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Title: Vortex generation and bubble dynamics in a microfluidic chamber with actuations
Authors: Shang, Xiaopeng
Keywords: DRNTU::Engineering::Mechanical engineering
Issue Date: 2017
Source: Shang, X. (2017). Vortex generation and bubble dynamics in a microfluidic chamber with actuations. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: This dissertation presents a comprehensive study on vortex generation, vortex control and bubble dynamics in a microfluidic chamber with PZT (lead-zirconate-titanate) actuations, which can be applied to mixing enhancement and manipulations of microparticles and nanoparticles in microfluidics. The microfluidic chamber is featured with an acoustic resonator shape, and the PZT disk is formed as part of the chamber walls. The working fluid (DI water) is pumped into the chamber with a controllable flowrate. Both vortices and bubbles are generated inside the chamber, depending on the actuation conditions. In the study on vortex generation, it is found that, at proper actuation conditions, large-scale vortices are formed inside the chamber, and the vortex generation and intensity can be regulated by the operation conditions, including actuation frequency, voltage and the flowrate through the microfluidic chamber. There is an actuation frequency window between 1.5 kHz and 3 kHz for the vortex generation and at the actuation frequency out of this window, the vortex is very weak or even unable to occur. The vortex intensity is found to increase with actuation voltage, and decreases with the flowrates. The direction of the generated vortex can, in principle, be clockwise or counter-clockwise due to the symmetry of the chamber and the PZT actuator. However, it is found that only one type of vortex can occur in an individual microfluidic chamber. This is probably due to the imperfection in the chamber fabrication. The control of the vortex direction has been carried out. The microfluidic chamber is modified by dividing the piezoelectric transducer into two parts which can be actuated separately, so that the actuation becomes asymmetric. In the modified microfluidic chamber, the vortex direction can be shifted on-demand from clockwise to counterclockwise by a switch to alter the working transducer. The mechanism of the vortex generation in the present study is investigated. It is found that the vibration of the PZT disk induces a large-amplitude pulsatile flow at the outlet channel of the chamber, where the geometry and configuration are similar to a sudden expansion from a narrow channel to a wide chamber. It is proposed that the vortex generation in the present study is due to the instability of the actuation-induced pulsatile flow through the sudden expansion part of the outlet channel at high Reynolds number. The flow velocity, corresponding to the actuation, at the outlet channel is measured to confirm that the flow Reynolds number at the working conditions is high enough for the vortex generation. Bubbles can be generated inside the chamber when the actuation voltage is larger than a threshold and the actuation frequency is within the range between 0.5 kHz and 5 kHz. The bubbles can be generated in forms of a single bubble, or as a cloud of bubbles, depending on the applied voltage. Such type of bubble generation is novel compared to the conventional bubble generations in other microfluidic devices reported in open literatures. The dynamics of multi-bubbles in the chamber with actuations at 1 kHz is investigated both experimentally and numerically. It is found that, under actuations, the bubbles are generated near the center of the chamber, grow up in size and move upstream against the hydrodynamic flow from inlets. Along with the bubbles’ motions, coalescences and breakups happen frequently and cause vigorous motions of surrounding liquids, which has a major contribution to the mixing enhancement. The pressure variation and distribution in the chamber are calculated by numerical simulation. The simulation results show that inside the chamber there is a low-pressure zone, which is consistent with the area of bubble generation observed in the experiment. The amplitude of the actuation exceeds the Blake threshold of nuclei (> 10 μm), which leads to the explosive growth of the nuclei in the working fluids, and hence the bubble are generated and observed in the experiments. The dynamics of a single bubble is investigated both experimentally and analytically. It is found that the bubble, after being generated, translates upstream against the flow at an average velocity of about 0.23 m/s, and undergoes significant expansion and compression with a ratio of the maximum to minimum volume up to 22. In order to understand the bubble translation under actuations, a simplified analytical model which describes the dynamics of a bubble nearby two intersected boundaries is constructed by using the Lagrangian scheme and the method of images. The analytical results of the model are found to be agreeable with the experimental observations in terms of the bubble expansion, compression and translation. It is concluded that the bubble translation is caused by the attractive secondary Bjerknes force of image bubbles behind the two inclined walls, which is the so-called wall effect.
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