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Acoustically actuated microbubbles in microchannels can be used as a versatile tool to directly manipulate fluids and particles in Lab-on-a-Chip devices for the purpose of fast microfluidic mixing, as well as the sorting of particles or cells based on their size and other physical properties. Many experimental investigations use such bubbles in microfluidic devices. However, the physics causing the streaming field are not understood in its details yet. Existing theoretical models describe the correlation of oscillating interfaces and the streaming field that they generate. The models are either based on the oscillation of rigid objects or interfaces that oscillate with simple oscillation modes. In the experiments of this work, much more complex oscillation modes were observed for an acoustically actuated sessile and hemi-cylindrical bubble in a microchannel. The bubble is resonantly driven at a frequency of 20 kHz, and periodic shape oscillations are recorded using a stroboscopic technique. With this technique, an equivalent frame rate of more than one million frames per second can easily be achieved without using high-speed imaging equipment. In contrast to the bubble interface, the motion of the surrounding fluid is not periodic and a stroboscopic technique cannot be applied. Therefore, a 256×256 pixel, high-speed imaging system at 180.000 frames per second is used to resolve the flow field by particle tracking velocimetry. The results of this work could help to revise current models for the shape oscillation of microbubbles in order to get a deeper understanding of the underlying physics. This could help to improve microfluidic applications that use acoustically actuated microbubbles as a tool for the manipulation of flows and particles in Lab-on-a-chip-devices.
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