Induction of Microstreaming by Nonspherical Bubble Oscillations in an Acoustic Levitation System

The understanding of bubble induced mechanical actions is crucial in many engineering and therapeutic applications. Controlling bubble oscillations and the induced flows in the vicinity is still a challenging task. The proposed techniques allows controlling the shape oscillation of a single bubble trapped in an acoustic levitator.

The induced flows are then visualized and correlated to the bubble dynamics. Estelle Mezianai, a student from Laboratory of Therapeutic Application of Ultrasound will be demonstrating the procedure. Begin with bubble generation by placing the water tank of the oscillator such that a focusing point of the laser is located inside the water tank leading to spark generation for every 5 to 10 millijoules laser pulse.

Switch on the ultrasound transducer and increase the applied voltage until the bubbles no longer rise vertically, but deviate toward the pressure anti-node and become trapped. Set the backlit illumination to the continuous light emitting diode and select the high speed camera to allow observation of the trapped bubble. To trap a bubble and capture its radial oscillations, set the frame size to 128 by 128 pixels and the acquisition rate to 180 kilohertz.

Record the bubble radial oscillations from 3 to 30 milliseconds under increasing applied transducer voltages from 0 to 8 volts. After the last recording, switch off the ultrasound transducer and capture one image of the background for post-analysis. For post-processing of the video series, run the VoltagePressure.

exe file. Specify the physical and experimental parameters and the values of the applied voltage for the series of recordings. In the bubble radius analysis panel, click load parameters and select the folder containing all the video series and background image files.

For each video file, the evolution of the bubble radius will be plotted over one acoustic period and a numerical fit will be superimposed. When all the videos have been processed, click linear regression to perform a linear fit of the pressure voltage curve. The data will be saved into a txt file within the current directory.

To induce bubble coalescence, switch on the ultrasound transducer and set the applied voltage high enough so that the corresponding acoustic pressure may trigger surface instability, nucleate a bubble which will then migrate to its trapping location. When a trapped bubble exhibits only spherical oscillations, generate a new laser spark. When the new bubble reaches the trapping location, coalescence occurs If the coalesced bubble exhibits only spherical oscillations after sparking, generate a new bubble.

But note that multiple coalescences may be necessary to reach the radius at which non-spherical deformations occur. Once the coalesced bubble exhibits non-spherical oscillations, record the bubble oscillations for approximately 3 to 30 milliseconds and use the figure to identify the mode number of the shape oscillations of the bubble. To perform fluid flow measurements, set the frame rate to 180 kilohertz, the frame size to 128 by 128 pixels, and the exposure time to 1 microsecond to record the dynamics of the bubble interface.

To record the motion of the dye tracers, set the frame size to 1024 by 768 pixels, frame rate to 600 hertz, and the exposure time to 1 millisecond. Adjust the position of the laser sheet so that the illuminated particles are visible to the camera and nucleate and trap a bubble as demonstrated. Adjust the position of the laser sheet further so that a shadow becomes visible behind the bubble and induce bubble coalescence until a stably oscillating shape mode is apparent.

Then acquire several recordings, switching between the bubble dynamics and micro streaming. For image processing and analysis, import the cine file containing the captured particle motion into image J and click image, adjust, brightness, contrast, and auto. An automatically optimized image will replace the dark background.

To display the resulting pattern, click image, stacks, and Z project, and select the max intensity option for the image projection. An output image with pixels containing the maximum value over all the images in the stack will be displayed. Shown here is a complete sequence of bubble coalescence leading to time stable symmetry controlled non-spherical oscillations that can be observed.

The approaching phase of two spherically oscillating bubbles ends when the thin liquid film between the two bubbles is ruptured. After the moment of coalescence, a single bubble exhibiting non-spherical oscillations with a complex shape remains, corresponding to the transient regime of oscillations following the excitation of any dynamical system. After a dozen to 100 acoustic periods, the oscillation shapes stabilize to a steady state oscillation.

Once a bubble is trapped and exhibits steady shape oscillations, the motion of the fluorescent tracers within the bubble vicinity can be captured. When shape oscillations occur, liquid motion is produced within the vicinity of the bubble interface. Alternative recording of the dynamics of the bubble interface at the acoustic timescale and of the motion of the particles at a lower timescale allow correlation of the micro streaming pattern to a given shape mode number.

If the dynamics of the bubble interface contain supplementary modes, then the micro streaming flow can be significantly modified due to the multiple interactions between the modes that would generate specific patterns. To safely associate the flow pattern to a given shape sedation, remember that it's necessary to alternatively capture the bubble dynamics and the flow motion. These findings may have practical use in neuropathic applications such as transform mediated drug delivery.

Indeed, acoustic bubbles are known to exert flow induced sheer stresses on cell membranes that lead to their permeation.