Acoustic Fluid Relocation for the Separation of Exosomes
The aim of the present work is to explore through the developed numerical model the separation of exosomes from a more complex mixture of particles i.e. red blood cells and white blood cells, through combined acoustofluidics and fluid relocation in a bulk acoustic wave microfluidic device. Theoretical and numerical model discussed by [1], are used as basis for the governing equations and are extended to simulate the fluid relocation phenomena. A rectangular cross section of the channel is chosen as control volume. Only the perturbation in the fluid domain is considered and the solid material of the channel is modeled as ideal rigid walls. The acoustic field excitation is modeled through an oscillating velocity boundary condition applied at the side walls at the resonance frequency, studied using a Parametric Sweep. The system is assumed as adiabatic. The starting concentration profile is defined with a Step Function, in which the fluid with higher impedance is placed at the side of the channel. The weak form of governing equations is implemented in COMSOL Multiphysics®. Since the acoustic field and the inertia, viscous relaxation and steady shear motion have two different time scales, the model is solved in two steps: first the faster acoustic time scale is solved keeping fixed the hydrodynamic degrees of freedom. The derived acoustic force is then used as a source term for solving, with a Time-Dependent Segregated Solver, the slower time scale that describes the motion of the fluid in the fluid channel. A rectangular mesh with about 45000 elements, is used for this study. For the fast time scale perturbation theory was used in the equation of conservation of momentum and mass and considered only the first order approximation for velocity, pressure and density. The oscillation-time-averaged acoustic momentum-flux-density tensor drives both the acoustic streaming and the fluid relocation and lead to an acoustic force that depends on the first order field and on the concentration field.
The hydrodynamics corresponding to slow time scale is studied as an advection-diffusion problem forced by the acoustic force and the gravity.
References
[1] J. T. Karlsen, W. Qiu, P. Augustsson, and H. Bruus, “Acoustic Streaming and Its Suppression in Inhomogeneous Fluids,” Phys. Rev. Lett., vol. 120, no. 5, p. 54501, 2018.
