Effect of High-frequency Acoustic Waves on the Fluid Interface PDF Download

Author: Shuai Zhang
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Languages : en
Pages : 0

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Book Description
High-frequency acoustic waves (above MHz) are widely applied in ultrasonic imaging and many microfluidic applications. For many cases, the response of interfaces to high-frequency acoustic waves can be simplified as a reflection and refraction process, where only the effect of the interfaces on acoustic wave propagation is considered. But for some physical processes, such as bubble and droplet manipulation, the response of the interfaces plays a key role. However, the interaction processes between high-frequency acoustic waves and interfaces, especially fluid interfaces, are mostly highly nonlinear. Most of the related work focused on acoustic waves with frequencies ranging from DC to kHz. There are continuously new phenomena found and reported showing the effect of high-frequency acoustic waves on fluid interfaces contrary to previously discovered theorems and laws. Yet many of the phenomena are poorly understood and explained. We briefly introduced the background of high-frequency ultrasonic devices, including the selection of material, design principles, physical models, and some of the important applications. Observations and measurements on the interface dynamics with small amplitudes and high frequencies have always been a challenge to researchers. Gas vesicles (GV) are bubbles wrapped by a protein layer with submicron sizes, which put forward an even higher requirement for the equipment and experiment design. We reported a precise non-contact technique to monitor GV's vibrational behavior using laser Doppler vibrometry. The resonance frequencies of the single GV and agglomerated GV were found and verified by the simulation results. We then focused on the behavior of the GV affected by lower frequencies (1-10 MHz) associated with medical and industrial applications. The change of GV's vibrational response to the acoustic wave with varying power input was observed. The mechanism of GV's function on ultrasonic signal amplification can be well explained with the measured linear and nonlinear behaviors (buckling and collapse). We then investigated the effect of high-frequency acoustic waves on the fluid-gas interface on the droplet's surface. We found that the sessile droplets with a small volume ( ~ [mu]L) produce visible, low-frequency vibrations (~ 100Hz) when exposed to high-frequency acoustic waves. However, there are no appropriate theories to predict capillary wave generation excited by high-frequency acoustic waves since the frequencies violate fundamental assumptions used in these theories. We brought up the acoustic radiation pressure-interface shape feedback mechanism to explain the energy transfer across vastly separate scales. The simulations based on the physical model are carefully compared with the direct observation of droplet interface dynamics from the high-speed digital holographic camera. The pressure-interface feedback model accurately predicts the vibration amplitude threshold at which capillary waves appear, the subsequent amplitude and frequency of the capillary waves, and the distribution of the standing wave pressure field within the sessile droplet responsible for the capillary waves.