A Study of Acoustic Radiation Force on Fluid Interface and Suspended Particles in Micro-Fluidic Devices

Abstract

Previous studies on cellular particle separation using acoustic radiation force have mainly focused on separation within a single fluid that needs a subsequent procedure to re-dilute separated particles into other media for cellular analysis. In this thesis, a new bi-fluid micro-flow methodology is proposed to combine the particle separation and re-dilution, also known as solvent exchange process, using the acoustic radiation force. The prototype experimental results show successful particle transport from its original solvent to the other solvent and particle collection at the outlet. This particle transport methodology extends the previous acoustic particle separation methods, most of which were performed within a single fluid. The transport process simplifies the cell preparation, resulting in less complex lab-on-chip systems. <br><br>A 2-D viscous hydrodynamic model governed by Stokes equations was firstly developed and solved by the boundary element method (BEM). The flow of two fluids in parallel in a micro-channel was studied by this model and verified by the experimental results. This 2-D model shows that the fluid viscosities, input flow rates and outlet pressures are the three major factors which affect the location of the fluid interface in the micro-channel. By changing these three factors, the fluid interface location can be controlled.<br><br>A methodology has been employed to transport particles between two parallel flows. In this methodology, the shift of the acoustic pressure node due to the different acoustic properties of the two fluids was studied. The fully developed fluid interface was designed to be offset from the shifted acoustic pressure node by adjusting the input flow rates. This offset between the interface and the pressure node enables particle transport from one fluid to the other using the acoustic radiation force. The experimental results obtained by the prototype micro-flow system proved that, for both the similar-fluid case (the pressure node is not shifted) and the dissimilar-fluid case (the pressure node is shifted significantly), this methodology could separate micro particles from one aquatic dilution, and simultaneously transport them into another one. The transported particles suspended in the second fluid flow could be collected downstream. Since the acoustic radiation force is a non-contact force which is based on the densities and compressibilities of the particles and fluids, this methodology provides a wide application potential, especially for cell separation integrated in lab-on-chip systems where aquatic dilutions are commonly used. <br><br>Finally, the deformation of the fully developed fluid interface due to the acoustic field was studied. The experimental results show that the directions of both the interface deformation and the acoustic radiation force agree with each other. The experimental results also indicate the frequency sensitivity of the interface deformation. Besides the experimental studies, a 2-D numerical model including the piezo-ceramic transducer, the microchannel structure and the bi-fluid flow was built to simulate the acoustic radiation force acting the interface. The analysis obtained indicates that the acoustic radiation force has caused the interface to be deformed from its original location. This estimation of the interface deformation is critical for the particle transportation.<br>