A Novel Method for Better Understanding the Behaviour of Acoustically Excited Bubbles

When you think of tiny bubbles spinning or traveling through liquid, likely a few common images come to mind: a wave crashing on the shore, a kitchen faucet filling a sink with water, even a glass of champagne or a gin and tonic. 

When Department of Physics PhD candidate Amin Jafari Sojahrood ponders the motion of bubbles, he thinks more about how to measure the dynamics of their oscillation when influenced by ultrasound. Why? Because bubbles under those conditions are the building block of several applications in fields that range from sonochemistry to material science, underwater exploration and medicine.

In the case of medicine, bubbles are currently being used to diagnose cancer and other pathologies at early stages in a fast, non-invasive and cost-effective manner. For example, tiny bubbles can bind to malignant tumours, making them visible to ultrasound imaging. They are also used to treat tumours non-invasively by delivering drugs directly to the tumour cells. In the case of brain disease, bubble applications can enhance the delivery of drugs past the brain barrier to tumours or other areas affected by conditions such as Alzheimer’s. 

But there are some obstacles to these bubble applications functioning at their full potential and with the greatest level of safety. One of those obstacles is that the dynamics of bubble oscillations when excited by ultrasound are nonlinear and complex. Understanding their complex behaviour is needed in order to control and optimize them. 

That’s where Sojahrood’s research comes in.

“Current tools of the nonlinear dynamics and chaos field are not very effective in gaining detailed knowledge about bubble oscillations,” he explains. “In the published study, the goal of my research was to develop a comprehensive tool that can reveal the oscillation properties of the bubbles without missing any details in an easy to understand and apply manner.”

Using the cutting-edge facilities of the Institute for Biomedical Engineering, Science and Technology (iBEST) (external link) , and under the supervision of Professors Michael Kolios and Raffi Karshafian, and with the contribution of graduate students Dana Wegierak and Hossein Haghi, Sojahrood proposed a novel method of constructing the bifurcation diagrams of the bubble behaviour. The research is published in the journal of Ultrasonics Sonochemistry (external link) .   

“Previous methods could not distinguish the super-harmonic and ultra-harmonic oscillations from linear and subharmonic oscillations,” explains Sojahrood. “The application of my method helped us to identify the super-harmonic and ultra-harmonic regimes of oscillations alongside the conventional subharmonic and fundamental resonances for the first time. We were able to clearly distinguish them from each other and link the optimized exposure parameters to the relevant applications for the first time.”

Providing a tool to better understand bubble dynamics and their effects on the surrounding medium has several implications. In general, Sojahrood’s method can be used to analyze the dynamics of any other nonlinear oscillators and better understand the behaviour of systems in areas such as engineering, economics and biology.

More specifically, the method has an impact on the fields of sonochemistry, industrial, and medical ultrasound. In medical ultrasound, for example, improvements can be made in high-resolution imaging and in more efficient localized drug delivery.

With cancer the leading cause of death in Canada and the increasingly complex challenges of treating brain disorders in an aging population, the impact of Sojahrood’s research in the field of medicine alone is noteworthy. His Vanier Canada Graduate Scholarship and funding from the National Sciences and Engineering Research Council of Canada (NSERC), the Canadian Institutes of Health Research (CIHR), and the Terry Fox New Frontiers Program Project Grants are further evidence of the promising nature of his work.

“Looking forward, I am thrilled to be developing new strategies for diagnosis and treatment in our iBEST lab,” concludes Sojahrood. “The applications of ultrasound-influenced bubbles are wide ranging and, for me, particularly exciting in the biomedical field.”