Vortex-Enhanced Microfluidic Chip for Efficient Mixing and Particle Capturing Combining Acoustics with Inertia.

Vortex-based microfluidics has received significant attention for its unique characteristics of high efficiency, flexible control, and label-free properties for the past decades. Herein, we present a vortex-based acousto-inertial chip that allows both fluid and particle manipulation within a significantly wider flow range and lower excitation voltage. Composed of contraction-expansion array structures and vibrating microstructures combined with bubbles and sharp edges, such a configuration results in more vigorous vortical fluid motions. The overall improvement in device performance comes from the synergistic effect of acoustics and inertia, as well as the positive feedback loop formed by vibrating bubbles and sharp edges. We characterize flow patterns in the microchannels by fluorescence particle tracer experiments and uncover single- and double-vortex modes over a range of sample flow rates and excitation voltages. On this basis, the ability of rapid and efficient sample homogenization up to a flow rate of 200 µL/min under an excitation voltage of 15 Vpp is verified by a two-fluid fluorescence mixing experiment. Moreover, the recirculation motion of particles in microvortices is investigated by using a high-speed imaging system. We also quantitatively measure the particle velocity variation on the trajectory and illustrate the capturing mechanism, which results from the interaction of the microvortices, particle dynamics, and composite microstructure perturbations. Further utilizing the shear forces derived by microvortices, our acousto-inertial chip is demonstrated to lysis red blood cells (RBCs) in a continuous, reagent-free manner. The high controllability and multifunction of this technology allow for the development of multistep miniaturized "lab-on-chip" analytical systems, which could significantly broaden the application of microvortex technology in biological, chemical, and clinical applications.

A multi-vortex micromixer based on the synergy of acoustics and inertia for nanoparticle synthesis.

Mixing is one of the most important steps in chemical reaction, sample preparation and emulsification. However, achieving complete mixing of fluids at high throughput is still a challenge for acoustic micromixers, which are limited by the intensity of the acoustic streaming. In this study, we proposed an acoustic-inertial micromixer based on multi-vortex synergy by introducing inertia into acoustic micromixer. The device contains side-wall sharp-edge structure and contraction-expansion array structure (SCEA) in the microchannel to enhance the acoustic streaming with inertial vortices. The mixing mechanism of SCEA was explored and the mixing process showed three modes: acoustic streaming dominant mode, acoustic-inertial synergy mode and inertial vortex dominant mode. On the basis of the "vortex seed" provided by the contraction-expansion structure, stronger chaotic advection was produced by the synergy of acoustic streaming and inertial vortices (including Dean vortex and horizontal vortex). Rapider mixing (0.20 m s) and wider operating ranges (0-3000 µL/min) were achieved in SCEA at lower driving voltages compared with conventional acoustic micromixers. Finally, more homogeneous and tunable chitosan nanoparticles and shellac nanoparticles were synthesized based on this device. The micromorphology, particle size distribution and drug loading properties of the products were measured and compared. This work provides a platform for control of mixing process in specific application environments with high operational flexibility, indicating potentially wider application of SCEA in multi-functional integration of lab-on-a-chip systems.

Degeneration of flow pattern in acousto-elastic flow through sharp-edge microchannels.

Acoustic streaming (AS) is the steady time-averaged flow generated by acoustic field, which has been widely used in enhancing mixing and particle manipulation. Current researches on acoustic streaming mainly focus on Newtonian fluids, while many biological and chemical solutions exhibit non-Newtonian properties. The acoustic streaming in viscoelastic fluids has been studied experimentally for the first time in this paper. We found that the addition of polyethylene oxide (PEO) polymer to the Newtonian fluid significantly altered the flow characteristics in the microchannel. The resulting acousto-elastic flow showed two modes: positive mode and negative mode. Specifically, the viscoelastic fluids under acousto-elastic flow exhibit mixing hysteresis features at low flow rates, and degeneration of flow pattern at high flow rates. Through quantitative analysis, the degeneration of flow pattern is further summarized as time fluctuation and spatial disturbance range reduction. The positive mode in acousto-elastic flow can be used for the mixing enhancement of viscoelastic fluids in the micromixer, while the negative mode provides a potential method for particle/cell manipulation in viscoelastic body fluids such as saliva by suppressing unstable flow.