires the planned interdisciplinary collaboration between the two applicants with their main expertise in measurement and numerical methods. Fundamental research questions have to be answered on both sides. The focus is on the numerical and experimental proof of a novel, step-by-step movement of a drop of liquid against the direction of gravity from a sound pressure node to a higher sound pressure node. Only with the support by the numerical models developed in this project and corresponding simulations, the planned experiments with liquid droplets can be carried out in a controlled, time-efficient, and predictable manner for a wide range of conditions. The measurement of all relevant quantities in turn allows the required step-by-step validation of the numerical models. Thus, we see the planned joint research project as a unique opportunity to establish fundamental techniques for a new, versatile non-contact manipulation of liquid droplets, with large potential for applications across various domains.

• DFG Programme: Research Grants