Alen Pavlic

Alen Pavlic

Dr. sc. ETH

Welcome!

About me

I am Alen Pavlic, a Slovenian postdoctoral researcher working in the Mechanics and Experimental Dynamics group of Prof. Jürg Dual at Institute for Mechanical Systems, ETH Zurich, in Switzerland.

The focus of my research (so far) lies in acoustophoresis and the exploration of the underlying forces. If you want to know what acoustophoresis is or learn more about my contributions to the field, keep reading. For a more detailed overview of my (research) activities, check out the rest of the website.

Summary of my acoustophoresis research at ETH Zurich

Acoustophoresis is the motion of objects in acoustic fields due to the action of acoustic forces. It can be used, for example, to separate, trap, and focus micrometer-sized objects, such as biological cells or metal nanoparticles. If the density and/or compressibility of an object and the surrounding medium differ, an acoustic wave will scatter at the object, which leads to the rise of the so-called radiation pressure on the object. Scattering and the deformation of the surface of the object lead also to the rise of microstreaming, which influences the stresses and the total acoustic radiation force (ARF) exerted on the object. If there are more objects in close proximity, or if the object consists of multiple parts, acoustic interaction force (AIF) arises from the re-scattering and from the interaction of multiple microstreaming fields. Looking at a broader picture, the fluid cavity which contains the acoustically-manipulated objects provides the acoustic field responsible for the manipulation. The interactions of the acoustic field with the cavity walls and the attenuation in the fluid bulk, lead to the rise of the environmental acoustic streaming (AS). The three forces – ARF, AIF and the drag from AS – mainly govern the acoustophoresis and are often the most important forces for acoustic manipulation in microfluidic systems.

We made several theoretical and experimental advances regarding the three forces, with a focus on the acoustic manipulation of small objects, such as bacteria, for which the effects of viscosity become important as the region of the viscosity-supported rotational acoustic velocity field close to the objects becomes comparable to the size of an individual object.

A numerical model that we developed has been used to resolve an open disagreement between two opposing theories in the field of the ARF in a standing wave, re-establishing the importance of viscosity for acoustic manipulation of small particles in a standing wave [1]. Extending the numerical model, we further elucidated the role of microstreaming and viscosity on the ARF [2] and on the AIF [3]. These studies will likely continue gaining on relevance, due to the continuous and promising efforts of the scientific community towards suppressing the disturbances of ARF-driven manipulation due to the environmental AS.

Equipped with the numerical estimation of viscous effects on the acoustophoresis of small objects in a standing wave, we investigated acoustophoresis of several bacteria in drinking water that are relevant for water safety [4]; specifically, several species of Legionella, Pseudomonas aeruginosa, and Escherichia coli. Using a see-through silicon-glass bulk acoustic wave (BAW) devices manufactured with cleanroom procedures, we were able to demonstrate acoustic focusing of all the tested bacterial species, as well as that of Acanthamoeba castellanii that can harbor L. pneumophila in water. We quantified the acoustic focusability of the majority of investigated species in a form of acoustic contrast factor to provide a standard parameter that can be used in designing devices for acoustic separation or up-concentration of bacteria in water. In addition, we theoretically and experimentally investigated the influence of concentration of bacteria on the acoustic focusability, discovering a phenomenon that we call collective hydrodynamic focusing (CHF).

In the context of acoustic fields and AS in fluid cavities, we analytically solved a classical problem posed by Lord Rayleigh more than 130 years ago – acoustic streaming in a Kundt’s tube. In his formulation [5], Rayleigh simplified the problem by assuming low viscosity and two-dimensional geometry (two infinite parallel plates at a distance). In our study [6], there is no restriction on the viscosity and the geometry of the tube is cylindrical with circular cross-section. The results are especially relevant for microscale acoustofluidic systems, wherein the viscosity can play an important role. An example of such a device relevant for 3D metal printing was recently developed in our group [7].

Lastly, we designed, fabricated, and characterized fluid cavities with sharp-edge structures that are known to produce a strong response to an acoustic field in a form of AS [8] and ARF [9]. We integrated such structures in chemically-resistant BAW devices at the openings of side channels in order to facilitate a pumping flow at the microscale [10]. Furthermore, the inherent nature of the sharp-edge AS enables simultaneous mixing of fluids, while the impedance mismatch between water and the BAW device material (silicon/glass) enables a generation of standing waves withing microfluidic channels for focusing or trapping of biological cells. These functionalities are programmable and can be tailored to a specific task. We explained the observed phenomena through numerical models that show promise for rapid adjustment and optimization of such multifunctional acoustofluidic devices in the future.

References

  1. Baasch, T.,* Pavlic, A.,* & Dual, J. (2019). Acoustic radiation force acting on a heavy particle in a standing wave can be dominated by the acoustic microstreaming. Physical Review E, 100(6), 061102. DOI: 10.1103/PhysRevE.100.061102

  2. Pavlic, A., Nagpure, P., Ermanni, L., & Dual, J. (2022). Influence of particle shape and material on the acoustic radiation force and microstreaming in a standing wave. Physical Review E, 106(1), 015105. DOI: 10.1103/PhysRevE.106.015105

  3. Pavlic, A., Ermanni, L., & Dual, J. (2022). Interparticle attraction along the direction of the pressure gradient in an acoustic standing wave. Physical Review E, 105(5), L053101. DOI: 10.1103/PhysRevE.105.L053101

  4. Pavlic, A., Veljkovic, M., Fieseler, L., & Dual, J. (2022). Acoustophoresis of Legionella species in water and the influence of collective hydrodynamic focusing. arXiv preprint arXiv:2211.11508. DOI: 10.48550/arXiv.2211.11508

  5. Strutt, J. W. (1884). I. On the circulation of air observed in Kundt’s tubes, and on some allied acoustical problems. Philosophical Transactions of the Royal Society of London, (175), 1-21. DOI: 10.1098/rstl.1884.0002

  6. Pavlic, A., & Dual, J. (2021). On the streaming in a microfluidic Kundt’s tube. Journal of Fluid Mechanics, 911. DOI: 10.1017/jfm.2020.1046

  7. Gerlt, M. S., Paeckel, A., Pavlic, A., Rohner, P., Poulikakos, D., & Dual, J. (2022). Focusing of Micrometer-Sized Metal Particles Enabled by Reduced Acoustic Streaming via Acoustic Forces in a Round Glass Capillary. Physical Review Applied, 17(1), 014043. DOI: 10.1103/PhysRevApplied.17.014043

  8. Doinikov, A. A., Gerlt, M. S., Pavlic, A., & Dual, J. (2020). Acoustic streaming produced by sharp-edge structures in microfluidic devices. Microfluidics and Nanofluidics, 24(5), 1-13. DOI: 10.1007/s10404-020-02335-5

  9. Doinikov, A. A., Gerlt, M. S., & Dual, J. (2020). Acoustic radiation forces produced by sharp-edge structures in microfluidic systems. Physical Review Letters, 124(15), 154501. DOI: 10.1103/PhysRevLett.124.154501

  10. Pavlic, A., Harshbarger, C. L., Rosenthaler, L., Snedeker, J. G., & Dual, J. (2023). Sharp-edge-based acoustofluidic chip capable of programmable pumping, mixing, cell focusing and trapping. Physics of Fluids. DOI: 10.1063/5.0133992