Theory of acoustic manipulation of microparticles influenced by thermal and viscous effects

Bjørn Gersholm Winckelmann

Research output: Book/ReportPh.D. thesis

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Acoustofluidics is an interdisciplinary field that integrates acoustic waves in microfluidic systems to apply forces to particles suspended in small fluid samples and to the fluid itself. The steady force experienced by a suspended particle from the scattering of the acoustic wave constitutes the primary working mechanism in acoustofluidic devices, where this so-called acoustic radiation force is used to manipulate small particles spatially. Through this, acoustofluidic devices trap, migrate, and concentrate bioparticles in watery solutions. Additionally, the acoustic forces acting on the fluid inside the microfluidic channel generate a steady flow, called acoustic streaming, that exerts a drag force on the suspended particles. The drag force tends to dominate the movement of smaller particles that are swirled around in the acoustic streaming patterns rather than being focused by the acoustic wave. Thus, acoustic streaming sets a lower limit for the size of particles that can be effectively manipulated in acoustofluidic systems, and it is, therefore, often an unwanted phenomenon.
Electroosmosis is another type of steady streaming applied to microfluidic systems by using external electrodes to manipulate ions in the fluid, thus generating an electric body force. In this thesis, theory and numerical simulations of combined electroosmosis and acoustic streaming are developed, and a method for suppressing the acoustic streaming in a typical acoustofluidic device by controlled electroosmosis is suggested. Subsequently, the focus of the thesis is shifted to a fundamental study of the acoustic radiation force on a single isolated particle, which extends previous theories by taking into account both particle vibrations, acoustic scattering, temperature- and density-dependent material parameters, and the acoustic streaming developing around the particle as well as inside the particle for the case of a fluid droplet. The extended theory predicts substantial deviations compared to the previous theories in the case of small particles, especially for particles made of materials with high mass density relative to the host fluid. For several cases, sign reversals in the acoustic radiation force are predicted for small particles compared to larger particles of the same material in the same fluid. Last, the theory of the acoustic radiation force is extended to study the case of a heated particle, and it is found that particle heating by an external source can significantly change the force. Again, sign reversals in the acoustic radiation force are predicted for certain combinations of particles and surrounding fluids, even for relatively small temperature increases. These theoretical discoveries could aid the development of new particle manipulation methods within the field of acoustofluidics.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages184
Publication statusPublished - 2023


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