Bubble dynamics

When exciting microbubbles with ultrasound, shape modes can be triggered in certain parameter ranges (of acoustic pressure, frequency and bubble size). We experimentally studied the triggering and temporal evolution of these shape modes. Among others, we were able to observe energy transfers from parametrically excited modes to other non-excited ones, which validated recent non-linear analytical models. Most importantly, I set up an experimental procedure exploiting bubble coalescence in an acoustic trap to entirely control the triggered shape modes by their mode number and axis of symmetry.

(Work conducted at Université de Lyon at LMFA laboratory.)

Microstreaming around acoustically excited bubbles

Steady surface oscillations of a bubble in a liquid induce a streaming flow in the vicinity of the bubble. This phenomenon is due to nonlinear viscous effects in the oscillating boundary layer. I was able to experimentally visualize different types of induced streaming patterns, classify them and conclude that characterizing the dominant shape mode is not sufficient to define the resulting pattern. Our new analytical model allowed us to confirm that even seemingly minor contributions to the modal content of the shape oscillations can strongly influence the streaming pattern.

(Work conducted at Université de Lyon at LMFA laboratory.)

Flow focusing

Microbubbles find current use as contrast agents in medical ultrasound applications. While commercial products usually consist of a large distribution of bubbles sizes it would be beneficial to use monodisperse bubbles. This motivated us to study monodisperse microbubble production via microfluidics, more precisely flow focusing with production rate of about one million bubbles per second, in order to understand today's limits of upscaling this process for industrial processes. An experimental and numerical study allowed us to discuss the role of liquid and gas properties, and achieve a time resolved characterization of the flow fields inside a flow focusing device.

(Work conducted at University of Twente at Physics of Fluids laboratory.)