Modeling of Acoustically Induced Rapid Mixing Processes in Microchannels Using Acoustic Streaming
Technical advancements in miniaturization in the last decades have, among others, brought forth the technology of so called “Lab-on-a-Chip” (LOC) systems. These systems provide the possibility to automate and accelerate certain laboratory processes, e.g., biomedical diagnostics, on the microscale. LOC systems usually consist of a network of sub-millimeter wide channels for processing fluids. Frequently, fluids need to be mixed, e.g., for diagnostical purposes. Due to the laminar nature of flow in those microchannels the mixing mechanism is governed by slow diffusion processes which would require unsuitably long channels to achieve homogeneous mixing. Actively introducing convection processes into the flow greatly enhances the mixing efficiency. Various publications in recent years have discussed the possibility of employing ultrasound in combination with sharp edged structures in microchannel walls to achieve rapid mixing. When actuating the channel with ultrasound the sharp edges vibrate and produce local acoustic streaming phenomena which in turn lead to substantially enhanced mixing of the fluids. Using acoustic frequencies in the low MHz-range, the wavelength is much larger than the channel width and thus unified actuation of the channel segment including the sharp edges can be assumed. Building upon this previous work, we employ the new Acoustic Streaming interface in the Acoustics Module of COMSOL Multiphysics® to simulate the mixing of two identical fluids with different species concentrations in a 2D or 3D segment of a straight microchannel containing sharp, evenly spaced, triangular edges. Our modeling pipeline combines the Acoustic Streaming interface for Pressure and Thermoviscous Acoustics with an additional Laminar Flow interface for the background flow and the Transport of Diluted Species interface to simulate two different species concentrations. The computational grid needs to be highly refined around the sharp edges to resolve the viscous boundary layer. The model is solved using four study steps, first solving the acoustics in the frequency domain, and subsequently calculating the stationary solution of the acoustic streaming flow, the laminar background flow as well as the concentration distribution. The obtained simulation results show a remarkable improvement of the mixing efficiency when introducing acoustically actuated sharp edges into the LOC-system. We used this setup to conduct parameter studies and determine optimal geometrical properties of the mentioned sharp edges, e.g., height, tip-angle and spacing, and other process parameters like actuation frequency and inlet velocity. The parameter study identified the sharp edges height and spacing as critical parameters of the geometrical setup. Furthermore, the inlet velocity could be determined as a critical parameter, improving the mixing quality with lower inlet velocities due to the longer residence time. Our results are comparable with previously published findings based on simulations employing the Weak Form PDE interface of COMSOL®.
