Microfluidics and low-Reynolds flow
How do wobbling bubbles fling off their particle coat?
Open access · cc by · source: Europe PMC
When ultrasound makes particle-coated bubbles wobble, particles shoot off only from the bulges where several wobble patterns line up in phase.
Study at a glance
- Design
- Other — Lab experiment: particle-coated air bubbles in a microscope chamber driven by 40 kHz ultrasound, filmed at 300,000 frames per second and decomposed into spatial Fourier modes.
- N
- The number of bubbles analysed is not stated; individual bubbles with radii of 40 to 100 micrometres are shown as examples.
- Population
- Air bubbles coated with 500 nm latex particles in salt water
- Outcome
- Shape-oscillation mode number versus bubble size, mode amplitudes and phases, timing and location of particle desorption
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Key findings
Unlike clean bubbles, coated bubbles showed no link between bubble size and wobble mode: modes from 2 to 8 appeared across the size range. Shape oscillations were subharmonic, repeating every two driving periods. Particles were expelled only from some antinodes, specifically where several coexisting modes (for example n = 2, 4 and 8) had their peaks aligned, at the moment of maximum curvature. Particles also migrated toward antinodes, raising local surface coverage from about 0.2 to about 0.6.
Methodology
The researchers coated air bubbles with 500 nm latex particles and drove them with ultrasound at 40 kHz in a thin microscope chamber. A high-speed camera recorded the bubble outline at 300 000 frames per second, and the outline was broken down into spatial Fourier modes to track each wobble pattern's size and orientation over time. They compared the forces that could push particles off the interface using dimensionless numbers, and imaged larger particles to watch where they gathered.
Limitations
The number of bubbles studied is not reported and the desorption threshold in the force phase diagram is not sharp, which the authors attribute to uncontrolled variation in initial particle coverage. The bubbles sat against a chamber wall, which distorted the measured mean radius and may alter the dynamics. The collective inertial explanation rests on an order-of-magnitude estimate, viscous drag was excluded only by visual timing, and the cause of particle migration to antinodes remains unknown.
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