Flow Control through ultrasound-driven microbubble streaming
Final Report Abstract
Microstreaming generated by acoustically actuated microbubbles in microchannels can be used for a variety of applications to sort/mix microparticles/cells based on their physical properties. By automating the laboratory equipment, a higher degree of reproducibility was achieved. In particle tracking and fluorescence microscopy, experimental techniques have been developed by using more powerful and homogeneous light sources, and by performing particle tracking experiments in brightfield mode, the acquisition speed has been increased by more than two orders of magnitude to frame rates of 180,000 f/s. Furthermore, an empirical formula was derived to calculate the density of aqueous solutions of glycerol, sodium chloride, and heavy water to make a solution with neutrally buoyant particles ideal for particle tracking velocimetry. The publication has gained high impact by considering the citations. In a PDMS microchannel, air can enter the channel and cause bubble growth. We found that by increasing the hydrostatic pressure in the channel, this effect can be compensated so that a stable bubble is formed. Based on these findings, an automated setup was developed that guarantees a constant bubble size. This approaches solves fundamental problems and allows to perform much more reliable and reproducible experiments. High-speed imaging was used to investigate the microstreaming characterization for bubbles of different sizes, and an optimal bubble size for achieving maximum flow velocities was determined. By understanding the flow field, the motions of particles of different sizes in the microstreaming could be described with a steric model. Results indicate while small particles move along the streamlines of the flow field, larger particles are trapped at specific locations within the flow structure. Another goal of this work was to develop a new microchannel that allows multiple identical bubbles to be confined in one microchannel to improve microfluidic mixing. It was observed that proper mixing in such a channel requires a sufficiently low flow velocity in the microchannel to allow the fluid elements to make multiple revolutions in the flow structure otherwise only a two-dimensional displacement is achieved, resulting in stratification of the separated fluids. Based on the project outcomes, we could eventually introduce an automated microfluidic bead sorter with up to ten micrometer accuracy in positioning individual microparticles. In our novel method, cells or microparticles can be automatically detected, tracked and sorted across the entire width of the microchannel. In this approach, controllable microstreaming is used as a sorting operator to actively locate target particles, which is not readily possible in applications that use only an external on-off force. Subsequently, the control algorithms were integrated into the operation to accurately position every single particle with an arbitrary initial position. Our approach is robust to different particle properties such as size, shape, density, compressibility, etc., making this technique universally applicable in broad areas of microfluidics which does not require any special maintenance or cleaning, as the concept does not rely on moving parts, but only on an oscillating bubble.
Publications
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Growth control of sessile microbubbles in PDMS devices. Lab on a Chip, 15(24):4607-4613, 2015
A.Volk, M. Rossi, C.J. Kähler, S. Hilgenfeldt, and A. Marin
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Density model for aqueous glycerol solutions. Experiments in Fluids, 59(5):75, 2018
A. Volk and C. J. Kähler
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Size control of sessile microbubbles for reproducibly driven acoustic streaming. Physical Review Applied, 9(5):054015, 2018
A. Volk and C. J. Kähler
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Experimental investigation of oscillation modes and streaming of an acoustically actuated bubble in a microchannel. In 32nd International Congress on High-Speed Imaging and Photonics, volume 11051, page 110510A. International Society for Optics and Photonics, 2019
A. Volk, M. Rossi, B. Mutsch, and C. J. Kähler
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Size-dependent particle migration and trapping in three-dimensional microbubble streaming flows. Phys. Rev. Fluids 5, 114201 – Published 19 November 2020
A.Volk, M. Rossi, B. Rallabandi, S. Hilgenfeldt, C.J. Kähler, and A. Marin
