In-situ synchrotron X-ray imaging of ultrasound (US)-generated bubbles: Influence of US frequency on microbubble cavitation for membrane fouling remediation

Abstract

Gaining an in-depth understanding of the characteristics and dynamics of ultrasound (US)--generated bubbles is crucial to effectively remediate membrane fouling. The goal of present study is to conduct in-situ visualization of US-generated microbubbles in water to examine the influence of US frequency on the dynamics of microbubbles. This study utilized synchrotron in-line phase contrast imaging (In-line PCI) available at the biomedical imaging and therapy (BMIT) beamlines at the Canadian Light Source (CLS) to enhance the contrast of liquid/air interfaces at different US frequencies of 20, 28 and 40 KHz at 60 Watts. A high-speed camera was used to capture 2,000 frames per second of the bubble cavitation generated in water under the ultrasound influence. Key parameters at the polychromatic beamlines were optimized to maximize the phase contrast of gas/liquid of the microbubbles with a minimum size of 5.5 µm. ImageJ software was used to analyze the bubble characteristics and their behavior under the US exposure including the microbubble number, size, and fraction of the total area occupied by the bubbles at each US frequency. Furthermore, the bubble characteristics over the US exposure time and at different distances from the transducer were studied. The qualitative and quantitative data analyses showed that the microbubble number or size did not change over time; however, it was observed that most bubbles were created at the middle of the frames and close to the US field. The number of bubbles created under the US exposure increased with the frequency from 20 kHz to 40 kHz (about 4.6 times). However, larger bubbles were generated at 20 kHz such that the average bubble radius at 20 kHz was about 6.8 times of that at 40 kHz. Microbubble movement/traveling through water was monitored, and it was observed that the bubble velocity increased as the frequency was increased from 20 kHz to 40 kHz. The small bubbles moved faster, and the majority of them traveled upward towards the US transducer location. The growth pattern (a correlation between the mean growth ratio and the exposure time) of bubbles at 20 kHz and 60 W was obtained by tracking the oscillation of 22 representative microbubbles over the 700 ms of imaging. The mean growth ratio model was also obtained.

Keywords: Bubble cavitation; Microbubble characteristics; Synchrotron X-ray imaging; Ultrasound (US) frequency; Ultrasound (US)irradiation.

Graphical abstract

Fig. 1

Schematic diagram of the PCI setup at the CLS, which was used to image ultrasound-induced cavitation microbubbles in water medium.

Fig. 2

Differences in detector response using in-line phase contrast imaging.

Fig. 3

Photon spectra vs. energy spectrum of white beam at the BMIT-BM 05B1-1 beamline.

Fig. 4

Ultrasound feed compartment design optimization to enhance photon flux and bubble visualization.

Fig. 5

Schematic diagram of the membrane module with ultrasonic transducer employed for bubble imaging at the feed compartment.

Fig. 6

Summary of microbubbles analysis approach.

Fig. 7

a) Raw image generated from the high-speed camera at CLS, b) Area of interest for image analysis using ImageJ, c) Moving microbubbles and bright pixels, d) Outliner function for noise removal, e) Image after subtracting the background and increasing the brightness, f) Image after using the Bandpass Filter, g) Image after applying the Sharpen function.

Fig. 8

The averaged value for each parameter during 2 s of imaging at the US frequency of 20 kHz, 28 kHz, and 40 kHz.

Fig. 9

Images with minimum, average, and maximum number of bubbles and bubble size distribution plots at different US frequencies.

Fig. 9

Images with minimum, average, and maximum number of bubbles and bubble size distribution plots at different US frequencies.

Fig. 9

Images with minimum, average, and maximum number of bubbles and bubble size distribution plots at different US frequencies.

Fig. 10

a) Moving and stationary bubbles at 20 kHz, b) Behaviour of transient and stable bubbles at 20 kHz (4 consecutive frames).

Fig. 11

Microbubble growth pattern at 20 kHz, a) initial bubble size, b) microbubbles after 700 ms, c) bubble size changes with time, d) the correlation between mean growth ratio and time.