In situ investigation on ultrasound (US)-generated bubbles' dynamics for membrane fouling control applications

Abstract

The purpose of this research article is to gain a comprehensive understanding of the influence of ultrasound (US) power intensity on the stationary and moving microbubbles close to the membrane surface and its application on membrane fouling control. Compared to other imaging techniques, synchrotron in-line Phase Contrast Imaging technique, available at the biomedical imaging and therapy (BMIT) beamlines at the Canadian Light Source (CLS), allowed for in-situ quantitative analysis and real-time imaging of microbubbles in water inside a membrane filtration unit. An advanced non-invasive synchrotron propagation-based imaging (PBI) technique was employed to improve the contrast between the liquid and gas inside the bubble (interface of liquid and gas), which was crucial for the qualitative and quantitative analyses. ImageJ software was employed for the quantitative analyses of cavitational bubbles dynamics and their characteristics at different US power intensities. The bubble-related parameters, including the number, size, and fraction of total area occupied by bubbles, were determined for all captured images at three different US power intensities. Bubble characteristics at different times of US exposure and distances from the US transducer were also studied. Then, MATLAB software was used to generate the plots for the bubble size distribution and probability density for each image. The effect of US power intensity on bubble velocity, and its application in membrane fouling control was also investigated. The generated bubbles were mostly located in the middle of the images, close to the membrane surface, where the US waves were detected. It was also observed that the velocity of smaller bubbles was much larger than that of the larger bubbles; and they tended to move toward the US transducer. Bubble growth was much faster at 100 W than at lower US power intensities. The experimental results were in good agreement with the in-situ observation of microbubbles. The mass transfer across the membrane was improved as the US power increased from 25 W to 100 W. This was likely due to more frequent bubble explosions and energy releases into the water medium, causing turbulence flow, and resulting in more membrane surface cleaning.