Active manipulation of inhomogeneous miscible fluid interfaces using acoustical tweezers

The manipulation of immiscible fluids at microscales has paved the way for a wide range of applications and technologies. In particular, ultrasounds serve as versatile tools for manipulating dispersed immiscible droplets. By shaping the acoustic field, droplets can be sorted, divided, merged, selected, and positioned. Typical approaches involve standing waves to attract and trap droplets at fixed pressure nodes/antinodes or traveling waves to displace droplets in the direction of the ultrasound wave propagation. Recent advancements have enabled precise manipulation of selected droplets using selective acoustic tweezers based on focused beams, or acoustical vortices. However, the ultrasound-based manipulation of miscible fluids has only recently been demonstrated theoretically by Karlsen, Augustsson, and Bruus [1]. In this case, the manipulation results from an acoustic force density appearing due to the inhomogeneity in density and sound speed between the two fluids. In this work, we experimentally demonstrate the application of acoustic tweezers to pattern, trap, and displace high-concentration miscible-fluid islands (Ficoll-PM400) within a lower-concentration medium (water) using ultrasounds. This research establishes the basis for ultrasound-based drug and chemical manipulation, with diverse applications in biology and medicine.