Abstract
Ultrasonic standing waves are increasingly applied in the manipulation and sorting of micrometer-sized particles in microfluidic cells. To optimize the performance of such devices, it is essential to know the exact forces that the particles experience in the acoustic wave. Although much progress has been made via analytical and numerical modeling, the reliability of these methods relies strongly on the assumptions used, e.g. the boundary conditions. Here, we have combined an acoustic flow cell with an optical laser trap to directly measure the force on a single spherical particle in two dimensions. While performing ultrasonic frequency scans, we measured the time-averaged forces on single particles that were moved with the laser trap through the microfluidic cell. The cell including piezoelectric transducers was modeled with finite element methods. We found that the experimentally obtained forces and the derived pressure fields confirm the predictions from theory and modeling. This novel approach can now be readily expanded to other particle, chamber, and fluid regimes and opens up the possibility of studying the effects of the presence of boundaries, acoustic streaming, and non-linear fluids.
Highlights
The arrangement of small objects with ultrasonic waves (US) finds widespread use in diverse fields such as chemistry, material sciences and medicine
We have combined an US microfluidic cell with an optical trap to directly measure the 2D forces that act on a single bead in the acoustic field
For specific research fields, such as the mechanotransduction in cells, the added possibility to literally “tune-in” to the cell type, while holding the specimen in the focal plane can open up new experimental possibilities
Summary
The arrangement of small objects with ultrasonic waves (US) finds widespread use in diverse fields such as chemistry, material sciences and medicine. Properties and material properties made the reproducible determination of fundamental parameters like pressure amplitude or force in a dedicated measurement system difficult.[6] Acoustic streaming phenomena make these issues even more complicated. For the performance of lab on a chip (LOC) devices, the acoustic energy related to the squared pressure is the decisive parameter; that is why it needs to be measured.[22] In the research field of device development, the acoustic pressure distribution is of essential interest to reach the targeted functions of the device. COMSOL simulations provided the information about the pressure distribution, and by experimental qualitative observations, the numerical results were proven. These observations do not provide a quantitative validation of the simulation and the real acoustic pressure amplitudes remain unknown. Our direct force measurements are highly relevant and close the gap of missing information about the acoustic pressure distribution inside a LOC device
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