Manipulating magnet-coated bubbles

Gas bubbles in a liquid driven by acoustic waves can be used in a variety of contexts—for example, as contrast agents for ultrasound imaging, as a delivery system for therapeutic drugs, as catalysts for sonochemical reactions, and as scrubbers of surfaces. The bubbles’ compressibility, which allows their volume to oscillate in response to the varying sound pressure, accounts for the wide applicability. Unfortunately, spatial gradients in the pressure field can make it nearly impossible to control the bubbles’ oscillations and position simultaneously. Researchers at Nanyang Technological University in Singapore have now found a simple recipe to dress the bubbles in a 1-µm-thick coat of magnetite nanoparticles. The coat, which self assembles in solution, stabilizes the bubbles (typically 40–350 µm in diameter) without sacrificing their compressibility; they remain intact for more than 6 months in a light-tight drawer and their position in solution can be controlled with a simple household magnet. The figure here illustrates an example: The oscillation of a single 130-µm-wide bubble, subject to sound at a frequency close to its resonant frequency, sets up eddies (signified by arrows) in the surrounding fluid. After the acoustic field is turned off, a permanent magnet nudged the bubble to the right. The authors expect magnetic bubbles to serve as remotely controlled microfluidic mixers and pumps, and, more generally, as tools to test fundamental fluid mechanics concepts. (X. Zhao, P. A. Quinto-Su, C.-D. Ohl, Phys. Rev. Lett. 102 , 024501, 2009 http://dx.doi.org/10.1103/PhysRevLett.102.024501 .)