No abstract

The transport of bubbles to a neighboring surface is very important in surface chemistry, bioengineering, and ultrasonic cleaning etc. This paper proposes a multi-bubble transport method by using an acoustic standing wave field and establishes a model which explains the multi-bubble translation by expressing the balance between Bjerknes forces and hydrodynamic forces on a bubble in a liquid medium. An uniform one-dimensional acoustic standing wave field was created by a multi-layered resonator which was designed based on a one-dimensional equivalent network model. A pair of modified Keller-Miksis equation and translation equations, which take into account the influence from boundary surfaces and neighboring bubbles, were used to simulate the bubble translations. The bubble translations were observed by a high speed camera system. Results indicated that the bubble translations were mainly influenced by the acoustic wave field, the boundary, neighboring bubbles, and buoyancy force from the surrounding liquid. The primary Bjerknes force generated by the sound field dominated the bubble translations at the beginning when the bubbles were far away from the boundary. A surge of attractive secondary Bjerknes force from the boundary was seen when the bubbles were approaching the boundary. The attraction force outweighed the primary Bjerknes force within a short distance of the surface and resulted in a faster bubble motion. Besides the forces generated from the acoustic field and the boundary, neighboring bubbles also exerted secondary Bjerknes forces on a target bubble and influenced its translation. Moreover, to optimize the bubble translation in a multi-bubble environment, a parametric study was carried out to investigate the influence of varied bubble size and acoustic pressure amplitudes on the bubble translation. It was found that increasing the size of a bubble can hardly alter its trajectory but only force it to move at a faster speed. An increase of pressure amplitude can also accelerate the bubble motion and enhance the bubble-bubble interaction. The secondary Bjerknes force between two bubbles can switch from an attractive one when they oscillate in phase, to a repulsive one when the bubble oscillations are out of phase. These findings provide an insight into the multi-bubble translation near a surface and can be applied to future bubble motion control studies, especially in drug delivery, sonoporation, and ultrasonic cleaning.

Sub-harmonic imaging using encapsulated microbubbles (EMs) improves the contrast of ultrasound imaging by taking advantage of increased contrast to the tissue signal. A dual-frequency excitation technique (DFET) is proposed for enhancing the sub-harmonic emission from EMs as compared with the conventional single frequency sinusoidal excitation technique (SFSET). This study includes theoretical simulation and in vitro experimental verification. A dual-frequency signal (2 and 4 MHz) is used to insonate EMs developed in our laboratory. Both theoretical and experimental studies indicate that the DFET may be able to improve the amplitude of the sub-harmonic component up to 13 dB over the SFSET. Increasing the value of the pulse repetition frequency or the number of cycles of ultrasound tone burst in the application of the DFET may increase the sub-harmonic emission. Furthermore, it is confirmed that the amplitude ratio of the second frequency (4 MHz) to the first frequency (2 MHz) and phase shift of the second frequency with respect to the first frequency also play an important role in sub-harmonic emission. A ratio of 0.5 and a phase shift around 180 degrees are found to be the optimum values.