I.H. Karampelas, J. Gómez-Pastora, M.J. Cowan, E. Bringas, I. Ortiz, E.P. Furlani
Flow Science Inc,
Keywords: acoustophoresis, standing acoustic wave, SAW, λ/2 channels, microchannel oscillations, nanoparticles, particle focusing
Summary:Acoustophoretic particle focusing is a modern and very attractive method of removing a variety of objects from solutions in a microfluidic channel . The process is inherently general and can be readily extended for multiple types of applications such as healthcare (e.g. malignant cell removal), academic research (e.g. nanoparticle separation), industrial (e.g. reclaiming of rare earths) and environmental applications (e.g. sequestration of suspended solids). According to classical understanding, particle separation is realized by acoustic forces i.e. pressure forces generated by standing wave field. Traditionally, the wavelength of the acoustic wave has to be fine-tuned to the channel width and specified to λ = 2Lchannel [2, 3]. However, particle focusing can also be achieved at other frequencies. This phenomenon may be due to the fact that particle motion occurs due to contributions from multiple forces including pressure gradient, added mass, fluidic drag and, potentially, impulse forces occurring due to collisions of the particles with the channel walls, given that the length amplitude microchannel oscillation is sufficiently large. This combination of factors acts to focus the particles undergoing acoustophoretic separation in the middle of the microchannel. To the best of our knowledge, this is the first computational study that takes into account all the force contributions previously mentioned. More specifically, preliminary results from numerical simulations performed with FLOW-3D, appear to indicate that channel oscillations at increased length amplitude generate a pressure gradient similar to what is observed in λ/2 channels even at off resonance frequencies. Depending on the phase of the oscillation, the pressure field changes accordingly, effectively pushing the particles to the center of the microfluidic channel. Moreover, impulse forces caused by collisions with the channel walls act to accelerate the process of particle focusing at high length amplitudes. The numerical results obtained with this method seem to indicate a level of separation exceeding 90% for an overall process time of less than 4 ms. This computational analysis can be generally applied to a multitude of acoustophoretic processes and should prove useful in promoting understanding of the phenomena involved in particle focusing.  Petersson, F., Åberg, L., Swärd-Nilsson, A.M. and Laurell, T., 2007. Free flow acoustophoresis: microfluidic-based mode of particle and cell separation. Analytical chemistry, 79(14), pp.5117-5123.  Barnkob, R., Augustsson, P., Laurell, T. and Bruus, H., 2010. Measuring the local pressure amplitude in microchannel acoustophoresis. Lab on a chip, 10(5), pp.563-570.  Muller, P.B., Barnkob, R., Jensen, M.J.H. and Bruus, H., 2012. A numerical study of microparticle acoustophoresis driven by acoustic radiation forces and streaming-induced drag forces. Lab on a Chip, 12(22), pp.4617-4627.