Micro Propulsion in Liquid by Oscillating BubblesFeng, Jian (2016) Micro Propulsion in Liquid by Oscillating Bubbles. Doctoral Dissertation, University of Pittsburgh. (Unpublished)
AbstractA number of attempts have been made to fabricate microswimmers that possibly navigate in vivo including the artificial magnetic bacteria flagella, chemical microswimmers and natural organism based microswimmers. This paper presents another propelling mechanism in micron scale that works by oscillating microbubbles in acoustic field. First of all, the propulsion mechanism is proven by two-dimensional computational fluid dynamics (CFD) simulations. Then, the microswimmer device is made on a parylene structure by photolithography. The underwater propulsion in one-dimensional is demonstrated and the propulsion mechanism is also confirmed by experiments. The relation of the propulsion speed/bubble oscillation amplitude and the input acoustic signal is measured. It is shown that the propulsion will happen when the bubble oscillation amplitude (or Reynolds number) gets large enough which is close to the system acoustic resonance. Around this resonance frequency (about 11 kHz), the measured propulsion speed is up to 45 mm/s and payload-carrying ability is realized. The one-directional rotation acoustic turbo is also made with a speed of about 75 rpm. This acoustic frequency dependence also becomes the foundation for two-dimensional propulsion. Then, the bi-directional motion and two-dimensional steering motion are realized by microbubbles with different lengths based on their different acoustic resonances. First of all, the frequency behavior for long (about 760 μm average length) and short (about 300 μm average length) bubbles at about 6 kHz and 11 kHz are measured, including oscillation amplitude and generated microstreaming. By adjusting input acoustic frequency, specific bubbles could be activated selectively. Then, when the different microbubbles are arranged into opposite directions, the bi-directional propulsion can be realized, including back/forth motion and clockwise/counter-clockwise rotation. The bi-directional motion mechanism is also confirmed by three-dimensional CFD simulations and the net force is calculated. The concept is further developed into two-dimensional propulsion by arranging long and short bubbles into orthogonal directions on the same device. By switching the input acoustic frequency, the controlled steering propulsion is illustrated on a two-dimensional plane. Carrying of objects in a T-junction microchannel is shown as well. The last part of this thesis is focused on developing the microswimmer into a biodegradable device, including long- and short-tem. The long-term biodegradable device is fabricated by polycaprolactone (PCL) by a simple dipping method, and propulsion in a minitube is shown. The short-term biodegradable device is fabricated by rolling up magnesium film based on building stress mismatch mechanically with help of a stretcher. The method could also be applied to aluminium and parylene film rollups. At last, the propulsion and biodegradable abilities of magnesium microtube are demonstrated. Share
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