Application of acoustophoresis to cells and particles separation in microchannel filled with fluid medium has been drawing increasing attention in many disciplines in the past decades due to the high precision and the minimum damages to the matters of interest. This study aims to use the technology on sub-micron particles manipulation with the goal of supporting the future development in aerosol measurement technique. Previous studies on particle separation often rely on the size-dependent feature of acoustic radiation force (ARF), while the acoustic streaming effect (ASE) is a hurdle as the size goes down. Sub-micron particles of different sizes circulate according to the streaming vortices and become inseparable. Adjusting the frequency of the soundwave, the aspect ratio of the microchannel and the viscosity of the fluid medium are suggested to enhance the acoustic field, thereby successfully controlling the movement of small particles. Instead of suppressing the ASE, this study intends to utilize the combined effect of ARF and ASE on sub-micron particles sorting by numerically investigating the streaming pattern in varies cross-sectional shapes of the microchannel. In the computational simulations, the governing perturbation equations for the thermoacoustic field are employed. The perturbation series of fluid density, pressure and velocity include three components each: the state in the absence of an acoustic field, first-order and second-order quantities. By putting them into the continuity and momentum equations together with the constitutive relation, perturbation equations with different orders are derived. Solving those equations give the acoustic field and the streaming velocity. The equation of motion of the particles are formed by considering the Stroke drag force from ASE and the ARF. The actuation of the ultrasound is modelled using linear velocity boundary condition on the base wall of the microchannel at 7.5MHz, while the other walls are modelled using no-slip boundary condition. No background flow is considered throughout the investigation. The roles of ARF and ASE on submicron particles of 0.1um and 0.5um in three different cross-sectional shapes are studied, which include: 1. rectangular, 2. trapezoidal, and 3. triangular. It is observed that changing the cross-sectional shapes alters the streaming patterns and the magnitude of streaming velocity. For all cases, the streaming pattern is symmetric among the left and right halves of the microchannel. However, among the top and bottom halves, the streaming pattern changes from symmetric in the rectangular cross-section to asymmetric in non-rectangular cross-section. Particularly, in the triangular microchannel, the vortices merge into one creating a greater streaming velocity gradient in the vertical direction. Regarding the particle motion, both 0.1um and 0.5um particles are dominated by ASE in the rectangular cross-section. Although the larger ones move slightly faster, the spatial distance between particles of different sizes is insufficient to separate them. In trapezoidal cross-section, most of the larger particles remain in the lower vortex but the smaller particles travel between the top and bottom vortices. Remarkable results are found in triangular cross-section that the larger particles are first driven towards the pressure node by the ARF and then dominated by the ASE, leading to a significant difference in the magnitude of velocity among the two types of particles and a clear separation between them at a few time instances. The results indicate that modifying the cross-sectional shape of the microchannel has the potential to enhance the manipulation of sub-micron particles and hence improve the efficiency of aerosol measurement. The study outcomes can be used to determine the length of the ultrasound actuation region. Dividing the flow at a suitable location and time instance can, therefore, separate submicron particles of different sizes.


Numerical Study on the Influence of Microchannel\u2019s Geometry on Sub-Micron Particles Separation Using Acoustophoresis

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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